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README.md

JavaScript Interview Questions & Answers

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Table of Contents

No. Questions
1 What are the possible ways to create objects in JavaScript
2 What is prototype chain
3 What is the difference between Call, Apply and Bind
4 What is JSON and its common operations
5 What is the purpose of the array slice method
6 What is the purpose of the array splice method
7 What is the difference between slice and splice
8 How do you compare Object and Map
9 What is the difference between == and === operators
10 What are lambda or arrow functions
11 What is a first class function
12 What is a first order function
13 What is a higher order function
14 What is a unary function
15 What is the currying function
16 What is a pure function
17 What is the purpose of the let keyword
18 What is the difference between let and var
19 What is the reason to choose the name let as a keyword
20 How do you redeclare variables in switch block without an error
21 What is the Temporal Dead Zone
22 What is IIFE(Immediately Invoked Function Expression)
23 How do you decode or encode a URL in JavaScript?
24 What is memoization
25 What is Hoisting
26 What are classes in ES6
27 What are closures
28 What are modules
29 Why do you need modules
30 What is scope in javascript
31 What is a service worker
32 How do you manipulate DOM using a service worker
33 How do you reuse information across service worker restarts
34 What is IndexedDB
35 What is web storage
36 What is a post message
37 What is a cookie
38 Why do you need a Cookie
39 What are the options in a cookie
40 How do you delete a cookie
41 What are the differences between cookie, local storage and session storage
42 What is the main difference between localStorage and sessionStorage
43 How do you access web storage
44 What are the methods available on session storage
45 What is a storage event and its event handler
46 Why do you need web storage
47 How do you check web storage browser support
48 How do you check web workers browser support
49 Give an example of a web worker
50 What are the restrictions of web workers on DOM
51 What is a promise
52 Why do you need a promise
53 What are the three states of promise
54 What is a callback function
55 Why do we need callbacks
56 What is a callback hell
57 What are server-sent events
58 How do you receive server-sent event notifications
59 How do you check browser support for server-sent events
60 What are the events available for server sent events
61 What are the main rules of promise
62 What is callback in callback
63 What is promise chaining
64 What is promise.all
65 What is the purpose of the race method in promise
66 What is a strict mode in javascript
67 Why do you need strict mode
68 How do you declare strict mode
69 What is the purpose of double exclamation
70 What is the purpose of the delete operator
71 What is typeof operator
72 What is undefined property
73 What is null value
74 What is the difference between null and undefined
75 What is eval
76 What is the difference between window and document
77 How do you access history in javascript
78 How do you detect caps lock key turned on or not
79 What is isNaN
80 What are the differences between undeclared and undefined variables
81 What are global variables
82 What are the problems with global variables
83 What is NaN property
84 What is the purpose of isFinite function
85 What is an event flow
86 What is event bubbling
87 What is event capturing
88 How do you submit a form using JavaScript
89 How do you find operating system details
90 What is the difference between document load and DOMContentLoaded events
91 What is the difference between native, host and user objects
92 What are the tools or techniques used for debugging JavaScript code
93 What are the pros and cons of promises over callbacks
94 What is the difference between an attribute and a property
95 What is same-origin policy
96 What is the purpose of void 0
97 Is JavaScript a compiled or interpreted language
98 Is JavaScript a case-sensitive language
99 Is there any relation between Java and JavaScript
100 What are events
101 Who created javascript
102 What is the use of preventDefault method
103 What is the use of stopPropagation method
104 What are the steps involved in return false usage
105 What is BOM
106 What is the use of setTimeout
107 What is the use of setInterval
108 Why is JavaScript treated as Single threaded
109 What is an event delegation
110 What is ECMAScript
111 What is JSON
112 What are the syntax rules of JSON
113 What is the purpose JSON stringify
114 How do you parse JSON string
115 Why do you need JSON
116 What are PWAs
117 What is the purpose of clearTimeout method
118 What is the purpose of clearInterval method
119 How do you redirect new page in javascript
120 How do you check whether a string contains a substring
121 How do you validate an email in javascript
122 How do you get the current url with javascript
123 What are the various url properties of location object
124 How do get query string values in javascript
125 How do you check if a key exists in an object
126 How do you loop through or enumerate javascript object
127 How do you test for an empty object
128 What is an arguments object
129 How do you make first letter of the string in an uppercase
130 What are the pros and cons of for loop
131 How do you display the current date in javascript
132 How do you compare two date objects
133 How do you check if a string starts with another string
134 How do you trim a string in javascript
135 How do you add a key value pair in javascript
136 Is the '!--' notation represents a special operator
137 How do you assign default values to variables
138 How do you define multiline strings
139 What is an app shell model
140 Can we define properties for functions
141 What is the way to find the number of parameters expected by a function
142 What is a polyfill
143 What are break and continue statements
144 What are js labels
145 What are the benefits of keeping declarations at the top
146 What are the benefits of initializing variables
147 What are the recommendations to create new object
148 How do you define JSON arrays
149 How do you generate random integers
150 Can you write a random integers function to print integers with in a range
151 What is tree shaking
152 What is the need of tree shaking
153 Is it recommended to use eval
154 What is a Regular Expression
155 What are the string methods available in Regular expression
156 What are modifiers in regular expression
157 What are regular expression patterns
158 What is a RegExp object
159 How do you search a string for a pattern
160 What is the purpose of exec method
161 How do you change the style of a HTML element
162 What would be the result of 1+2+'3'
163 What is a debugger statement
164 What is the purpose of breakpoints in debugging
165 Can I use reserved words as identifiers
166 How do you detect a mobile browser
167 How do you detect a mobile browser without regexp
168 How do you get the image width and height using JS
169 How do you make synchronous HTTP request
170 How do you make asynchronous HTTP request
171 How do you convert date to another timezone in javascript
172 What are the properties used to get size of window
173 What is a conditional operator in javascript
174 Can you apply chaining on conditional operator
175 What are the ways to execute javascript after page load
176 What is the difference between proto and prototype
177 Give an example where do you really need semicolon
178 What is a freeze method
179 What is the purpose of freeze method
180 Why do I need to use freeze method
181 How do you detect a browser language preference
182 How to convert string to title case with javascript
183 How do you detect javascript disabled in the page
184 What are various operators supported by javascript
185 What is a rest parameter
186 What happens if you do not use rest parameter as a last argument
187 What are the bitwise operators available in javascript
188 What is a spread operator
189 How do you determine whether object is frozen or not
190 How do you determine two values same or not using object
191 What is the purpose of using object is method
192 How do you copy properties from one object to other
193 What are the applications of assign method
194 What is a proxy object
195 What is the purpose of seal method
196 What are the applications of seal method
197 What are the differences between freeze and seal methods
198 How do you determine if an object is sealed or not
199 How do you get enumerable key and value pairs
200 What is the main difference between Object.values and Object.entries method
201 How can you get the list of keys of any object
202 How do you create an object with prototype
203 What is a WeakSet
204 What are the differences between WeakSet and Set
205 List down the collection of methods available on WeakSet
206 What is a WeakMap
207 What are the differences between WeakMap and Map
208 List down the collection of methods available on WeakMap
209 What is the purpose of uneval
210 How do you encode an URL
211 How do you decode an URL
212 How do you print the contents of web page
213 What is the difference between uneval and eval
214 What is an anonymous function
215 What is the precedence order between local and global variables
216 What are javascript accessors
217 How do you define property on Object constructor
218 What is the difference between get and defineProperty
219 What are the advantages of Getters and Setters
220 Can I add getters and setters using defineProperty method
221 What is the purpose of switch-case
222 What are the conventions to be followed for the usage of switch case
223 What are primitive data types
224 What are the different ways to access object properties
225 What are the function parameter rules
226 What is an error object
227 When you get a syntax error
228 What are the different error names from error object
229 What are the various statements in error handling
230 What are the two types of loops in javascript
231 What is nodejs
232 What is an Intl object
233 How do you perform language specific date and time formatting
234 What is an Iterator
235 How does synchronous iteration works
236 What is an event loop
237 What is call stack
238 What is an event queue
239 What is a decorator
240 What are the properties of Intl object
241 What is an Unary operator
242 How do you sort elements in an array
243 What is the purpose of compareFunction while sorting arrays
244 How do you reversing an array
245 How do you find min and max value in an array
246 How do you find min and max values without Math functions
247 What is an empty statement and purpose of it
248 How do you get metadata of a module
249 What is a comma operator
250 What is the advantage of a comma operator
251 What is typescript
252 What are the differences between javascript and typescript
253 What are the advantages of typescript over javascript
254 What is an object initializer
255 What is a constructor method
256 What happens if you write constructor more than once in a class
257 How do you call the constructor of a parent class
258 How do you get the prototype of an object
259 What happens If I pass string type for getPrototype method
260 How do you set prototype of one object to another
261 How do you check whether an object can be extendable or not
262 How do you prevent an object to extend
263 What are the different ways to make an object non-extensible
264 How do you define multiple properties on an object
265 What is MEAN in javascript
266 What Is Obfuscation in javascript
267 Why do you need Obfuscation
268 What is Minification
269 What are the advantages of minification
270 What are the differences between Obfuscation and Encryption
271 What are the common tools used for minification
272 How do you perform form validation using javascript
273 How do you perform form validation without javascript
274 What are the DOM methods available for constraint validation
275 What are the available constraint validation DOM properties
276 What are the list of validity properties
277 Give an example usage of rangeOverflow property
278 Is enums feature available in javascript
279 What is an enum
280 How do you list all properties of an object
281 How do you get property descriptors of an object
282 What are the attributes provided by a property descriptor
283 How do you extend classes
284 How do I modify the url without reloading the page
285 How do you check whether an array includes a particular value or not
286 How do you compare scalar arrays
287 How to get the value from get parameters
288 How do you print numbers with commas as thousand separators
289 What is the difference between java and javascript
290 Does javascript supports namespace
291 How do you declare namespace
292 How do you invoke javascript code in an iframe from parent page
293 How do get the timezone offset from date
294 How do you load CSS and JS files dynamically
295 What are the different methods to find HTML elements in DOM
296 What is jQuery
297 What is V8 JavaScript engine
298 Why do we call javascript as dynamic language
299 What is a void operator
300 How to set the cursor to wait
301 How do you create an infinite loop
302 Why do you need to avoid with statement
303 What is the output of below for loops
304 List down some of the features of ES6
305 What is ES6
306 Can I redeclare let and const variables
307 Is const variable makes the value immutable
308 What are default parameters
309 What are template literals
310 How do you write multi-line strings in template literals
311 What are nesting templates
312 What are tagged templates
313 What are raw strings
314 What is destructuring assignment
315 What are default values in destructuring assignment
316 How do you swap variables in destructuring assignment
317 What are enhanced object literals
318 What are dynamic imports
319 What are the use cases for dynamic imports
320 What are typed arrays
321 What are the advantages of module loaders
322 What is collation
323 What is for...of statement
324 What is the output of below spread operator array
325 Is PostMessage secure
326 What are the problems with postmessage target origin as wildcard
327 How do you avoid receiving postMessages from attackers
328 Can I avoid using postMessages completely
329 Is postMessages synchronous
330 What paradigm is Javascript
331 What is the difference between internal and external javascript
332 Is JavaScript faster than server side script
333 How do you get the status of a checkbox
334 What is the purpose of double tilde operator
335 How do you convert character to ASCII code
336 What is ArrayBuffer
337 What is the output of below string expression
338 What is the purpose of Error object
339 What is the purpose of EvalError object
340 What are the list of cases error thrown from non-strict mode to strict mode
341 Do all objects have prototypes
342 What is the difference between a parameter and an argument
343 What is the purpose of some method in arrays
344 How do you combine two or more arrays
345 What is the difference between Shallow and Deep copy
346 How do you create specific number of copies of a string
347 How do you return all matching strings against a regular expression
348 How do you trim a string at the beginning or ending
349 What is the output of below console statement with unary operator
350 Does javascript uses mixins
351 What is a thunk function
352 What are asynchronous thunks
353 What is the output of below function calls
354 How to remove all line breaks from a string
355 What is the difference between reflow and repaint
356 What happens with negating an array
357 What happens if we add two arrays
358 What is the output of prepend additive operator on falsy values
359 How do you create self string using special characters
360 How do you remove falsy values from an array
361 How do you get unique values of an array
362 What is destructuring aliases
363 How do you map the array values without using map method
364 How do you empty an array
365 How do you rounding numbers to certain decimals
366 What is the easiest way to convert an array to an object
367 How do you create an array with some data
368 What are the placeholders from console object
369 Is it possible to add CSS to console messages
370 What is the purpose of dir method of console object
371 Is it possible to debug HTML elements in console
372 How do you display data in a tabular format using console object
373 How do you verify that an argument is a Number or not
374 How do you create copy to clipboard button
375 What is the shortcut to get timestamp
376 How do you flattening multi dimensional arrays
377 What is the easiest multi condition checking
378 How do you capture browser back button
379 How do you disable right click in the web page
380 What are wrapper objects
381 What is AJAX
382 What are the different ways to deal with Asynchronous Code
383 How to cancel a fetch request
384 What is web speech API
385 What is minimum timeout throttling
386 How do you implement zero timeout in modern browsers
387 What are tasks in event loop
388 What is microtask
389 What are different event loops
390 What is the purpose of queueMicrotask
391 How do you use javascript libraries in typescript file
392 What are the differences between promises and observables
393 What is heap
394 What is an event table
395 What is a microTask queue
396 What is the difference between shim and polyfill
397 How do you detect primitive or non primitive value type
398 What is babel
399 Is Node.js completely single threaded
400 What are the common use cases of observables
401 What is RxJS
402 What is the difference between Function constructor and function declaration
403 What is a Short circuit condition
404 What is the easiest way to resize an array
405 What is an observable
406 What is the difference between function and class declarations
407 What is an async function
408 How do you prevent promises swallowing errors
409 What is deno
410 How do you make an object iterable in javascript
411 What is a Proper Tail Call
412 How do you check an object is a promise or not
413 How to detect if a function is called as constructor
414 What are the differences between arguments object and rest parameter
415 What are the differences between spread operator and rest parameter
416 What are the different kinds of generators
417 What are the built-in iterables
418 What are the differences between for...of and for...in statements
419 How do you define instance and non-instance properties
420 What is the difference between isNaN and Number.isNaN?
421 How to invoke an IIFE without any extra brackets?
422 Is that possible to use expressions in switch cases?
423 What is the easiest way to ignore promise errors?
424 How do style the console output using CSS?
425 What is nullish coalescing operator (??)?
426 How do you group and nest console output?
427 What is the difference between dense and sparse arrays?
428 What are the different ways to create sparse arrays?
429 What is the difference between setTimeout, setImmediate and process.nextTick?
430 How do you reverse an array without modifying original array?
431 How do you create custom HTML element?
432 What is global execution context?
433 What is function execution context?
434 What is debouncing?
435 What is throttling?
436 What is optional chaining?
  1. What are the possible ways to create objects in JavaScript

