What the f*ck JavaScript?

A list of funny and tricky examples of JavaScript.

JavaScript is a great language. It has a simple syntax, large ecosystem and, what is the most important, great community.

At the same time, all we know that JavaScript is a quite funny language with tricky parts. Some of them can quickly turn our everyday job into hell, some of them can make us laugh out loud.

Motivation

Just for fun

— “Just for Fun: The Story of an Accidental Revolutionary”, Linus Torvalds

The primary goal of this list is to collect some crazy examples and explain how they work, if possible. Just because it's fun to learn something that we didn't know before.

If you are a beginner, you can use this notes to get deeper dive into the JavaScript. I hope this notes will motivate you to spend more time reading the specification.

If you are a professional developer, you can consider these examples as a great resource for interview questions and quizzes for newcomers in your company. At the same time, these examples would be handy while preparing for the interview.

In any case, just read this. Probably you're going to find something new for yourself.

Notation

// -> is used to show the result of an expression. For example:

1 + 1 // -> 2

// > means the result of console.log or other output. For example:

console.log('hello, world!') // > hello, world!

[] is truly, but not true

An array is a truthy value, however, it's not equal true.

!![]       // -> true
[] == true // -> false

💡 Explanation:

Here are liks to the corresponding sections in the ECMA-262 specification:

• 12.5.9 Logical NOT Operator (!)
• 7.2.13 Abstract Equality Comparison

null is falsy, but not false

Despite the fact that null is falsy value, it's not equal false.

!!null        // -> false
null == false // -> false

At the same time, other falsy values, like 0 or '' are equal false.

0 == false  // -> true
'' == false // -> true

💡 Explanation:

The explanation is the same as for previous example. Here's a corresponding link:

• 7.2.13 Abstract Equality Comparison

undefined and Number

If we don't pass any argument into a Number constructor, we'll get 0. undefined is a value assigned to formal arguments which there are no actual arguments, so you might expect that Number without arguments takes undefined as a value of its parameter. However, when we pass undefined, we will get NaN.

Number()          // -> 0
Number(undefined) // -> NaN

💡 Explanation:

According to the specification:

1. If no arguments were passed to this function invocation, let n be +0.
2. Else, let n be ? ToNumber(value).
3. In case with undefined, ToNumber(undefined) should return NaN.

Here's a corresponding section:

• 20.1.1 The Number Constructor
• 7.1.3 ToNumber(argument)

Math with true and false

Let's do some math:

true + true // -> 2
(true + true) * (true + true) - true // -> 3

Hmmm… 🤔

💡 Explanation:

We can coerce values to numbers with Number constructor. It's quite obvious that true will be coerced to 1:

Number(true) // -> 1

The unary plus operator attempts to convert its value into a number. It can convert string representations of integers and floats, as well as the non-string values true, false, and null. If it cannot parse a particular value, it will evaluate to NaN. That means we can coerce true to 1 easier:

+true // -> 1

When you're performing addition or multiplication, ToNumber method invokes. In according to the specification, this method returns:

If argument is true, return 1. If argument is false, return +0.

That's why we can add boolean values as regular numbers and get correct results.

Corresponding sections:

• 12.5.6 Unary + Operator
• 12.8.3 The Addition Operator (+)
• 7.1.3 ToNumber(argument)

HTML comments are valid in JavaScript

You will be impressed, but <!-- (which is known as HTML comment) is a valid comment in JavaScript.

// valid comment
<!-- valid comment too

💡 Explanation:

Impressed? HTML-like comments were intended to allow browsers that didn't understand the <script> tag to degrade gracefully. These browsers, eg. Netscape 1.x are no longer popular. So there is really no point in putting HTML comments in your script tags anymore.

Since Node.js is based on V8 engine, HTML-like comments are supported in the Node.js runtime too. Moreover, they're a part of specification:

• B.1.3 HTML-like Comments

NaN is not a number

Despite the fact that type of NaN is a 'number', NaN is not instance of number

typeof NaN            // -> 'number'
NaN instanceof Number // -> false

💡 Explanation:

TODO

[] and null are objects

typeof []   // -> 'object'
typeof null // -> 'object'

💡 Explanation:

TODO. However, you can check this using toString method.

