What is the difference between == and === in JavaScript?

Double equals (==) is abstract equality comparison operator, which transforms the operands to the same type before making the comparison. 

For example,

4    ==  4        // true
'4'  ==  4        //true
4    == '4'       // true
0    == false     // true

Triple equals (===) are strict equality comparison operator, which returns false for different types and different content.

For example,

4 === 4  // true
4 === '4' // false
var v1 = {'value':'key'};
var v2 = {'value': 'key'};
v1 === v2 //false