    There are many ways to create objects in javascript as below

    1. Object constructor:

      The simplest way to create an empty object is using the Object constructor. Currently this approach is not recommended.

      var object = new Object();
    2. Object's create method:

      The create method of Object creates a new object by passing the prototype object as a parameter

      var object = Object.create(null);
    3. Object literal syntax:

      The object literal syntax (or object initializer), is a comma-separated set of name-value pairs wrapped in curly braces.

      var object = {
           name: "Sudheer",
           age: 34
      };
      
      Object literal property values can be of any data type, including array, function, and nested object.

      Note: This is an easiest way to create an object

    4. Function constructor:

      Create any function and apply the new operator to create object instances,

      function Person(name) {
        this.name = name;
        this.age = 21;
      }
      var object = new Person("Sudheer");
    5. Function constructor with prototype:

      This is similar to function constructor but it uses prototype for their properties and methods,

      function Person() {}
      Person.prototype.name = "Sudheer";
      var object = new Person();

      This is equivalent to an instance created with an object create method with a function prototype and then call that function with an instance and parameters as arguments.

      function func() {};
      
      new func(x, y, z);

      (OR)

      // Create a new instance using function prototype.
      var newInstance = Object.create(func.prototype)
      
      // Call the function
      var result = func.call(newInstance, x, y, z),
      
      // If the result is a non-null object then use it otherwise just use the new instance.
      console.log(result && typeof result === 'object' ? result : newInstance);
    6. ES6 Class syntax:

      ES6 introduces class feature to create the objects

      class Person {
        constructor(name) {
          this.name = name;
        }
      }
      
      var object = new Person("Sudheer");
    7. Singleton pattern:

      A Singleton is an object which can only be instantiated one time. Repeated calls to its constructor return the same instance and this way one can ensure that they don't accidentally create multiple instances.

      var object = new (function () {
        this.name = "Sudheer";
      })();

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  2. What is a prototype chain

    Prototype chaining is used to build new types of objects based on existing ones. It is similar to inheritance in a class based language.

    The prototype on object instance is available through Object.getPrototypeOf(object) or **proto** property whereas prototype on constructors function is available through Object.prototype.

    Screenshot

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  3. What is the difference between Call, Apply and Bind

    The difference between Call, Apply and Bind can be explained with below examples,

    Call: The call() method invokes a function with a given this value and arguments provided one by one

    var employee1 = { firstName: "John", lastName: "Rodson" };
    var employee2 = { firstName: "Jimmy", lastName: "Baily" };
    
    function invite(greeting1, greeting2) {
      console.log(
        greeting1 + " " + this.firstName + " " + this.lastName + ", " + greeting2
      );
    }
    
    invite.call(employee1, "Hello", "How are you?"); // Hello John Rodson, How are you?
    invite.call(employee2, "Hello", "How are you?"); // Hello Jimmy Baily, How are you?

    Apply: Invokes the function with a given this value and allows you to pass in arguments as an array

    var employee1 = { firstName: "John", lastName: "Rodson" };
    var employee2 = { firstName: "Jimmy", lastName: "Baily" };
    
    function invite(greeting1, greeting2) {
      console.log(
        greeting1 + " " + this.firstName + " " + this.lastName + ", " + greeting2
      );
    }
    
    invite.apply(employee1, ["Hello", "How are you?"]); // Hello John Rodson, How are you?
    invite.apply(employee2, ["Hello", "How are you?"]); // Hello Jimmy Baily, How are you?

    bind: returns a new function, allowing you to pass any number of arguments

    var employee1 = { firstName: "John", lastName: "Rodson" };
    var employee2 = { firstName: "Jimmy", lastName: "Baily" };
    
    function invite(greeting1, greeting2) {
      console.log(
        greeting1 + " " + this.firstName + " " + this.lastName + ", " + greeting2
      );
    }
    
    var inviteEmployee1 = invite.bind(employee1);
    var inviteEmployee2 = invite.bind(employee2);
    inviteEmployee1("Hello", "How are you?"); // Hello John Rodson, How are you?
    inviteEmployee2("Hello", "How are you?"); // Hello Jimmy Baily, How are you?

    Call and apply are pretty interchangeable. Both execute the current function immediately. You need to decide whether it’s easier to send in an array or a comma separated list of arguments. You can remember by treating Call is for comma (separated list) and Apply is for Array.

    Whereas Bind creates a new function that will have this set to the first parameter passed to bind().

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  4. What is JSON and its common operations

    JSON is a text-based data format following JavaScript object syntax, which was popularized by Douglas Crockford. It is useful when you want to transmit data across a network and it is basically just a text file with an extension of .json, and a MIME type of application/json

    Parsing: Converting a string to a native object

    JSON.parse(text);

    Stringification: converting a native object to a string so it can be transmitted across the network

    JSON.stringify(object);

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  5. What is the purpose of the array slice method

    The slice() method returns the selected elements in an array as a new array object. It selects the elements starting at the given start argument, and ends at the given optional end argument without including the last element. If you omit the second argument then it selects till the end.

    Some of the examples of this method are,

    let arrayIntegers = [1, 2, 3, 4, 5];
    let arrayIntegers1 = arrayIntegers.slice(0, 2); // returns [1,2]
    let arrayIntegers2 = arrayIntegers.slice(2, 3); // returns [3]
    let arrayIntegers3 = arrayIntegers.slice(4); //returns [5]

    Note: Slice method won't mutate the original array but it returns the subset as a new array.

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  6. What is the purpose of the array splice method

    The splice() method is used either adds/removes items to/from an array, and then returns the removed item. The first argument specifies the array position for insertion or deletion whereas the optional second argument indicates the number of elements to be deleted. Each additional argument is added to the array.

    Some of the examples of this method are,

    let arrayIntegersOriginal1 = [1, 2, 3, 4, 5];
    let arrayIntegersOriginal2 = [1, 2, 3, 4, 5];
    let arrayIntegersOriginal3 = [1, 2, 3, 4, 5];
    
    let arrayIntegers1 = arrayIntegersOriginal1.splice(0, 2); // returns [1, 2]; original array: [3, 4, 5]
    let arrayIntegers2 = arrayIntegersOriginal2.splice(3); // returns [4, 5]; original array: [1, 2, 3]
    let arrayIntegers3 = arrayIntegersOriginal3.splice(3, 1, "a", "b", "c"); //returns [4]; original array: [1, 2, 3, "a", "b", "c", 5]

    Note: Splice method modifies the original array and returns the deleted array.

