JavaScript syntax: Difference between revisions

Revision as of 09:34, 19 May 2019

Revision as of 11:40, 5 June 2019

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Note: There is no built-in language literal for undefined. Thus <{{code>|lang=javascript|code=(x == undefined)</code>}} is not a foolproof way to check whether a variable is undefined, because in versions before ECMAScript 5, it is legal for someone to write <{{code>|lang=javascript|code=var undefined = "I'm defined now";</code>}}. A more robust approach is to compare using <{{code>|lang=javascript|code=(typeof x === 'undefined')</code>}}.

Functions like this won't work as expected:

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Unlike undefined, [[Null Object pattern|null]] is often set to indicate that something has been declared, but has been defined to be empty. In a Boolean context, the value of null is considered a false value in JavaScript.

Note: Null is a true primitive-type within the JavaScript language, of which {{mono|null}} (note case) is the single value. As such, when performing checks that enforce type checking, the null value will not equal other false types. Surprisingly, {{mono|null}} is considered an object by {{mono|[[typeof]]}}.

<syntaxhighlight lang="javascript">