A list of funny and tricky JavaScript examples

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

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

The original idea for WTFJS belongs to Brian Leroux. This list is highly inspired by his talk “WTFJS” at dotJS 2012:

You can install this handbook using npm. Just run:

$ npm install -g wtfjs

You should be able to run wtfjs at the command line now. This will open the manual in your selected $PAGER. Otherwise, you may continue reading on here.

The source is available here: https://github.com/denysdovhan/wtfjs

Currently, there are these translations of wtfjs:

• 中文
• हिंदी
• Français
• Português do Brasil
• Polski
• Italiano
• Russian (on Habr.com)
• 한국어

Help translating to your language

Note: Translations are maintained by their translators. They may not contain every example, and existing examples may be outdated.

• 💪🏻 Motivation
• ✍🏻 Notation
• 👀 Examples
  • [] is equal ![]
  • true is not equal ![], but not equal [] too
  • true is false
  • baNaNa
  • NaN is not a NaN
  • Object.is() and === weird cases
  • It's a fail
  • [] is truthy, but not true
  • null is falsy, but not false
  • document.all is an object, but it is undefined
  • Minimal value is greater than zero
  • function is not a function
  • Adding arrays
  • Trailing commas in array
  • Array equality is a monster
  • undefined and Number
  • parseInt is a bad guy
  • Math with true and false
  • HTML comments are valid in JavaScript
  • NaN is not a number
  • [] and null are objects
  • Magically increasing numbers
  • Precision of 0.1 + 0.2
  • Patching numbers
  • Comparison of three numbers
  • Funny math
  • Addition of RegExps
  • Strings aren't instances of String
  • Calling functions with backticks
  • Call call call
  • A constructor property
  • Object as a key of object's property
  • Accessing prototypes with __proto__
  • `${{Object}}`
  • Destructuring with default values
  • Dots and spreading
  • Labels
  • Nested labels
  • Insidious try..catch
  • Is this multiple inheritance?
  • A generator which yields itself
  • A class of class
  • Non-coercible objects
  • Tricky arrow functions
  • Arrow functions can not be a constructor
  • arguments and arrow functions
  • Tricky return
  • Chaining assignments on object
  • Accessing object properties with arrays
  • Number.toFixed() display different numbers
  • Math.max() less than Math.min()
  • Comparing null to 0
  • Same variable redeclaration
  • Default behavior Array.prototype.sort()
  • resolve() won't return Promise instance
  • {}{} is undefined
  • arguments binding
  • An alert from hell
  • An infinite timeout
  • A setTimeout object
  • Double dot
  • Extra Newness
  • Why you should use semicolons
  • Split a string by a space
  • A stringified string
  • Non-strict comparison of a number to true
• 📚 Other resources
• 🤝 Supporting
• 🎓 License

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 these notes to get a deeper dive into JavaScript. I hope these 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 reference for all of the quirks and unexpected edges of our beloved JavaScript.

In any case, just read this. You're probably going to find something new.

⚠️ Note: If you enjoy reading this document, please, consider supporting the author of this collection.

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

1 + 1; // -> 2

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

console.log("hello, world!"); // > hello, world!

// is just a comment used for explanations. Example:

// Assigning a function to foo constant
const foo = function() {};

Array is equal not array:

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

The abstract equality operator converts both sides to numbers to compare them, and both sides become the number 0 for different reasons. Arrays are truthy, so on the right, the opposite of a truthy value is false, which is then coerced to 0. On the left, however, an empty array is coerced to a number without becoming a boolean first, and empty arrays are coerced to 0, despite being truthy.

Here is how this expression simplifies:

+[] == +![];
0 == +false;
0 == 0;
true;

See also [] is truthy, but not true.

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

Array is not equal true, but not Array is not equal true too; Array is equal false, not Array is equal false too:

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

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

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

// According to the specification

true == []; // -> false

toNumber(true); // -> 1
toNumber([]); // -> 0

1 == 0; // -> false

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

![]; // -> false

true == false; // -> false

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

// According to the specification

false == []; // -> true

toNumber(false); // -> 0
toNumber([]); // -> 0

0 == 0; // -> true

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

![]; // -> false

false == false; // -> true

• 7.2.15 Abstract Equality Comparison

!!"false" == !!"true"; // -> true
!!"false" === !!"true"; // -> true

Consider this step-by-step:

// true is 'truthy' and represented by value 1 (number), 'true' in string form is NaN.
true == "true"; // -> false
false == "false"; // -> false

// 'false' is not the empty string, so it's a truthy value
!!"false"; // -> true
!!"true"; // -> true

• 7.2.15 Abstract Equality Comparison

"b" + "a" + +"a" + "a"; // -> 'baNaNa'

This is an old-school joke in JavaScript, but remastered. Here's the original one:

"foo" + +"bar"; // -> 'fooNaN'

The expression is evaluated as 'foo' + (+'bar'), which converts 'bar' to not a number.

