Difference between == and === in javascript

In almost every programming language the easiest way for comparison is "==". But in javascript there is "===" also available. If you ask anyone what is the difference between == and === is, they will say the same thing, that is === also does the type checking. Well this is absolutely correct, but still the doubt is, how does it work?
Here are few examples -
0 == false -- > true
0 === false -- > false

1 == true --> true
1 === true --> false

2 == true --> false

In order to understand the behavior of == and ===, we need to know few internal methods of javascripts.

ToPrimitive -
Return a default value for the Object. The default value of an object is retrieved by calling the [[DefaultValue]] internal method of the object.

[[DefaultValue]] - get the output of "toString" or "valueOf" method on the object. If both method not available, then throw exception.

ToNumber -

Argument Type	Result
Undefined	NaN
Null	0
Boolean	true - > 1, false - 0
Number	No change
String	if it is number as string, then return the number, else NaN
Object	Apply ToPrimitive, then apply ToNumber and get the output

Lets try to understand == first.

Example -> x== y

If the typeOf x and y are same, then there is no confusion, because its normal comparison, but the trick comes when the types are different.

Key Concept - Javascript internally converts x and y both into Numbers, if the Types are different and then do the comparison.

Lets try to practice it by example -

Variable Type	ToNumber	Example
String	Numeric number in string -> corresponding number Nan, for everything else	'5' == 5 -> true 'a' == 2 -> false
Object	Apply toprimitive first, and then use above mentioned conversions for comparison	new String('1') == 1 -> true new Boolean(false) == 0 -> true
Number	No change	1==2 -> false 3==3 -> true
Boolean	true - > 1 false -> 0	true == 1 -> true ; false == 0 -> true true == 3 -> false ; false == 1 -> false

In case of x and y being null or undefined, they are compared with following rules -

null == undefined -> true

 

In case of ===, the very first check javascript does is, to check the types of both x and y. Both are of different types then return false, else go ahead with further comparison.