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

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.