Why is tab equal to false in JavaScript? [duplicate]

Question

I thought I knew about the quirks with == and all the strange type casts in JavaScript, but today I stumbled upon one thing that doesn't make any sense to me:

'\t' == false // => true 

Why is that?

Apparently, '\t' is not a falsy value, and if combined with || it works as excpected:

'\t' || 42 // => '\t' 

On the other hand, a toString is also not called on false, see:

'\t' == 'false' // => false 

This led me to thinking that the tab maybe gets converted to a boolean, but:

Boolean('\t') == false // => false 

So, the question is: Why is that?

Answer 1

See Abstract Equality Comparison::

The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:

So, in your situation, x is a string, and y is a boolean. The first condition that is fulfilled here is:

9. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).

Turning the check into

'\t' == 0 

Which then fulfills:

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

And ToNumber('\t') === 0:

console.log(Number('\t'));

Turning the check into

0 == 0 

which is the same as

0 === 0 

or true.

Note that while a string composed of all whitespace is == false, calling Boolean on such a string will return true, because the string has a non-zero length:

console.log(    Boolean(' '),    Boolean('\t')  );

Of course, it would be best to always avoid == - use === instead, and you won't have to worry about these silly coercion rules.