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  7. What is the difference between slice and splice

    Some of the major difference in a tabular form

    Slice Splice
    Doesn't modify the original array(immutable) Modifies the original array(mutable)
    Returns the subset of original array Returns the deleted elements as array
    Used to pick the elements from array Used to insert or delete elements to/from array

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  8. How do you compare Object and Map

    Objects are similar to Maps in that both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Due to this reason, Objects have been used as Maps historically. But there are important differences that make using a Map preferable in certain cases.

    1. The keys of an Object are Strings and Symbols, whereas they can be any value for a Map, including functions, objects, and any primitive.
    2. The keys in Map are ordered while keys added to Object are not. Thus, when iterating over it, a Map object returns keys in order of insertion.
    3. You can get the size of a Map easily with the size property, while the number of properties in an Object must be determined manually.
    4. A Map is an iterable and can thus be directly iterated, whereas iterating over an Object requires obtaining its keys in some fashion and iterating over them.
    5. An Object has a prototype, so there are default keys in the map that could collide with your keys if you're not careful. As of ES5 this can be bypassed by using map = Object.create(null), but this is seldom done.
    6. A Map may perform better in scenarios involving frequent addition and removal of key pairs.

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  9. What is the difference between == and === operators

    JavaScript provides both strict(===, !==) and type-converting(==, !=) equality comparison. The strict operators take type of variable in consideration, while non-strict operators make type correction/conversion based upon values of variables. The strict operators follow the below conditions for different types,

    1. Two strings are strictly equal when they have the same sequence of characters, same length, and same characters in corresponding positions.
    2. Two numbers are strictly equal when they are numerically equal. i.e, Having the same number value. There are two special cases in this,
      1. NaN is not equal to anything, including NaN.
      2. Positive and negative zeros are equal to one another.
    3. Two Boolean operands are strictly equal if both are true or both are false.
    4. Two objects are strictly equal if they refer to the same Object.
    5. Null and Undefined types are not equal with ===, but equal with ==. i.e, null===undefined --> false but null==undefined --> true

    Some of the example which covers the above cases,

    0 == false   // true
    0 === false  // false
    1 == "1"     // true
    1 === "1"    // false
    null == undefined // true
    null === undefined // false
    '0' == false // true
    '0' === false // false
    []==[] or []===[] //false, refer different objects in memory
    {}=={} or {}==={} //false, refer different objects in memory

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  10. What are lambda or arrow functions

    An arrow function is a shorter syntax for a function expression and does not have its own this, arguments, super, or new.target. These functions are best suited for non-method functions, and they cannot be used as constructors.

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  11. What is a first class function

    In Javascript, functions are first class objects. First-class functions means when functions in that language are treated like any other variable.

    For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable. For example, in the below example, handler functions assigned to a listener

    const handler = () => console.log("This is a click handler function");
    document.addEventListener("click", handler);

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  12. What is a first order function

    First-order function is a function that doesn’t accept another function as an argument and doesn’t return a function as its return value.

    const firstOrder = () => console.log("I am a first order function!");

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  13. What is a higher order function

    Higher-order function is a function that accepts another function as an argument or returns a function as a return value or both.

    const firstOrderFunc = () =>
      console.log("Hello, I am a First order function");
    const higherOrder = (ReturnFirstOrderFunc) => ReturnFirstOrderFunc();
    higherOrder(firstOrderFunc);

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  14. What is a unary function

    Unary function (i.e. monadic) is a function that accepts exactly one argument. It stands for a single argument accepted by a function.

    Let us take an example of unary function,

    const unaryFunction = (a) => console.log(a + 10); // Add 10 to the given argument and display the value

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  15. What is the currying function

    Currying is the process of taking a function with multiple arguments and turning it into a sequence of functions each with only a single argument. Currying is named after a mathematician Haskell Curry. By applying currying, a n-ary function turns it into a unary function.

    Let's take an example of n-ary function and how it turns into a currying function,

    const multiArgFunction = (a, b, c) => a + b + c;
    console.log(multiArgFunction(1, 2, 3)); // 6
    
    const curryUnaryFunction = (a) => (b) => (c) => a + b + c;
    curryUnaryFunction(1); // returns a function: b => c =>  1 + b + c
    curryUnaryFunction(1)(2); // returns a function: c => 3 + c
    curryUnaryFunction(1)(2)(3); // returns the number 6

    Curried functions are great to improve code reusability and functional composition.

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  16. What is a pure function

    A Pure function is a function where the return value is only determined by its arguments without any side effects. i.e, If you call a function with the same arguments 'n' number of times and 'n' number of places in the application then it will always return the same value.

    Let's take an example to see the difference between pure and impure functions,

    //Impure
    let numberArray = [];
    const impureAddNumber = (number) => numberArray.push(number);
    //Pure
    const pureAddNumber = (number) => (argNumberArray) =>
      argNumberArray.concat([number]);
    
    //Display the results
    console.log(impureAddNumber(6)); // returns 1
    console.log(numberArray); // returns [6]
    console.log(pureAddNumber(7)(numberArray)); // returns [6, 7]
    console.log(numberArray); // returns [6]

    As per the above code snippets, the Push function is impure itself by altering the array and returning a push number index independent of the parameter value. . Whereas Concat on the other hand takes the array and concatenates it with the other array producing a whole new array without side effects. Also, the return value is a concatenation of the previous array.

    Remember that Pure functions are important as they simplify unit testing without any side effects and no need for dependency injection. They also avoid tight coupling and make it harder to break your application by not having any side effects. These principles are coming together with Immutability concept of ES6 by giving preference to const over let usage.

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  17. What is the purpose of the let keyword

    The let statement declares a block scope local variable. Hence the variables defined with let keyword are limited in scope to the block, statement, or expression on which it is used. Whereas variables declared with the var keyword used to define a variable globally, or locally to an entire function regardless of block scope.

    Let's take an example to demonstrate the usage,

    let counter = 30;
    if (counter === 30) {
      let counter = 31;
      console.log(counter); // 31
    }
    console.log(counter); // 30 (because the variable in if block won't exist here)

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  18. What is the difference between let and var

    You can list out the differences in a tabular format

    var let
    It is been available from the beginning of JavaScript Introduced as part of ES6
    It has function scope It has block scope
    Variables will be hoisted Hoisted but not initialized

    Let's take an example to see the difference,

    function userDetails(username) {
      if (username) {
        console.log(salary); // undefined due to hoisting
        console.log(age); // ReferenceError: Cannot access 'age' before initialization
        let age = 30;
        var salary = 10000;
      }
      console.log(salary); //10000 (accessible to due function scope)
      console.log(age); //error: age is not defined(due to block scope)
    }
    userDetails("John");

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  19. What is the reason to choose the name let as a keyword

    let is a mathematical statement that was adopted by early programming languages like Scheme and Basic. It has been borrowed from dozens of other languages that use let already as a traditional keyword as close to var as possible.

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  20. How do you redeclare variables in switch block without an error

    If you try to redeclare variables in a switch block then it will cause errors because there is only one block. For example, the below code block throws a syntax error as below,

    let counter = 1;
    switch (x) {
      case 0:
        let name;
        break;
    
      case 1:
        let name; // SyntaxError for redeclaration.
        break;
    }

    To avoid this error, you can create a nested block inside a case clause and create a new block scoped lexical environment.

    let counter = 1;
    switch (x) {
      case 0: {
        let name;
        break;
      }
      case 1: {
        let name; // No SyntaxError for redeclaration.
        break;
      }
    }

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  21. What is the Temporal Dead Zone

    The Temporal Dead Zone is a behavior in JavaScript that occurs when declaring a variable with the let and const keywords, but not with var. In ECMAScript 6, accessing a let or const variable before its declaration (within its scope) causes a ReferenceError. The time span when that happens, between the creation of a variable’s binding and its declaration, is called the temporal dead zone.

    Let's see this behavior with an example,

    function somemethod() {
      console.log(counter1); // undefined
      console.log(counter2); // ReferenceError
      var counter1 = 1;
      let counter2 = 2;
    }

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  22. What is IIFE(Immediately Invoked Function Expression)

    IIFE (Immediately Invoked Function Expression) is a JavaScript function that runs as soon as it is defined. The signature of it would be as below,

    (function () {
      // logic here
    })();

    The primary reason to use an IIFE is to obtain data privacy because any variables declared within the IIFE cannot be accessed by the outside world. i.e, If you try to access variables with IIFE then it throws an error as below,

    (function () {
      var message = "IIFE";
      console.log(message);
    })();
    console.log(message); //Error: message is not defined

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  23. How do you decode or encode a URL in JavaScript?

    encodeURI() function is used to encode an URL. This function requires a URL string as a parameter and return that encoded string. decodeURI() function is used to decode an URL. This function requires an encoded URL string as parameter and return that decoded string.

    Note: If you want to encode characters such as / ? : @ & = + $ # then you need to use encodeURIComponent().

    let uri = "employeeDetails?name=john&occupation=manager";
    let encoded_uri = encodeURI(uri);
    let decoded_uri = decodeURI(encoded_uri);

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  24. What is memoization

    Memoization is a programming technique which attempts to increase a function’s performance by caching its previously computed results. Each time a memoized function is called, its parameters are used to index the cache. If the data is present, then it can be returned, without executing the entire function. Otherwise the function is executed and then the result is added to the cache. Let's take an example of adding function with memoization,

    const memoizAddition = () => {
      let cache = {};
      return (value) => {
        if (value in cache) {
          console.log("Fetching from cache");
          return cache[value]; // Here, cache.value cannot be used as property name starts with the number which is not a valid JavaScript  identifier. Hence, can only be accessed using the square bracket notation.
        } else {
          console.log("Calculating result");
          let result = value + 20;
          cache[value] = result;
          return result;
        }
      };
    };
    // returned function from memoizAddition
    const addition = memoizAddition();
    console.log(addition(20)); //output: 40 calculated
    console.log(addition(20)); //output: 40 cached

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  25. What is Hoisting

    Hoisting is a JavaScript mechanism where variables, function declarations and classes are moved to the top of their scope before code execution. Remember that JavaScript only hoists declarations, not initialisation. Let's take a simple example of variable hoisting,

    console.log(message); //output : undefined
    var message = "The variable Has been hoisted";

    The above code looks like as below to the interpreter,

    var message;
    console.log(message);
    message = "The variable Has been hoisted";

    In the same fashion, function declarations are hoisted too

    message("Good morning"); //Good morning
    
    function message(name) {
      console.log(name);
    }

    This hoisting makes functions to be safely used in code before they are declared.

