'' equals false in Javascript? What would be the safest way to distinguish between '' and boolean false?

Question

We use an external API whcih returns '' or boolean false while Javascript seems to find the both equal. for example:

var x = '';
if (!x) {
  alert(x); // the alert box is shown - empty

}
if (x=='') {
  alert(x); // the alert box is shown here too  - empty

}
var z = false;
if (!z) {
  alert(z); // the alert box is shown - displays 'false'

}
if (z=='') {
  alert(z); // the alert box is shown here too - displays 'false'

}

How can we distinguish between the two?

Answer 1

Use the triple equal

if (x===false) //false as expected
if (z==='') //false as expected

A double equal will do type casting, while triple equal will not. So:

0 == "0" //true
0 === "0" //false, since the first is an int, and the second is a string

Answer 2

as mentioned by peirix: tripple equal signs check both the value and the type

1 == '1' // true
1 === '1' // false

Answer 3

var typeX = typeof(x);
var typeZ = typeof(z);

if (typeX == 'string' && x == '')
else if (typeX == 'boolean' && !typeX)

I like Peirix's answer too, but here is an alternative.

Answer 4

For avoid this questions use jslint validator. It helps for find unsafe operations.