Object.prototype.toString.call([])
// -> '[object Array]'

Object.prototype.toString.call(new Date)
// -> '[object Date]'

Object.prototype.toString.call(null)
// -> '[object Null]'

Magicaly increasing numbers

999999999999999  // -> 999999999999999
9999999999999999 // -> 10000000000000000

💡 Explanation:

TODO

Precision of 0.1 + 0.2

Well known joke from JavaScript. An addition of 0.1 and 0.2 is deadly precise:

0.1 + 0.2 // -> 0.30000000000000004

💡 Explanation:

TODO

Patching numbers

You can add own methods to wrapper objects like Number or String.

Number.prototype.isOne = function () {
  return Number(this) === 1
}

1.0.isOne() // -> true
1..isOne()  // -> true
2.0.isOne() // -> false
(7).isOne() // -> false

However, it's not recommended if the behavior of defined method is not a part of the specification.

💡 Explanation:

TODO

Comparation of there numbers

1 < 2 < 3 // -> true
3 > 2 > 1 // -> false

💡 Explanation:

Why does this work that way? Well, the problem is in the first part of an expression. Here's how it works:

1 < 2 < 3 // 1 < 2 -> true
true  < 3 // true -> 1
1     < 3 // -> true

3 > 2 > 1 // 3 > 2 -> true
true  > 1 // true -> 1
1     > 1 // -> false

We can fix this with Greater than or equal operator (>=). TODO(add link to spec):

3 > 2 >= 1 // true

Funny addition

Often the results of an addition operation in JavaScript might be quite unexpectable. Consider these examples:

 3  - 1    // -> 2
 3  + 1    // -> 4
'3' - 1  // -> 2
'3' + 1  // -> '31'

'' + '' // -> ''
[] + [] // -> ''
{} + [] // -> 0
[] + {} // -> '[object Object]'
{} + {} // -> '[object Object][object Object]'

💡 Explanation:

What's happening in the first four examples? Here's a small table to understand addition in JavaScript:

Number  + Number  -> addition
Boolean + Number  -> addition
Boolean + Boolean -> addition
Number  + String  -> concatenation
String  + Boolean -> concatenation
String  + String  -> concatenation

What about the rest examples? A toString method is being implicitly called for [] and {} before addition. TODO

Strings aren't instances of String

'str' // -> 'str'
typeof 'str' // -> 'string'
'str' instanceof String // -> false

💡 Explanation:

The String construnctor returns a string:

typeof String('str')   // -> 'string'
String('str')          // -> 'str'
String('str') == 'str' // -> true

Let's try with a new:

new String('str') == 'str' // -> true
typeof new String('str')   // -> 'object'

Object? What's that?

new String('str') // -> [String: 'str']

Calling functions with backticks

Let's declare a function which logs all params into the console:

function f(...args) {
  return args
}

No doubt, you know you can call this function like this:

f(1, 2, 3) // -> [ 1, 2, 3 ]

But did you know you can call any function with backticks?

f`true is ${true}, false is ${false}, array is ${[1,2,3]}`
// -> [ [ 'true is ', ', false is ', ', array is ', '' ],
// ->   true,
// ->   false,
// ->   [ 1, 2, 3 ] ]

💡 Explanation:

Well, this is not magic at all if you're familiar with Tagged template literals TODO(link to spec). In the example above, f function is a tag for template literal. Tags before template literal allow you to parse template literals with a function. The first argument of a tag function contains an array of string values. The remaining arguments are related to the expressions. Example:

function template(strings, ...keys) {
  // do something with strings and keys…
}

This is the magic behind famous library called styled-components, which is popular in React-community.

A constructor property

const c = 'constructor'
c[c][c]('console.log("WTF?")')() // > WTF?