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  26. What are classes in ES6

    In ES6, Javascript classes are primarily syntactic sugar over JavaScript’s existing prototype-based inheritance. For example, the prototype based inheritance written in function expression as below,

    function Bike(model, color) {
      this.model = model;
      this.color = color;
    }
    
    Bike.prototype.getDetails = function () {
      return this.model + " bike has" + this.color + " color";
    };

    Whereas ES6 classes can be defined as an alternative

    class Bike {
      constructor(color, model) {
        this.color = color;
        this.model = model;
      }
    
      getDetails() {
        return this.model + " bike has" + this.color + " color";
      }
    }

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  27. What are closures

    A closure is the combination of a function and the lexical environment within which that function was declared. i.e, It is an inner function that has access to the outer or enclosing function’s variables. The closure has three scope chains

    1. Own scope where variables defined between its curly brackets
    2. Outer function’s variables
    3. Global variables

    Let's take an example of closure concept,

    function Welcome(name) {
      var greetingInfo = function (message) {
        console.log(message + " " + name);
      };
      return greetingInfo;
    }
    var myFunction = Welcome("John");
    myFunction("Welcome "); //Output: Welcome John
    myFunction("Hello Mr."); //output: Hello Mr.John

    As per the above code, the inner function(i.e, greetingInfo) has access to the variables in the outer function scope(i.e, Welcome) even after the outer function has returned.

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  28. What are modules

    Modules refer to small units of independent, reusable code and also act as the foundation of many JavaScript design patterns. Most of the JavaScript modules export an object literal, a function, or a constructor

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  29. Why do you need modules

    Below are the list of benefits using modules in javascript ecosystem

    1. Maintainability
    2. Reusability
    3. Namespacing

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  30. What is scope in javascript

    Scope is the accessibility of variables, functions, and objects in some particular part of your code during runtime. In other words, scope determines the visibility of variables and other resources in areas of your code.

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  31. What is a service worker

    A Service worker is basically a script (JavaScript file) that runs in the background, separate from a web page and provides features that don't need a web page or user interaction. Some of the major features of service workers are Rich offline experiences(offline first web application development), periodic background syncs, push notifications, intercept and handle network requests and programmatically managing a cache of responses.

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  32. How do you manipulate DOM using a service worker

    Service worker can't access the DOM directly. But it can communicate with the pages it controls by responding to messages sent via the postMessage interface, and those pages can manipulate the DOM.

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  33. How do you reuse information across service worker restarts

    The problem with service worker is that it gets terminated when not in use, and restarted when it's next needed, so you cannot rely on global state within a service worker's onfetch and onmessage handlers. In this case, service workers will have access to IndexedDB API in order to persist and reuse across restarts.

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  34. What is IndexedDB

    IndexedDB is a low-level API for client-side storage of larger amounts of structured data, including files/blobs. This API uses indexes to enable high-performance searches of this data.

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  35. What is web storage

    Web storage is an API that provides a mechanism by which browsers can store key/value pairs locally within the user's browser, in a much more intuitive fashion than using cookies. The web storage provides two mechanisms for storing data on the client.

    1. Local storage: It stores data for current origin with no expiration date.
    2. Session storage: It stores data for one session and the data is lost when the browser tab is closed.

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  36. What is a post message

    Post message is a method that enables cross-origin communication between Window objects.(i.e, between a page and a pop-up that it spawned, or between a page and an iframe embedded within it). Generally, scripts on different pages are allowed to access each other if and only if the pages follow same-origin policy(i.e, pages share the same protocol, port number, and host).

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  37. What is a Cookie

    A cookie is a piece of data that is stored on your computer to be accessed by your browser. Cookies are saved as key/value pairs. For example, you can create a cookie named username as below,

    document.cookie = "username=John";

    Screenshot

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  38. Why do you need a Cookie

    Cookies are used to remember information about the user profile(such as username). It basically involves two steps,

    1. When a user visits a web page, the user profile can be stored in a cookie.
    2. Next time the user visits the page, the cookie remembers the user profile.

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  39. What are the options in a cookie

    There are few below options available for a cookie,

    1. By default, the cookie is deleted when the browser is closed but you can change this behavior by setting expiry date (in UTC time).
    document.cookie = "username=John; expires=Sat, 8 Jun 2019 12:00:00 UTC";
    1. By default, the cookie belongs to a current page. But you can tell the browser what path the cookie belongs to using a path parameter.
    document.cookie = "username=John; path=/services";

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  40. How do you delete a cookie

    You can delete a cookie by setting the expiry date as a passed date. You don't need to specify a cookie value in this case. For example, you can delete a username cookie in the current page as below.

    document.cookie =
      "username=; expires=Fri, 07 Jun 2019 00:00:00 UTC; path=/;";

    Note: You should define the cookie path option to ensure that you delete the right cookie. Some browsers doesn't allow to delete a cookie unless you specify a path parameter.

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  41. What are the differences between cookie, local storage and session storage

    Below are some of the differences between cookie, local storage and session storage,

    Feature Cookie Local storage Session storage
    Accessed on client or server side Both server-side & client-side client-side only client-side only
    Lifetime As configured using Expires option until deleted until tab is closed
    SSL support Supported Not supported Not supported
    Maximum data size 4KB 5 MB 5MB

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  42. What is the main difference between localStorage and sessionStorage

    LocalStorage is the same as SessionStorage but it persists the data even when the browser is closed and reopened(i.e it has no expiration time) whereas in sessionStorage data gets cleared when the page session ends.

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  43. How do you access web storage

    The Window object implements the WindowLocalStorage and WindowSessionStorage objects which has localStorage(window.localStorage) and sessionStorage(window.sessionStorage) properties respectively. These properties create an instance of the Storage object, through which data items can be set, retrieved and removed for a specific domain and storage type (session or local). For example, you can read and write on local storage objects as below

    localStorage.setItem("logo", document.getElementById("logo").value);
    localStorage.getItem("logo");

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  44. What are the methods available on session storage

    The session storage provided methods for reading, writing and clearing the session data

    // Save data to sessionStorage
    sessionStorage.setItem("key", "value");
    
    // Get saved data from sessionStorage
    let data = sessionStorage.getItem("key");
    
    // Remove saved data from sessionStorage
    sessionStorage.removeItem("key");
    
    // Remove all saved data from sessionStorage
    sessionStorage.clear();

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  45. What is a storage event and its event handler

    The StorageEvent is an event that fires when a storage area has been changed in the context of another document. Whereas onstorage property is an EventHandler for processing storage events. The syntax would be as below

    window.onstorage = functionRef;

    Let's take the example usage of onstorage event handler which logs the storage key and it's values

    window.onstorage = function (e) {
      console.log(
        "The " +
          e.key +
          " key has been changed from " +
          e.oldValue +
          " to " +
          e.newValue +
          "."
      );
    };

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  46. Why do you need web storage

    Web storage is more secure, and large amounts of data can be stored locally, without affecting website performance. Also, the information is never transferred to the server. Hence this is a more recommended approach than Cookies.

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  47. How do you check web storage browser support

    You need to check browser support for localStorage and sessionStorage before using web storage,

    if (typeof Storage !== "undefined") {
      // Code for localStorage/sessionStorage.
    } else {
      // Sorry! No Web Storage support..
    }

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  48. How do you check web workers browser support

    You need to check browser support for web workers before using it

    if (typeof Worker !== "undefined") {
      // code for Web worker support.
    } else {
      // Sorry! No Web Worker support..
    }

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  49. Give an example of a web worker

    You need to follow below steps to start using web workers for counting example

    1. Create a Web Worker File: You need to write a script to increment the count value. Let's name it as counter.js
    let i = 0;
    
    function timedCount() {
      i = i + 1;
      postMessage(i);
      setTimeout("timedCount()", 500);
    }
    
    timedCount();

    Here postMessage() method is used to post a message back to the HTML page

    1. Create a Web Worker Object: You can create a web worker object by checking for browser support. Let's name this file as web_worker_example.js
    if (typeof w == "undefined") {
      w = new Worker("counter.js");
    }

    and we can receive messages from web worker

    w.onmessage = function (event) {
      document.getElementById("message").innerHTML = event.data;
    };
    1. Terminate a Web Worker: Web workers will continue to listen for messages (even after the external script is finished) until it is terminated. You can use the terminate() method to terminate listening to the messages.
    w.terminate();
    1. Reuse the Web Worker: If you set the worker variable to undefined you can reuse the code
    w = undefined;

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  50. What are the restrictions of web workers on DOM

    WebWorkers don't have access to below javascript objects since they are defined in an external files

    1. Window object
    2. Document object
    3. Parent object

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  51. What is a promise

    A promise is an object that may produce a single value some time in the future with either a resolved value or a reason that it’s not resolved(for example, network error). It will be in one of the 3 possible states: fulfilled, rejected, or pending.

    The syntax of Promise creation looks like below,

    const promise = new Promise(function (resolve, reject) {
      // promise description
    });

    The usage of a promise would be as below,

    const promise = new Promise(
      (resolve) => {
        setTimeout(() => {
          resolve("I'm a Promise!");
        }, 5000);
      },
      (reject) => {}
    );
    
    promise.then((value) => console.log(value));

    The action flow of a promise will be as below,

    Screenshot

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  52. Why do you need a promise

    Promises are used to handle asynchronous operations. They provide an alternative approach for callbacks by reducing the callback hell and writing the cleaner code.

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  53. What are the three states of promise

    Promises have three states:

    1. Pending: This is an initial state of the Promise before an operation begins
    2. Fulfilled: This state indicates that the specified operation was completed.
    3. Rejected: This state indicates that the operation did not complete. In this case an error value will be thrown.

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  54. What is a callback function

    A callback function is a function passed into another function as an argument. This function is invoked inside the outer function to complete an action. Let's take a simple example of how to use callback function

    function callbackFunction(name) {
      console.log("Hello " + name);
    }
    
    function outerFunction(callback) {
      let name = prompt("Please enter your name.");
      callback(name);
    }
    
    outerFunction(callbackFunction);

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  55. Why do we need callbacks

    The callbacks are needed because javascript is an event driven language. That means instead of waiting for a response javascript will keep executing while listening for other events. Let's take an example with the first function invoking an API call(simulated by setTimeout) and the next function which logs the message.

    function firstFunction() {
      // Simulate a code delay
      setTimeout(function () {
        console.log("First function called");
      }, 1000);
    }
    function secondFunction() {
      console.log("Second function called");
    }
    firstFunction();
    secondFunction();
    
    Output;
    // Second function called
    // First function called

    As observed from the output, javascript didn't wait for the response of the first function and the remaining code block got executed. So callbacks are used in a way to make sure that certain code doesn’t execute until the other code finishes execution.

