JavaScript syntax: Difference between revisions

Revision as of 20:40, 7 August 2023

Revision as of 20:42, 7 August 2023

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console.log(undefined === null); // enforce type during check, displays false

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Try out this example code on the [https://jsonlinecompiler.com/javascript-online-editor/ JavaScript Online Compiler & Editor].

Note: There is no built-in language literal for undefined. Thus {{code|lang=javascript|code=(x === undefined)}} 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";}}. A more robust approach is to compare using {{code|lang=javascript|code=(typeof x === 'undefined')}}.

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function isUndefined(x) { return (typeof x) === "undefined"; } // ... or that third one

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Try out this example code on the [https://jsonlinecompiler.com/javascript-online-editor/ JavaScript Online Compiler & Editor].

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