Funny JavaScript

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.

Let’s start:

a) [] is equal ![].

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

Explanation:

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.

Also please check :

1): 12.5.9 – Logical NOT Operator(!)

2): 7.2.13 – Abstract Equality Comparison

Try to play:

+[] == +![]

0 == +false

b) true is not equal ![], but not equal [] too.

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

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

Explanation:

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

// According to the specification

true == []; // -> false

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

1 == 0; // -> false

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

![]; // -> false

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

// According to the specification

false == []; // -> true

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

0 == 0; // -> true

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

![]; // -> false

false == false; // -> true

c) NaN is a Number

typeof NaN; // -> 'number'
instanceof NaN; // -> 'SyntaxError'

Check:

1.12.5.5 – typeof

2.12.10.4 – instanceof

Try to play:

NaN === NaN; -> ?

d) null and [] are objects

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

null instanceof Object; // false

Explanation:

As section c) described, For null, ordinary, standard exotic and non-standard exotic objects, which do not implement [[Call]], it returns the string object.

So we can use toString function to check the type of an object.

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

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

e) null and relational operators

null > 0; // false
null == 0; // false

null >= 0; // true

Explanation:

if null is less than 0 is false, then null >= 0 is true. Read in-depth explanation for this here.

Have fun, please check here for more details, also about Denys Dovhan.