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  56. What is a callback hell

    Callback Hell is an anti-pattern with multiple nested callbacks which makes code hard to read and debug when dealing with asynchronous logic. The callback hell looks like below,

    async1(function(){
        async2(function(){
            async3(function(){
                async4(function(){
                    ....
                });
            });
        });
    });

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  57. What are server-sent events

    Server-sent events (SSE) is a server push technology enabling a browser to receive automatic updates from a server via HTTP connection without resorting to polling. These are a one way communications channel - events flow from server to client only. This has been used in Facebook/Twitter updates, stock price updates, news feeds etc.

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  58. How do you receive server-sent event notifications

    The EventSource object is used to receive server-sent event notifications. For example, you can receive messages from server as below,

    if (typeof EventSource !== "undefined") {
      var source = new EventSource("sse_generator.js");
      source.onmessage = function (event) {
        document.getElementById("output").innerHTML += event.data + "<br>";
      };
    }

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  59. How do you check browser support for server-sent events

    You can perform browser support for server-sent events before using it as below,

    if (typeof EventSource !== "undefined") {
      // Server-sent events supported. Let's have some code here!
    } else {
      // No server-sent events supported
    }

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  60. What are the events available for server sent events

    Below are the list of events available for server sent events

    Event Description
    onopen It is used when a connection to the server is opened
    onmessage This event is used when a message is received
    onerror It happens when an error occurs

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  61. What are the main rules of promise

    A promise must follow a specific set of rules,

    1. A promise is an object that supplies a standard-compliant .then() method
    2. A pending promise may transition into either fulfilled or rejected state
    3. A fulfilled or rejected promise is settled and it must not transition into any other state.
    4. Once a promise is settled, the value must not change.

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  62. What is callback in callback

    You can nest one callback inside in another callback to execute the actions sequentially one by one. This is known as callbacks in callbacks.

    loadScript("/script1.js", function (script) {
      console.log("first script is loaded");
    
      loadScript("/script2.js", function (script) {
        console.log("second script is loaded");
    
        loadScript("/script3.js", function (script) {
          console.log("third script is loaded");
          // after all scripts are loaded
        });
      });
    });

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  63. What is promise chaining

    The process of executing a sequence of asynchronous tasks one after another using promises is known as Promise chaining. Let's take an example of promise chaining for calculating the final result,

    new Promise(function (resolve, reject) {
      setTimeout(() => resolve(1), 1000);
    })
      .then(function (result) {
        console.log(result); // 1
        return result * 2;
      })
      .then(function (result) {
        console.log(result); // 2
        return result * 3;
      })
      .then(function (result) {
        console.log(result); // 6
        return result * 4;
      });

    In the above handlers, the result is passed to the chain of .then() handlers with the below work flow,

    1. The initial promise resolves in 1 second,
    2. After that .then handler is called by logging the result(1) and then return a promise with the value of result * 2.
    3. After that the value passed to the next .then handler by logging the result(2) and return a promise with result * 3.
    4. Finally the value passed to the last .then handler by logging the result(6) and return a promise with result * 4.

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  64. What is promise.all

    Promise.all is a promise that takes an array of promises as an input (an iterable), and it gets resolved when all the promises get resolved or any one of them gets rejected. For example, the syntax of promise.all method is below,

    Promise.all([Promise1, Promise2, Promise3]) .then(result) => {   console.log(result) }) .catch(error => console.log(`Error in promises ${error}`))

    Note: Remember that the order of the promises(output the result) is maintained as per input order.

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  65. What is the purpose of the race method in promise

    Promise.race() method will return the promise instance which is firstly resolved or rejected. Let's take an example of race() method where promise2 is resolved first

    var promise1 = new Promise(function (resolve, reject) {
      setTimeout(resolve, 500, "one");
    });
    var promise2 = new Promise(function (resolve, reject) {
      setTimeout(resolve, 100, "two");
    });
    
    Promise.race([promise1, promise2]).then(function (value) {
      console.log(value); // "two" // Both promises will resolve, but promise2 is faster
    });

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  66. What is a strict mode in javascript

    Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a “strict” operating context. This way it prevents certain actions from being taken and throws more exceptions. The literal expression "use strict"; instructs the browser to use the javascript code in the Strict mode.

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  67. Why do you need strict mode

    Strict mode is useful to write "secure" JavaScript by notifying "bad syntax" into real errors. For example, it eliminates accidentally creating a global variable by throwing an error and also throws an error for assignment to a non-writable property, a getter-only property, a non-existing property, a non-existing variable, or a non-existing object.

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  68. How do you declare strict mode

    The strict mode is declared by adding "use strict"; to the beginning of a script or a function. If declared at the beginning of a script, it has global scope.

    "use strict";
    x = 3.14; // This will cause an error because x is not declared

    and if you declare inside a function, it has local scope

    x = 3.14; // This will not cause an error.
    myFunction();
    
    function myFunction() {
      "use strict";
      y = 3.14; // This will cause an error
    }

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  69. What is the purpose of double exclamation

    The double exclamation or negation(!!) ensures the resulting type is a boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, true. For example, you can test IE version using this expression as below,

    let isIE8 = false;
    isIE8 = !!navigator.userAgent.match(/MSIE 8.0/);
    console.log(isIE8); // returns true or false

    If you don't use this expression then it returns the original value.

    console.log(navigator.userAgent.match(/MSIE 8.0/)); // returns either an Array or null

    Note: The expression !! is not an operator, but it is just twice of ! operator.

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  70. What is the purpose of the delete operator

    The delete keyword is used to delete the property as well as its value.

    var user = { name: "John", age: 20 };
    delete user.age;
    
    console.log(user); // {name: "John"}

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  71. What is typeof operator

    You can use the JavaScript typeof operator to find the type of a JavaScript variable. It returns the type of a variable or an expression.

    typeof "John Abraham"; // Returns "string"
    typeof (1 + 2); // Returns "number"

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  72. What is undefined property

    The undefined property indicates that a variable has not been assigned a value, or declared but not initialized at all. The type of undefined value is undefined too.

    var user; // Value is undefined, type is undefined
    console.log(typeof user); //undefined

    Any variable can be emptied by setting the value to undefined.

    user = undefined;

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  73. What is null value

    The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values. The type of null value is object. You can empty the variable by setting the value to null.

    var user = null;
    console.log(typeof user); //object

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  74. What is the difference between null and undefined

    Below are the main differences between null and undefined,

    Null Undefined
    It is an assignment value which indicates that variable points to no object. It is not an assignment value where a variable has been declared but has not yet been assigned a value.
    Type of null is object Type of undefined is undefined
    The null value is a primitive value that represents the null, empty, or non-existent reference. The undefined value is a primitive value used when a variable has not been assigned a value.
    Indicates the absence of a value for a variable Indicates absence of variable itself
    Converted to zero (0) while performing primitive operations Converted to NaN while performing primitive operations

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  75. What is eval

    The eval() function evaluates JavaScript code represented as a string. The string can be a JavaScript expression, variable, statement, or sequence of statements.

    console.log(eval("1 + 2")); //  3

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  76. What is the difference between window and document

    Below are the main differences between window and document,

    Window Document
    It is the root level element in any web page It is the direct child of the window object. This is also known as Document Object Model(DOM)
    By default window object is available implicitly in the page You can access it via window.document or document.
    It has methods like alert(), confirm() and properties like document, location It provides methods like getElementById, getElementsByTagName, createElement etc

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  77. How do you access history in javascript

    The window.history object contains the browser's history. You can load previous and next URLs in the history using back() and next() methods.

    function goBack() {
      window.history.back();
    }
    function goForward() {
      window.history.forward();
    }

    Note: You can also access history without window prefix.

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  78. How do you detect caps lock key turned on or not

    The mouseEvent getModifierState() is used to return a boolean value that indicates whether the specified modifier key is activated or not. The modifiers such as CapsLock, ScrollLock and NumLock are activated when they are clicked, and deactivated when they are clicked again.

    Let's take an input element to detect the CapsLock on/off behavior with an example,

    <input type="password" onmousedown="enterInput(event)" />
    
    <p id="feedback"></p>
    
    <script>
      function enterInput(e) {
        var flag = e.getModifierState("CapsLock");
        if (flag) {
          document.getElementById("feedback").innerHTML = "CapsLock activated";
        } else {
          document.getElementById("feedback").innerHTML =
            "CapsLock not activated";
        }
      }
    </script>

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  79. What is isNaN

    The isNaN() function is used to determine whether a value is an illegal number (Not-a-Number) or not. i.e, This function returns true if the value equates to NaN. Otherwise it returns false.

    isNaN("Hello"); //true
    isNaN("100"); //false

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  80. What are the differences between undeclared and undefined variables

    Below are the major differences between undeclared(not defined) and undefined variables,

    undeclared undefined
    These variables do not exist in a program and are not declared These variables declared in the program but have not assigned any value
    If you try to read the value of an undeclared variable, then a runtime error is encountered If you try to read the value of an undefined variable, an undefined value is returned.

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  81. What are global variables

    Global variables are those that are available throughout the length of the code without any scope. The var keyword is used to declare a local variable but if you omit it then it will become global variable

    msg = "Hello"; // var is missing, it becomes global variable

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  82. What are the problems with global variables

    The problem with global variables is the conflict of variable names of local and global scope. It is also difficult to debug and test the code that relies on global variables.

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  83. What is NaN property

    The NaN property is a global property that represents "Not-a-Number" value. i.e, It indicates that a value is not a legal number. It is very rare to use NaN in a program but it can be used as return value for few cases

    Math.sqrt(-1);
    parseInt("Hello");

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  84. What is the purpose of isFinite function

    The isFinite() function is used to determine whether a number is a finite, legal number. It returns false if the value is +infinity, -infinity, or NaN (Not-a-Number), otherwise it returns true.

    isFinite(Infinity); // false
    isFinite(NaN); // false
    isFinite(-Infinity); // false
    
    isFinite(100); // true

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  85. What is an event flow

    Event flow is the order in which event is received on the web page. When you click an element that is nested in various other elements, before your click actually reaches its destination, or target element, it must trigger the click event for each of its parent elements first, starting at the top with the global window object. There are two ways of event flow

    1. Top to Bottom(Event Capturing)
    2. Bottom to Top (Event Bubbling)

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  86. What is event bubbling

    Event bubbling is a type of event propagation where the event first triggers on the innermost target element, and then successively triggers on the ancestors (parents) of the target element in the same nesting hierarchy till it reaches the outermost DOM element.