💡 Explanation:

Let's consider this example step-by-step:

// Declare a new constant which is a string 'constructor'
const c = 'constructor'

// c is a string
c // -> 'constructor'

// Getting a constructor of string
c[c] // -> [Function: String]

// Getting a constructor of constructor
c[c][c] // -> [Function: Function]

// Call the Function constructor and pass
// the body of new function as an argument
c[c][c]('console.log("WTF?")') // -> [Function: anonymous]

// And then call this anonymous function
// The result is console-logging a string 'WTF'
c[c][c]('console.log("WTF?")')() // > WTF

Object as a key of object's property

{ [{}]: {} } // -> { '[object Object]': {} }

💡 Explanation:

Why does this work so? Here we're using a Computed property name TODO(add link to spec). When you pass an object between those brackets, it coerces object to a string, so we get a property key '[object Object]' and value {}.

The same way we can make brackets hell like this:

({[{}]:{[{}]:{}}})[{}][{}] // -> {}

// structure:
// {
//   '[object Object]': {
//     '[object Object]': {}
//   }
// }

Accessing prototypes with __proto__

As we know, primitives don't have prototypes. However, if we try to get a value of __proto__ for primitives, we would get this:

(1).__proto__.__proto__.__proto__ // -> null

💡 Explanation:

It happens because of when primitive doesn't have a prototype, it will be wrapped in a wrapper object. So, spet-by-step:

(1).__proto__ // -> [Number: 0]
(1).__proto__.__proto__ // -> {}
(1).__proto__.__proto__.__proto__ // -> null

TODO(add more comprehensive explanation)

`${{Object}}`

What the result of the expression below?

`${{Object}}`

The answer is:

// -> '[object Object]'

💡 Explanation:

We defined an object with a property Object using Shorthand property notation TODO(add link to spec). Then we've passed this object to the template literal, so the toString method calls for that object. That's why we get string '[object Object]'.

Destructoring with default values

Consider this example:

let x, { x: y = 1 } = { x }; y;

The example above is a great task for an interview. What the value of y? The answer is:

// -> 1

💡 Explanation:

TODO

Dots and spreading

Interesting examples could be composed with spreading of arrays. Consider this:

[...[...'...']].lenght // -> 3

💡 Explanation:

Why 3? When we use spread operator TODO(link to spec), the @@iterator method calls, and the returned iterator is used to obtain the values to be iterated. The default iterator for string spreads string by character. After spreading, we're packing this characters into an array. Then spreading this array again and packing back to the array.

A '...' string consists with three ., so the length of resulting array will be 3.

Now, step-by-step:

[...'...']             // -> [ '.', '.', '.' ]
[...[...'...']]        // -> [ '.', '.', '.' ]
[...[...'...']].length // -> 3

Obviously, we can spread and wrap the elements of array as many times as we want:

[...'...']                 // -> [ '.', '.', '.' ]
[...[...'...']]            // -> [ '.', '.', '.' ]
[...[...[...'...']]]       // -> [ '.', '.', '.' ]
[...[...[...[...'...']]]]  // -> [ '.', '.', '.' ]
// and so on …

Labels

Not so many programmers know about labels in JavaScript. They are kind of interesting:

foo: {
  console.log('first');
  break foo;
  console.log('second');
}

// > first
// -> undefined

💡 Explanation:

The labeled statement is used with break or continue statements. You can use a label to identify a loop, and then use the break or continue statements to indicate whether a program should interrupt the loop or continue its execution.

In the example above, we identify a label foo. Then console.log('first'); executes and then we interrupt execution.

Nested labels

a: b: c: d: e: f: g: 1, 2, 3, 4, 5; // -> 5

💡 Explanation:

TODO

Insidious try..catch

What will this expression return? 2 or 3?

(() => {
  try {
    return 2;
  } finally {
    return 3;
  }
})()

The answer is 3. Surprised?

💡 Explanation:

TODO

License

© Denys Dovhan