The == and != operators do type coercion before comparing. This is bad because it causes ' \t\r\n' == 0 to be true. This can mask type errors. It is best to not use == and != and to always use the more reliable === and !== operators instead.

Exceptions

Even if testing the equality of a variable against null doesn't do exactly what most JavaScript developers believe, usage of == or != is tolerated in such context. In the following case, if foo hasn't been initialized, its default value is not null but undefined. Nevertheless undefined == null, so JavaScript developers get the expected behavior.

if(foo == null) {...}