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  87. What is event capturing

    Event capturing is a type of event propagation where the event is first captured by the outermost element, and then successively triggers on the descendants (children) of the target element in the same nesting hierarchy till it reaches the innermost DOM element.

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  88. How do you submit a form using JavaScript

    You can submit a form using document.forms[0].submit(). All the form input's information is submitted using onsubmit event handler

    function submit() {
      document.forms[0].submit();
    }

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  89. How do you find operating system details

    The window.navigator object contains information about the visitor's browser OS details. Some of the OS properties are available under platform property,

    console.log(navigator.platform);

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  90. What is the difference between document load and DOMContentLoaded events

    The DOMContentLoaded event is fired when the initial HTML document has been completely loaded and parsed, without waiting for assets(stylesheets, images, and subframes) to finish loading. Whereas The load event is fired when the whole page has loaded, including all dependent resources(stylesheets, images).

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  91. What is the difference between native, host and user objects

    Native objects are objects that are part of the JavaScript language defined by the ECMAScript specification. For example, String, Math, RegExp, Object, Function etc core objects defined in the ECMAScript spec. Host objects are objects provided by the browser or runtime environment (Node). For example, window, XmlHttpRequest, DOM nodes etc are considered as host objects. User objects are objects defined in the javascript code. For example, User objects created for profile information.

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  92. What are the tools or techniques used for debugging JavaScript code

    You can use below tools or techniques for debugging javascript

    1. Chrome Devtools
    2. debugger statement
    3. Good old console.log statement

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  93. What are the pros and cons of promises over callbacks

    Below are the list of pros and cons of promises over callbacks,

    Pros:

    1. It avoids callback hell which is unreadable
    2. Easy to write sequential asynchronous code with .then()
    3. Easy to write parallel asynchronous code with Promise.all()
    4. Solves some of the common problems of callbacks(call the callback too late, too early, many times and swallow errors/exceptions)

    Cons:

    1. It makes little complex code
    2. You need to load a polyfill if ES6 is not supported

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  94. What is the difference between an attribute and a property

    Attributes are defined on the HTML markup whereas properties are defined on the DOM. For example, the below HTML element has 2 attributes type and value,

    <input type="text" value="Name:">

    You can retrieve the attribute value as below,

    const input = document.querySelector("input");
    console.log(input.getAttribute("value")); // Good morning
    console.log(input.value); // Good morning

    And after you change the value of the text field to "Good evening", it becomes like

    console.log(input.getAttribute("value")); // Good evening
    console.log(input.value); // Good evening

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  95. What is same-origin policy

    The same-origin policy is a policy that prevents JavaScript from making requests across domain boundaries. An origin is defined as a combination of URI scheme, hostname, and port number. If you enable this policy then it prevents a malicious script on one page from obtaining access to sensitive data on another web page using Document Object Model(DOM).

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  96. What is the purpose of void 0

    Void(0) is used to prevent the page from refreshing. This will be helpful to eliminate the unwanted side-effect, because it will return the undefined primitive value. It is commonly used for HTML documents that use href="JavaScript:Void(0);" within an <a> element. i.e, when you click a link, the browser loads a new page or refreshes the same page. But this behavior will be prevented using this expression. For example, the below link notify the message without reloading the page

    <a href="JavaScript:void(0);" onclick="alert('Well done!')">
      Click Me!
    </a>

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  97. Is JavaScript a compiled or interpreted language

    JavaScript is an interpreted language, not a compiled language. An interpreter in the browser reads over the JavaScript code, interprets each line, and runs it. Nowadays modern browsers use a technology known as Just-In-Time (JIT) compilation, which compiles JavaScript to executable bytecode just as it is about to run.

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  98. Is JavaScript a case-sensitive language

    Yes, JavaScript is a case sensitive language. The language keywords, variables, function & object names, and any other identifiers must always be typed with a consistent capitalization of letters.

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  99. Is there any relation between Java and JavaScript

    No, they are entirely two different programming languages and have nothing to do with each other. But both of them are Object Oriented Programming languages and like many other languages, they follow similar syntax for basic features(if, else, for, switch, break, continue etc).

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  100. What are events

    Events are "things" that happen to HTML elements. When JavaScript is used in HTML pages, JavaScript can react on these events. Some of the examples of HTML events are,

    1. Web page has finished loading
    2. Input field was changed
    3. Button was clicked

    Let's describe the behavior of click event for button element,

    <!doctype html>
    <html>
     <head>
       <script>
         function greeting() {
           alert('Hello! Good morning');
         }
       </script>
     </head>
     <body>
       <button type="button" onclick="greeting()">Click me</button>
     </body>
    </html>

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  101. Who created javascript

    JavaScript was created by Brendan Eich in 1995 during his time at Netscape Communications. Initially it was developed under the name Mocha, but later the language was officially called LiveScript when it first shipped in beta releases of Netscape.

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  102. What is the use of preventDefault method

    The preventDefault() method cancels the event if it is cancelable, meaning that the default action or behaviour that belongs to the event will not occur. For example, prevent form submission when clicking on submit button and prevent opening the page URL when clicking on hyperlink are some common use cases.

    document
      .getElementById("link")
      .addEventListener("click", function (event) {
        event.preventDefault();
      });

    Note: Remember that not all events are cancelable.

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  103. What is the use of stopPropagation method

    The stopPropagation method is used to stop the event from bubbling up the event chain. For example, the below nested divs with stopPropagation method prevents default event propagation when clicking on nested div(Div1)

    <p>Click DIV1 Element</p>
    <div onclick="secondFunc()">DIV 2
      <div onclick="firstFunc(event)">DIV 1</div>
    </div>
    
    <script>
    function firstFunc(event) {
      alert("DIV 1");
      event.stopPropagation();
    }
    
    function secondFunc() {
      alert("DIV 2");
    }
    </script>

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  104. What are the steps involved in return false usage

    The return false statement in event handlers performs the below steps,

    1. First it stops the browser's default action or behaviour.
    2. It prevents the event from propagating the DOM
    3. Stops callback execution and returns immediately when called.

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  105. What is BOM

    The Browser Object Model (BOM) allows JavaScript to "talk to" the browser. It consists of the objects navigator, history, screen, location and document which are children of the window. The Browser Object Model is not standardized and can change based on different browsers.

    Screenshot

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  106. What is the use of setTimeout

    The setTimeout() method is used to call a function or evaluate an expression after a specified number of milliseconds. For example, let's log a message after 2 seconds using setTimeout method,

    setTimeout(function () {
      console.log("Good morning");
    }, 2000);

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  107. What is the use of setInterval

    The setInterval() method is used to call a function or evaluate an expression at specified intervals (in milliseconds). For example, let's log a message after 2 seconds using setInterval method,

    setInterval(function () {
      console.log("Good morning");
    }, 2000);

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  108. Why is JavaScript treated as Single threaded

    JavaScript is a single-threaded language. Because the language specification does not allow the programmer to write code so that the interpreter can run parts of it in parallel in multiple threads or processes. Whereas languages like java, go, C++ can make multi-threaded and multi-process programs.

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  109. What is an event delegation

    Event delegation is a technique for listening to events where you delegate a parent element as the listener for all of the events that happen inside it.

    For example, if you wanted to detect field changes in inside a specific form, you can use event delegation technique,

    var form = document.querySelector("#registration-form");
    
    // Listen for changes to fields inside the form
    form.addEventListener(
      "input",
      function (event) {
        // Log the field that was changed
        console.log(event.target);
      },
      false
    );

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  110. What is ECMAScript

    ECMAScript is the scripting language that forms the basis of JavaScript. ECMAScript standardized by the ECMA International standards organization in the ECMA-262 and ECMA-402 specifications. The first edition of ECMAScript was released in 1997.

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  111. What is JSON

    JSON (JavaScript Object Notation) is a lightweight format that is used for data interchanging. It is based on a subset of JavaScript language in the way objects are built in JavaScript.

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  112. What are the syntax rules of JSON

    Below are the list of syntax rules of JSON

    1. The data is in name/value pairs
    2. The data is separated by commas
    3. Curly braces hold objects
    4. Square brackets hold arrays

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  113. What is the purpose JSON stringify

    When sending data to a web server, the data has to be in a string format. You can achieve this by converting JSON object into a string using stringify() method.

    var userJSON = { name: "John", age: 31 };
    var userString = JSON.stringify(userJSON);
    console.log(userString); //"{"name":"John","age":31}"

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  114. How do you parse JSON string

    When receiving the data from a web server, the data is always in a string format. But you can convert this string value to a javascript object using parse() method.

    var userString = '{"name":"John","age":31}';
    var userJSON = JSON.parse(userString);
    console.log(userJSON); // {name: "John", age: 31}

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  115. Why do you need JSON

    When exchanging data between a browser and a server, the data can only be text. Since JSON is text only, it can easily be sent to and from a server, and used as a data format by any programming language.

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  116. What are PWAs

    Progressive web applications (PWAs) are a type of mobile app delivered through the web, built using common web technologies including HTML, CSS and JavaScript. These PWAs are deployed to servers, accessible through URLs, and indexed by search engines.

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  117. What is the purpose of clearTimeout method

    The clearTimeout() function is used in javascript to clear the timeout which has been set by setTimeout()function before that. i.e, The return value of setTimeout() function is stored in a variable and it’s passed into the clearTimeout() function to clear the timer.

    For example, the below setTimeout method is used to display the message after 3 seconds. This timeout can be cleared by the clearTimeout() method.

    <script>
    var msg;
    function greeting() {
       alert('Good morning');
    }
    function start() {
      msg =setTimeout(greeting, 3000);
    
    }
    
    function stop() {
        clearTimeout(msg);
    }
    </script>

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  118. What is the purpose of clearInterval method

    The clearInterval() function is used in javascript to clear the interval which has been set by setInterval() function. i.e, The return value returned by setInterval() function is stored in a variable and it’s passed into the clearInterval() function to clear the interval.

    For example, the below setInterval method is used to display the message for every 3 seconds. This interval can be cleared by the clearInterval() method.

