6

I was wondering when the Javascript if expression actually evaluates to false and when to true. When is the if statement false, and is that true for all JS interpreters?

I guess the condition is false on

  • false
  • undefined
  • null
  • 0

otherwise true. Is that correct for all implementations (tested in Safari/WebKit console), or am I better off with explicit checking like (typeof a === "undefined")?

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2 Answers 2

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9

The following values will evaluate to false:

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

https://developer.mozilla.org/en/JavaScript/Guide/Statements#if...else_Statement

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2 Comments

Thank you! Did overlook that!
Just to know -- What is the order of evaluation in multiple JS if() conditions? Left to right or right to left?
0

If you want to check for variable existence, and a is not declared anywhere in your script, typeof a === 'undefined' is the way, or you can use if (!window.a). Otherwise a ReferenceError is thrown.

Your guessing seems to be correct. An empty string and NaN also evaluate to false (or, as some like it falsy). The fact that 0 evaluates to false may be tricky, but it's handy too, in statements like while (i--) (if i has the value of 0 it evaluates to false and the while loop stops).

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