• 12.8.3 The Addition Operator (+)
• 12.5.6 Unary + Operator

NaN === NaN; // -> false

The specification strictly defines the logic behind this behavior:

1. If Type(x) is different from Type(y), return false.
2. If Type(x) is Number, then
   1. If x is NaN, return false.
   2. If y is NaN, return false.
   3. … … …

— 7.2.14 Strict Equality Comparison

Following the definition of NaN from the IEEE:

Four mutually exclusive relations are possible: less than, equal, greater than, and unordered. The last case arises when at least one operand is NaN. Every NaN shall compare unordered with everything, including itself.

— “What is the rationale for all comparisons returning false for IEEE754 NaN values?” at StackOverflow

Object.is() determines if two values have the same value or not. It works similar to the === operator but there are a few weird cases:

Object.is(NaN, NaN); // -> true
NaN === NaN; // -> false

Object.is(-0, 0); // -> false
-0 === 0; // -> true

Object.is(NaN, 0 / 0); // -> true
NaN === 0 / 0; // -> false

In JavaScript lingo, NaN and NaN are the same value but they're not strictly equal. NaN === NaN being false is apparently due to historical reasons so it would probably be better to accept it as it is.

Similarly, -0 and 0 are strictly equal, but they're not the same value.

For more details about NaN === NaN, see the above case.

• Here are the TC39 specs about Object.is
• Equality comparisons and sameness on MDN

You would not believe, but …

(![] + [])[+[]] +
  (![] + [])[+!+[]] +
  ([![]] + [][[]])[+!+[] + [+[]]] +
  (![] + [])[!+[] + !+[]];
// -> 'fail'

By breaking that mass of symbols into pieces, we notice that the following pattern occurs often:

![] + []; // -> 'false'
![]; // -> false

So we try adding [] to false. But due to a number of internal function calls (binary + Operator -> ToPrimitive -> [[DefaultValue]]) we end up converting the right operand to a string:

![] + [].toString(); // 'false'

Thinking of a string as an array we can access its first character via [0]:

"false"[0]; // -> 'f'

The rest is obvious, but the i is tricky. The i in fail is grabbed by generating the string 'falseundefined' and grabbing the element on index ['10'].

More examples:

+![]          // -> 0
+!![]         // -> 1
!![]          // -> true
![]           // -> false
[][[]]        // -> undefined
+!![] / +![]  // -> Infinity
[] + {}       // -> "[object Object]"
+{}           // -> NaN

• Brainfuck beware: JavaScript is after you!
• Writing a sentence without using the Alphabet — generate any phrase using JavaScript

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

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

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

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

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

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

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

0 == false; // -> true
"" == false; // -> true

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

• 7.2.15 Abstract Equality Comparison

⚠️ This is part of the Browser API and won't work in a Node.js environment ⚠️

Despite the fact that document.all is an array-like object and it gives access to the DOM nodes in the page, it responds to the typeof function as undefined.

document.all instanceof Object; // -> true
typeof document.all; // -> 'undefined'

At the same time, document.all is not equal to undefined.

document.all === undefined; // -> false
document.all === null; // -> false

But at the same time:

document.all == null; // -> true

document.all used to be a way to access DOM elements, in particular with old versions of IE. While it has never been a standard it was broadly used in the old age JS code. When the standard progressed with new APIs (such as document.getElementById) this API call became obsolete and the standard committee had to decide what to do with it. Because of its broad use they decided to keep the API but introduce a willful violation of the JavaScript specification. The reason why it responds to false when using the Strict Equality Comparison with undefined while true when using the Abstract Equality Comparison is due to the willful violation of the specification that explicitly allows that.

— “Obsolete features - document.all” at WhatWG - HTML spec — “Chapter 4 - ToBoolean - Falsy values” at YDKJS - Types & Grammar

Number.MIN_VALUE is the smallest number, which is greater than zero:

Number.MIN_VALUE > 0; // -> true

Number.MIN_VALUE is 5e-324, i.e. the smallest positive number that can be represented within float precision, i.e. that's as close as you can get to zero. It defines the best resolution that floats can give you.

Now the overall smallest value is Number.NEGATIVE_INFINITY although it's not really numeric in a strict sense.

— “Why is 0 less than Number.MIN_VALUE in JavaScript?” at StackOverflow

• 20.1.2.9 Number.MIN_VALUE

⚠️ A bug present in V8 v5.5 or lower (Node.js <=7) ⚠️

All of you know about the annoying undefined is not a function, but what about this?

// Declare a class which extends null
class Foo extends null {}
// -> [Function: Foo]

new Foo() instanceof null;
// > TypeError: function is not a function
// >     at … … …

This is not a part of the specification. It's just a bug that has now been fixed, so there shouldn't be a problem with it in the future.