    <script>
    var msg;
    function greeting() {
       alert('Good morning');
    }
    function start() {
      msg = setInterval(greeting, 3000);
    
    }
    
    function stop() {
        clearInterval(msg);
    }
    </script>

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  119. How do you redirect new page in javascript

    In vanilla javascript, you can redirect to a new page using the location property of window object. The syntax would be as follows,

    function redirect() {
      window.location.href = "newPage.html";
    }

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  120. How do you check whether a string contains a substring

    There are 3 possible ways to check whether a string contains a substring or not,

    1. Using includes: ES6 provided String.prototype.includes method to test a string contains a substring
    var mainString = "hello",
      subString = "hell";
    mainString.includes(subString);
    1. Using indexOf: In an ES5 or older environment, you can use String.prototype.indexOf which returns the index of a substring. If the index value is not equal to -1 then it means the substring exists in the main string.
    var mainString = "hello",
      subString = "hell";
    mainString.indexOf(subString) !== -1;
    1. Using RegEx: The advanced solution is using Regular expression's test method(RegExp.test), which allows for testing for against regular expressions
    var mainString = "hello",
      regex = /hell/;
    regex.test(mainString);

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  121. How do you validate an email in javascript

    You can validate an email in javascript using regular expressions. It is recommended to do validations on the server side instead of the client side. Because the javascript can be disabled on the client side.

    function validateEmail(email) {
      var re =
        /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
      return re.test(String(email).toLowerCase());
    }

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    The above regular expression accepts unicode characters.

  122. How do you get the current url with javascript

    You can use window.location.href expression to get the current url path and you can use the same expression for updating the URL too. You can also use document.URL for read-only purposes but this solution has issues in FF.

    console.log("location.href", window.location.href); // Returns full URL

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  123. What are the various url properties of location object

    The below Location object properties can be used to access URL components of the page,

    1. href - The entire URL
    2. protocol - The protocol of the URL
    3. host - The hostname and port of the URL
    4. hostname - The hostname of the URL
    5. port - The port number in the URL
    6. pathname - The path name of the URL
    7. search - The query portion of the URL
    8. hash - The anchor portion of the URL

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  124. How do get query string values in javascript

    You can use URLSearchParams to get query string values in javascript. Let's see an example to get the client code value from URL query string,

    const urlParams = new URLSearchParams(window.location.search);
    const clientCode = urlParams.get("clientCode");

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  125. How do you check if a key exists in an object

    You can check whether a key exists in an object or not using three approaches,

    1. Using in operator: You can use the in operator whether a key exists in an object or not
    "key" in obj;

    and If you want to check if a key doesn't exist, remember to use parenthesis,

    !("key" in obj);
    1. Using hasOwnProperty method: You can use hasOwnProperty to particularly test for properties of the object instance (and not inherited properties)
    obj.hasOwnProperty("key"); // true
    1. Using undefined comparison: If you access a non-existing property from an object, the result is undefined. Let’s compare the properties against undefined to determine the existence of the property.
    const user = {
      name: "John",
    };
    
    console.log(user.name !== undefined); // true
    console.log(user.nickName !== undefined); // false

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  126. How do you loop through or enumerate javascript object

    You can use the for-in loop to loop through javascript object. You can also make sure that the key you get is an actual property of an object, and doesn't come from the prototype using hasOwnProperty method.

    var object = {
      k1: "value1",
      k2: "value2",
      k3: "value3",
    };
    
    for (var key in object) {
      if (object.hasOwnProperty(key)) {
        console.log(key + " -> " + object[key]); // k1 -> value1 ...
      }
    }

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  127. How do you test for an empty object

    There are different solutions based on ECMAScript versions

    1. Using Object entries(ECMA 7+): You can use object entries length along with constructor type.
    Object.entries(obj).length === 0 && obj.constructor === Object; // Since date object length is 0, you need to check constructor check as well
    1. Using Object keys(ECMA 5+): You can use object keys length along with constructor type.
    Object.keys(obj).length === 0 && obj.constructor === Object; // Since date object length is 0, you need to check constructor check as well
    1. Using for-in with hasOwnProperty(Pre-ECMA 5): You can use a for-in loop along with hasOwnProperty.
    function isEmpty(obj) {
      for (var prop in obj) {
        if (obj.hasOwnProperty(prop)) {
          return false;
        }
      }
    
      return JSON.stringify(obj) === JSON.stringify({});
    }

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  128. What is an arguments object

    The arguments object is an Array-like object accessible inside functions that contains the values of the arguments passed to that function. For example, let's see how to use arguments object inside sum function,

    function sum() {
      var total = 0;
      for (var i = 0, len = arguments.length; i < len; ++i) {
        total += arguments[i];
      }
      return total;
    }
    
    sum(1, 2, 3); // returns 6

    Note: You can't apply array methods on arguments object. But you can convert into a regular array as below.

    var argsArray = Array.prototype.slice.call(arguments);

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  129. How do you make first letter of the string in an uppercase

    You can create a function which uses a chain of string methods such as charAt, toUpperCase and slice methods to generate a string with the first letter in uppercase.

    function capitalizeFirstLetter(string) {
      return string.charAt(0).toUpperCase() + string.slice(1);
    }

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  130. What are the pros and cons of for loop

    The for-loop is a commonly used iteration syntax in javascript. It has both pros and cons

    Pros

    1. Works on every environment
    2. You can use break and continue flow control statements

    Cons

    1. Too verbose
    2. Imperative
    3. You might face one-by-off errors

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  131. How do you display the current date in javascript

    You can use new Date() to generate a new Date object containing the current date and time. For example, let's display the current date in mm/dd/yyyy

    var today = new Date();
    var dd = String(today.getDate()).padStart(2, "0");
    var mm = String(today.getMonth() + 1).padStart(2, "0"); //January is 0!
    var yyyy = today.getFullYear();
    
    today = mm + "/" + dd + "/" + yyyy;
    document.write(today);

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  132. How do you compare two date objects

    You need to use date.getTime() method to compare date values instead of comparison operators (==, !=, ===, and !== operators)

    var d1 = new Date();
    var d2 = new Date(d1);
    console.log(d1.getTime() === d2.getTime()); //True
    console.log(d1 === d2); // False

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  133. How do you check if a string starts with another string

    You can use ECMAScript 6's String.prototype.startsWith() method to check if a string starts with another string or not. But it is not yet supported in all browsers. Let's see an example to see this usage,

    "Good morning".startsWith("Good"); // true
    "Good morning".startsWith("morning"); // false

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  134. How do you trim a string in javascript

    JavaScript provided a trim method on string types to trim any whitespaces present at the beginning or ending of the string.

    "  Hello World   ".trim(); //Hello World

    If your browser(<IE9) doesn't support this method then you can use below polyfill.

    if (!String.prototype.trim) {
      (function () {
        // Make sure we trim BOM and NBSP
        var rtrim = /^[\s\uFEFF\xA0]+|[\s\uFEFF\xA0]+$/g;
        String.prototype.trim = function () {
          return this.replace(rtrim, "");
        };
      })();
    }

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  135. How do you add a key value pair in javascript

    There are two possible solutions to add new properties to an object. Let's take a simple object to explain these solutions.

    var object = {
      key1: value1,
      key2: value2,
    };
    1. Using dot notation: This solution is useful when you know the name of the property
    object.key3 = "value3";
    1. Using square bracket notation: This solution is useful when the name of the property is dynamically determined.
    obj["key3"] = "value3";

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  136. Is the !-- notation represents a special operator

    No,that's not a special operator. But it is a combination of 2 standard operators one after the other,

    1. A logical not (!)
    2. A prefix decrement (--)

    At first, the value decremented by one and then tested to see if it is equal to zero or not for determining the truthy/falsy value.

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  137. How do you assign default values to variables

    You can use the logical or operator || in an assignment expression to provide a default value. The syntax looks like as below,

    var a = b || c;

    As per the above expression, variable 'a 'will get the value of 'c' only if 'b' is falsy (if is null, false, undefined, 0, empty string, or NaN), otherwise 'a' will get the value of 'b'.

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  138. How do you define multiline strings

    You can define multiline string literals using the '\' character followed by line terminator.

    var str =
      "This is a \
    very lengthy \
    sentence!";

    But if you have a space after the '\' character, the code will look exactly the same, but it will raise a SyntaxError.

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  139. What is an app shell model

    An application shell (or app shell) architecture is one way to build a Progressive Web App that reliably and instantly loads on your users' screens, similar to what you see in native applications. It is useful for getting some initial HTML to the screen fast without a network.

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  140. Can we define properties for functions

    Yes, We can define properties for functions because functions are also objects.

    fn = function (x) {
      //Function code goes here
    };
    
    fn.name = "John";
    
    fn.profile = function (y) {
      //Profile code goes here
    };

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  141. What is the way to find the number of parameters expected by a function

    You can use function.length syntax to find the number of parameters expected by a function. Let's take an example of sum function to calculate the sum of numbers,

    function sum(num1, num2, num3, num4) {
      return num1 + num2 + num3 + num4;
    }
    sum.length; // 4 is the number of parameters expected.

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  142. What is a polyfill

    A polyfill is a piece of JS code used to provide modern functionality on older browsers that do not natively support it. For example, Silverlight plugin polyfill can be used to mimic the functionality of an HTML Canvas element on Microsoft Internet Explorer 7.

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  143. What are break and continue statements

    The break statement is used to "jump out" of a loop. i.e, It breaks the loop and continues executing the code after the loop.

    for (i = 0; i < 10; i++) {
      if (i === 5) {
        break;
      }
      text += "Number: " + i + "<br>";
    }

    The continue statement is used to "jump over" one iteration in the loop. i.e, It breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.

    for (i = 0; i < 10; i++) {
      if (i === 5) {
        continue;
      }
      text += "Number: " + i + "<br>";
    }

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  144. What are js labels

    The label statement allows us to name loops and blocks in JavaScript. We can then use these labels to refer back to the code later. For example, the below code with labels avoids printing the numbers when they are same,

    var i, j;
    
    loop1: for (i = 0; i < 3; i++) {
      loop2: for (j = 0; j < 3; j++) {
        if (i === j) {
          continue loop1;
        }
        console.log("i = " + i + ", j = " + j);
      }
    }
    
    // Output is:
    //   "i = 1, j = 0"
    //   "i = 2, j = 0"
    //   "i = 2, j = 1"

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  145. What are the benefits of keeping declarations at the top

    It is recommended to keep all declarations at the top of each script or function. The benefits of doing this are,

    1. Gives cleaner code
    2. It provides a single place to look for local variables
    3. Easy to avoid unwanted global variables
    4. It reduces the possibility of unwanted re-declarations

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  146. What are the benefits of initializing variables

    It is recommended to initialize variables because of the below benefits,

    1. It gives cleaner code
    2. It provides a single place to initialize variables
    3. Avoid undefined values in the code

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  147. What are the recommendations to create new object

    It is recommended to avoid creating new objects using new Object(). Instead you can initialize values based on it's type to create the objects.

