Question

My app behaves abnormally and figured the code below is going to else statement unexpectedly.

code

if(" " != 0) {
    console.log("whitespace is not zero");
}
else {
    console.log("bar");
}

Firebug output

bar

I thought whitespace is a String and comparison against integer zero should return false like the case above but I don’t know why it is going to else statement.

Could anyone explain why?

Answer 1

In JS, " " == 0 equals true with loose/lenient comparison, you should use strict equality operator ===/!== instead:

" " !== 0

To get to first condition.

---

Tests:

console.log(" " == 0); // true
console.log(" " === 0); // false

---

Loose Comparison Chart:

""           ==   "0"           // false
0            ==   ""            // true
0            ==   "0"           // true
false        ==   "false"       // false
false        ==   "0"           // true
false        ==   undefined     // false
false        ==   null          // false
null         ==   undefined     // true
" \t\r\n"    ==   0             // true

Strict Comparison Chart:

""           ===   "0"           // false
0            ===   ""            // false
0            ===   "0"           // false
false        ===   "false"       // false
false        ===   "0"           // false
false        ===   undefined     // false
false        ===   null          // false
null         ===   undefined     // false
" \t\r\n"    ===   0             // false

(Examples by Douglas Crockford)

---

Good Practice:

Whenever possible, use strict equality operator because with loose equality operator, JS does type coercion which is a performance hit and doesn’t always yield expected results as shown in above comparison charts.