It's continuation of story with previous bug in modern environment (tested with Chrome 71 and Node.js v11.8.0).

class Foo extends null {}
new Foo() instanceof null;
// > TypeError: Super constructor null of Foo is not a constructor

This is not a bug because:

Object.getPrototypeOf(Foo.prototype); // -> null

If the class has no constructor the call from prototype chain. But in the parent has no constructor. Just in case, I’ll clarify that null is an object:

typeof null === "object";

Therefore, you can inherit from it (although in the world of the OOP for such terms would have beaten me). So you can't call the null constructor. If you change this code:

class Foo extends null {
  constructor() {
    console.log("something");
  }
}

You see the error:

ReferenceError: Must call super constructor in derived class before accessing 'this' or returning from derived constructor

And if you add super:

class Foo extends null {
  constructor() {
    console.log(111);
    super();
  }
}

JS throws an error:

TypeError: Super constructor null of Foo is not a constructor

• An explanation of this issue by @geekjob

What if you try to add two arrays?

[1, 2, 3] + [4, 5, 6]; // -> '1,2,34,5,6'

The concatenation happens. Step-by-step, it looks like this:

[1, 2, 3] +
  [4, 5, 6][
    // call toString()
    (1, 2, 3)
  ].toString() +
  [4, 5, 6].toString();
// concatenation
"1,2,3" + "4,5,6";
// ->
("1,2,34,5,6");

You've created an array with 4 empty elements. Despite all, you'll get an array with three elements, because of trailing commas:

let a = [, , ,];
a.length; // -> 3
a.toString(); // -> ',,'

Trailing commas (sometimes called "final commas") can be useful when adding new elements, parameters, or properties to JavaScript code. If you want to add a new property, you can simply add a new line without modifying the previously last line if that line already uses a trailing comma. This makes version-control diffs cleaner and editing code might be less troublesome.

— Trailing commas at MDN

Array equality is a monster in JS, as you can see below:

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

[null] == ''      // true
[null] == 0       // true
[undefined] == '' // true
[undefined] == 0  // true

[[]] == 0  // true
[[]] == '' // true

[[[[[[]]]]]] == '' // true
[[[[[[]]]]]] == 0  // true

[[[[[[ null ]]]]]] == 0  // true
[[[[[[ null ]]]]]] == '' // true

[[[[[[ undefined ]]]]]] == 0  // true
[[[[[[ undefined ]]]]]] == '' // true

You should watch very carefully for the above examples! The behaviour is described in section 7.2.15 Abstract Equality Comparison of the specification.

If we don't pass any arguments into the Number constructor, we'll get 0. The value undefined is assigned to formal arguments when 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

According to the specification:

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

Here's the corresponding section:

• 20.1.1 The Number Constructor
• 7.1.3 ToNumber(argument)

parseInt is famous by its quirks:

parseInt("f*ck"); // -> NaN
parseInt("f*ck", 16); // -> 15

💡 Explanation: This happens because parseInt will continue parsing character-by-character until it hits a character it doesn't know. The f in 'f*ck' is the hexadecimal digit 15.

Parsing Infinity to integer is something…

//
parseInt("Infinity", 10); // -> NaN
// ...
parseInt("Infinity", 18); // -> NaN...
parseInt("Infinity", 19); // -> 18
// ...
parseInt("Infinity", 23); // -> 18...
parseInt("Infinity", 24); // -> 151176378
// ...
parseInt("Infinity", 29); // -> 385849803
parseInt("Infinity", 30); // -> 13693557269
// ...
parseInt("Infinity", 34); // -> 28872273981
parseInt("Infinity", 35); // -> 1201203301724
parseInt("Infinity", 36); // -> 1461559270678...
parseInt("Infinity", 37); // -> NaN

Be careful with parsing null too:

parseInt(null, 24); // -> 23

💡 Explanation:

It's converting null to the string "null" and trying to convert it. For radixes 0 through 23, there are no numerals it can convert, so it returns NaN. At 24, "n", the 14th letter, is added to the numeral system. At 31, "u", the 21st letter, is added and the entire string can be decoded. At 37 on there is no longer any valid numeral set that can be generated and NaN is returned.

— “parseInt(null, 24) === 23… wait, what?” at StackOverflow

Don't forget about octals:

parseInt("06"); // 6
parseInt("08"); // 8 if support ECMAScript 5
parseInt("08"); // 0 if not support ECMAScript 5

💡 Explanation: If the input string begins with "0", radix is eight (octal) or 10 (decimal). Exactly which radix is chosen is implementation-dependent. ECMAScript 5 specifies that 10 (decimal) is used, but not all browsers support this yet. For this reason always specify a radix when using parseInt.

parseInt always convert input to string:

parseInt({ toString: () => 2, valueOf: () => 1 }); // -> 2
Number({ toString: () => 2, valueOf: () => 1 }); // -> 1

Be careful while parsing floating point values

parseInt(0.000001); // -> 0
parseInt(0.0000001); // -> 1
parseInt(1 / 1999999); // -> 5

💡 Explanation: ParseInt takes a string argument and returns an integer of the specified radix. ParseInt also strips anything after and including the first non-digit in the string parameter. 0.000001 is converted to a string "0.000001" and the parseInt returns 0. When 0.0000001 is converted to a string it is treated as "1e-7" and hence parseInt returns 1. 1/1999999 is interpreted as 5.00000250000125e-7 and parseInt returns 5.

Let's do some math:

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

Hmmm… 🤔

We can coerce values to numbers with the 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, the ToNumber method is invoked. 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)

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