    1. Assign {} instead of new Object()
    2. Assign "" instead of new String()
    3. Assign 0 instead of new Number()
    4. Assign false instead of new Boolean()
    5. Assign [] instead of new Array()
    6. Assign /()/ instead of new RegExp()
    7. Assign function (){} instead of new Function()

    You can define them as an example,

    var v1 = {};
    var v2 = "";
    var v3 = 0;
    var v4 = false;
    var v5 = [];
    var v6 = /()/;
    var v7 = function () {};

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  148. How do you define JSON arrays

    JSON arrays are written inside square brackets and arrays contain javascript objects. For example, the JSON array of users would be as below,

    "users":[
      {"firstName":"John", "lastName":"Abrahm"},
      {"firstName":"Anna", "lastName":"Smith"},
      {"firstName":"Shane", "lastName":"Warn"}
    ]

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  149. How do you generate random integers

    You can use Math.random() with Math.floor() to return random integers. For example, if you want generate random integers between 1 to 10, the multiplication factor should be 10,

    Math.floor(Math.random() * 10) + 1; // returns a random integer from 1 to 10
    Math.floor(Math.random() * 100) + 1; // returns a random integer from 1 to 100

    Note: Math.random() returns a random number between 0 (inclusive), and 1 (exclusive)

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  150. Can you write a random integers function to print integers with in a range

    Yes, you can create a proper random function to return a random number between min and max (both included)

    function randomInteger(min, max) {
      return Math.floor(Math.random() * (max - min + 1)) + min;
    }
    randomInteger(1, 100); // returns a random integer from 1 to 100
    randomInteger(1, 1000); // returns a random integer from 1 to 1000

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  151. What is tree shaking

    Tree shaking is a form of dead code elimination. It means that unused modules will not be included in the bundle during the build process and for that it relies on the static structure of ES2015 module syntax,( i.e. import and export). Initially this has been popularized by the ES2015 module bundler rollup.

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  152. What is the need of tree shaking

    Tree Shaking can significantly reduce the code size in any application. i.e, The less code we send over the wire the more performant the application will be. For example, if we just want to create a “Hello World” Application using SPA frameworks then it will take around a few MBs, but by tree shaking it can bring down the size to just a few hundred KBs. Tree shaking is implemented in Rollup and Webpack bundlers.

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  153. Is it recommended to use eval

    No, it allows arbitrary code to be run which causes a security problem. As we know that the eval() function is used to run text as code. In most of the cases, it should not be necessary to use it.

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  154. What is a Regular Expression

    A regular expression is a sequence of characters that forms a search pattern. You can use this search pattern for searching data in a text. These can be used to perform all types of text search and text replace operations. Let's see the syntax format now,

    /pattern/modifiers;

    For example, the regular expression or search pattern with case-insensitive username would be,

    /John/i;

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  155. What are the string methods available in Regular expression

    Regular Expressions has two string methods: search() and replace(). The search() method uses an expression to search for a match, and returns the position of the match.

    var msg = "Hello John";
    var n = msg.search(/John/i); // 6

    The replace() method is used to return a modified string where the pattern is replaced.

    var msg = "Hello John";
    var n = msg.replace(/John/i, "Buttler"); // Hello Buttler

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  156. What are modifiers in regular expression

    Modifiers can be used to perform case-insensitive and global searches. Let's list down some of the modifiers,

    Modifier Description
    i Perform case-insensitive matching
    g Perform a global match rather than stops at first match
    m Perform multiline matching

    Let's take an example of global modifier,

    var text = "Learn JS one by one";
    var pattern = /one/g;
    var result = text.match(pattern); // one,one

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  157. What are regular expression patterns

    Regular Expressions provide a group of patterns in order to match characters. Basically they are categorized into 3 types,

    1. Brackets: These are used to find a range of characters. For example, below are some use cases,
      1. [abc]: Used to find any of the characters between the brackets(a,b,c)
      2. [0-9]: Used to find any of the digits between the brackets
      3. (a|b): Used to find any of the alternatives separated with |
    2. Metacharacters: These are characters with a special meaning For example, below are some use cases,
      1. \d: Used to find a digit
      2. \s: Used to find a whitespace character
      3. \b: Used to find a match at the beginning or ending of a word
    3. Quantifiers: These are useful to define quantities For example, below are some use cases,
      1. n+: Used to find matches for any string that contains at least one n
      2. n*: Used to find matches for any string that contains zero or more occurrences of n
      3. n?: Used to find matches for any string that contains zero or one occurrences of n

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  158. What is a RegExp object

    RegExp object is a regular expression object with predefined properties and methods. Let's see the simple usage of RegExp object,

    var regexp = new RegExp("\\w+");
    console.log(regexp);
    // expected output: /\w+/

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  159. How do you search a string for a pattern

    You can use the test() method of regular expression in order to search a string for a pattern, and return true or false depending on the result.

    var pattern = /you/;
    console.log(pattern.test("How are you?")); //true

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  160. What is the purpose of exec method

    The purpose of exec method is similar to test method but it executes a search for a match in a specified string and returns a result array, or null instead of returning true/false.

    var pattern = /you/;
    console.log(pattern.exec("How are you?")); //["you", index: 8, input: "How are you?", groups: undefined]

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  161. How do you change the style of a HTML element

    You can change inline style or classname of a HTML element using javascript

    1. Using style property: You can modify inline style using style property
    document.getElementById("title").style.fontSize = "30px";
    1. Using ClassName property: It is easy to modify element class using className property
    document.getElementById("title").className = "custom-title";

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  162. What would be the result of 1+2+'3'

    The output is going to be 33. Since 1 and 2 are numeric values, the result of the first two digits is going to be a numeric value 3. The next digit is a string type value because of that the addition of numeric value 3 and string type value 3 is just going to be a concatenation value 33.

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  163. What is a debugger statement

    The debugger statement invokes any available debugging functionality, such as setting a breakpoint. If no debugging functionality is available, this statement has no effect. For example, in the below function a debugger statement has been inserted. So execution is paused at the debugger statement just like a breakpoint in the script source.

    function getProfile() {
      // code goes here
      debugger;
      // code goes here
    }

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  164. What is the purpose of breakpoints in debugging

    You can set breakpoints in the javascript code once the debugger statement is executed and the debugger window pops up. At each breakpoint, javascript will stop executing, and let you examine the JavaScript values. After examining values, you can resume the execution of code using the play button.

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  165. Can I use reserved words as identifiers

    No, you cannot use the reserved words as variables, labels, object or function names. Let's see one simple example,

    var else = "hello"; // Uncaught SyntaxError: Unexpected token else

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  166. How do you detect a mobile browser

    You can use regex which returns a true or false value depending on whether or not the user is browsing with a mobile.

    window.mobilecheck = function () {
      var mobileCheck = false;
      (function (a) {
        if (
          /(android|bb\d+|meego).+mobile|avantgo|bada\/|blackberry|blazer|compal|elaine|fennec|hiptop|iemobile|ip(hone|od)|iris|kindle|lge |maemo|midp|mmp|mobile.+firefox|netfront|opera m(ob|in)i|palm( os)?|phone|p(ixi|re)\/|plucker|pocket|psp|series(4|6)0|symbian|treo|up\.(browser|link)|vodafone|wap|windows ce|xda|xiino/i.test(
            a
          ) ||
          /1207|6310|6590|3gso|4thp|50[1-6]i|770s|802s|a wa|abac|ac(er|oo|s\-)|ai(ko|rn)|al(av|ca|co)|amoi|an(ex|ny|yw)|aptu|ar(ch|go)|as(te|us)|attw|au(di|\-m|r |s )|avan|be(ck|ll|nq)|bi(lb|rd)|bl(ac|az)|br(e|v)w|bumb|bw\-(n|u)|c55\/|capi|ccwa|cdm\-|cell|chtm|cldc|cmd\-|co(mp|nd)|craw|da(it|ll|ng)|dbte|dc\-s|devi|dica|dmob|do(c|p)o|ds(12|\-d)|el(49|ai)|em(l2|ul)|er(ic|k0)|esl8|ez([4-7]0|os|wa|ze)|fetc|fly(\-|_)|g1 u|g560|gene|gf\-5|g\-mo|go(\.w|od)|gr(ad|un)|haie|hcit|hd\-(m|p|t)|hei\-|hi(pt|ta)|hp( i|ip)|hs\-c|ht(c(\-| |_|a|g|p|s|t)|tp)|hu(aw|tc)|i\-(20|go|ma)|i230|iac( |\-|\/)|ibro|idea|ig01|ikom|im1k|inno|ipaq|iris|ja(t|v)a|jbro|jemu|jigs|kddi|keji|kgt( |\/)|klon|kpt |kwc\-|kyo(c|k)|le(no|xi)|lg( g|\/(k|l|u)|50|54|\-[a-w])|libw|lynx|m1\-w|m3ga|m50\/|ma(te|ui|xo)|mc(01|21|ca)|m\-cr|me(rc|ri)|mi(o8|oa|ts)|mmef|mo(01|02|bi|de|do|t(\-| |o|v)|zz)|mt(50|p1|v )|mwbp|mywa|n10[0-2]|n20[2-3]|n30(0|2)|n50(0|2|5)|n7(0(0|1)|10)|ne((c|m)\-|on|tf|wf|wg|wt)|nok(6|i)|nzph|o2im|op(ti|wv)|oran|owg1|p800|pan(a|d|t)|pdxg|pg(13|\-([1-8]|c))|phil|pire|pl(ay|uc)|pn\-2|po(ck|rt|se)|prox|psio|pt\-g|qa\-a|qc(07|12|21|32|60|\-[2-7]|i\-)|qtek|r380|r600|raks|rim9|ro(ve|zo)|s55\/|sa(ge|ma|mm|ms|ny|va)|sc(01|h\-|oo|p\-)|sdk\/|se(c(\-|0|1)|47|mc|nd|ri)|sgh\-|shar|sie(\-|m)|sk\-0|sl(45|id)|sm(al|ar|b3|it|t5)|so(ft|ny)|sp(01|h\-|v\-|v )|sy(01|mb)|t2(18|50)|t6(00|10|18)|ta(gt|lk)|tcl\-|tdg\-|tel(i|m)|tim\-|t\-mo|to(pl|sh)|ts(70|m\-|m3|m5)|tx\-9|up(\.b|g1|si)|utst|v400|v750|veri|vi(rg|te)|vk(40|5[0-3]|\-v)|vm40|voda|vulc|vx(52|53|60|61|70|80|81|83|85|98)|w3c(\-| )|webc|whit|wi(g |nc|nw)|wmlb|wonu|x700|yas\-|your|zeto|zte\-/i.test(
            a.substr(0, 4)
          )
        )
          mobileCheck = true;
      })(navigator.userAgent || navigator.vendor || window.opera);
      return mobileCheck;
    };

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