Links

• See also: isNaN
• Bad Parts (from JavaScript: The Good Parts - Douglas Crockford)
• Truthy and Falsy: When All is Not Equal in JavaScript

True and False values

From: http://www.sitepoint.com/javascript-truthy-falsy/

Always falsy values

• false
• 0 (zero)
• "" (empty string)
• null
• undefined
• NaN

Everything else is truthy including "0" (string), etc.

Comparison with ==

From: http://oreilly.com/javascript/excerpts/javascript-good-parts/bad-parts.html

The operators == and != attempt coercion on operands of different types.

For example: ' \t\t ' == false,  yet ' \t\t ' is true in a conditional.

In theory, it is better to use === and !==, which do not attempt coercion.

• null and undefined are not equivalent to anything except themselves.
• NaN is not equivalent to anything including itself.  But see isNaN

Snippets

function test(o, name) {
  console.log("Testing: %s", name);
  console.log((o == false) ? "== false", "!= false");
  console.log((o) ? "is NOT falsey", "is falsey");
}

// null == false ?
test(null, "null");
test('', "''");