JavaScript syntax: Difference between revisions

Revision as of 10:41, 26 June 2015

Revision as of 10:45, 26 June 2015

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Note: There is no built-in language literal for undefined. Thus <ttcode>(x == undefined)</ttcode> 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 <ttcode>var undefined = "I'm defined now";</ttcode>. A more robust approach is to compare using <ttcode>(typeof x === 'undefined')</ttcode>.

Functions like this won't work as expected:

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Here, calling <ttcode>isUndefined(my_var)</ttcode> raises a <tt>{{mono|ReferenceError</tt>}}, if <tt>{{mono|my_var</tt>}} is an unknown identifier, whereas <ttcode>typeof my_var === 'undefined'</ttcode> doesn't.

===Null===