JavaScript questions

1. What is var let and Const in Javascript?
   All of those are diffrenet ways to define variables while var is in global scope, let and const are in block scope. const is useable when you are not willing to change your variable in comparison to let.
2. What is hoisting?
   Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution. Inevitably, this means that no matter where functions and variables are declared, they are moved to the top of their scope regardless of whether their scope is global or local.
3. What is critical rendering?
   The Critical Rendering Path is the sequence of steps the browser goes through to convert the HTML, CSS, and JavaScript into pixels on the screen. Optimizing the critical render path improves render performance.The critical rendering path includes the Document Object Model (DOM), CSS Object Model (CSSOM), render tree and layout.
4. What is the difference between promise and observable?
   a Promise is always asynchronous, while an Observable can be either synchronous or asynchronous, a Promise can provide a single value, whereas an Observable is a stream of values (from 0 to multiple values), you can apply RxJS operators to an Observable to get a new tailored stream.
5. What is DOM tree JavaScript?
   According to the Document Object Model (DOM), every HTML tag is an object. ... Nested tags are “children” of the enclosing one. The text inside a tag is an object as well. All these objects are accessible using JavaScript, and we can use them to modify the page.
6. Is JavaScript multithreaded?
   JavaScript is single threaded and only one line of code can be executed at any given time.
7. Let's say you have complex calculation where will you calculate it?
   Web Workers makes it possible to run a script operation in a background thread separate from the main execution thread of a web application.
8. What is debounce function?
   The debounce() function forces a function to wait a certain amount of time before running again. The function is built to limit the number of times a function is called. A quick example: you have a resize listener on the window which does some element dimension calculations and (possibly) repositions a few elements. That isn't a heavy task in itself but being repeatedly fired after numerous resizes will really slow your site down.
9. What is the difference between classical inheritance and prototypal inheritance?\
   • Class Inheritance: instances inherit from classes (like a blueprint — a description of the class), and create sub-class relationships: hierarchical class taxonomies. Instances are typically instantiated via constructor functions with the new keyword. Class inheritance may or may not use the class keyword from ES6.
   • Prototypal Inheritance: instances inherit directly from other objects. Instances are typically instantiated via factory functions or Object.create(). Instances may be composed from many different objects, allowing for easy selective inheritance.
10. Angular vs React
    React seems simpler at first sight, and it takes less time to start working on a React project. However, that simplicity as the main advantage of React is neutralized because you have to learn to work with additional JavaScript frameworks and tools. Angular itself is more complex and takes quite some time to master.
11. Suppose the server sends a ping every so often. How will the client communicate with him?
    setTimeout, Promises, async await.
12. What is Axios?
    Axios is a popular, promise-based HTTP client that supports an easy-to-use API and can be used in both the browser and Node. ... Making HTTP requests to fetch or save data is one of the most common tasks a client-side JavaScript application will need to do.
13. What is cors?
    Cross-Origin Resource Sharing (CORS) is a mechanism that uses additional HTTP headers to tell browsers to give a web application running at one origin, access to selected resources from a different origin.
14. What is AJAX?
    AJAX = Asynchronous JavaScript And XM. A browser built-in XMLHttpRequest object (to request data from a web server) JavaScript and HTML DOM (to display or use the data)