Why javascript treats 0 equal to empty string? [duplicate]

Question

Possible Duplicate:
Implied string comparison, 0='', but 1='1'

Executing the following case in javascript, I get 0 equal to '' (an empty string)

var a   =   0; var b   =   '';//empty string if(a==b){     console.log('equal');//this is printed in console }else{     console.log('not equal'); } 

Could anyone guide that what is the reason behind this?

Answer 1

Quoting the doc (MDN):

Equal (==)

If the two operands are not of the same type, JavaScript converts the operands then applies strict comparison. If either operand is a number or a boolean, the operands are converted to numbers if possible; else if either operand is a string, the other operand is converted to a string if possible.

As a operand type here is Number, b gets converted to Number as well. And Number('') evaluates to 0.

This can be quite surprising sometimes. Consider this, for example:

console.log(0 == '0');  // true console.log(0 == '');   // true console.log('' == '0'); // O'RLY? 

... or this:

console.log(false == undefined); // false console.log(false == null);      // false console.log(null == undefined);  // fal.... NO WAIT! 

...and that's exactly why it's almost always recommended to use === (strict equality) operator instead.

Answer 2

0, "" (Empty String), false all technically have the same value, when typecasted. If you need to strictly treat them, you can use ===.

It is the same with similar programming languages, like PHP.

var a = 0; var b = ''; //empty string if(a == b){     console.log('equal'); //this is printed in console }else{     console.log('not equal'); } 

To make a strict comparison:

if(a === b){     console.log('equal'); }else{     console.log('not equal'); //this is printed in console }