Question

alert (0 == ''); // true
alert (0 == '0'); // true

JSFiddle Proof

I understand that == in javascript performs a conversion and then checks for equality, but how does it perform the conversion in the statements above? Does it convert 0 to ” or ” to 0? Or something else perhaps? Is there a spec somewhere that explains the implementation?

Answer 1

It uses the Abstract Equality Comparison Algorithm.

Specifically for your example

If Type(x) is Number and Type(y) is String,
return the result of the comparison x == ToNumber(y).

So then you’ll end up with:

0 == 0

…in both cases because an empty string converts to the number 0, and a numeric string converts to the number given.

From 9.3.1 ToNumber Applied to the String Type:

A StringNumericLiteral that is empty or contains only white space is converted to +0

Since we’re now doing a comparison of the same Type on the second pass, it will do the following:

If Type(x) is the same as Type(y), then

…
If Type(x) is Number, then

…
If x is the same Number value as y, return true.

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To test a toNumber conversion, you can use the unary + operator.

console.log( +'' );   // 0
console.log( +'0' );  // 0