Info

There are a few different ways to check if an object is undefined. Which is the best for speed and also size.

Preparation code

<script>
  var a;
  var b = 42;
  var undefinedVar;
  var undefinedStringCached = "undefined";
</script>

Test runner

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if (typeof a === "undefined") {}
if (typeof b === "undefined") {}

if (typeof a === undefinedStringCached) {}
if (typeof b === undefinedStringCached) {}

if (a === undefined) {}
if (b === undefined) {}

if (a === void(0)) {}
if (b === void(0)) {}

if (a === undefinedVar) {}
if (b === undefinedVar) {}

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Revisions

You can edit these tests or add even more tests to this page by appending /edit to the URL. Here’s a list of current revisions for this page:

• Revision 1: published by Corban Brook on 14th August 2010

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