Why does JSLint tell me to use "=== undefined" instead of "typeof ... === 'undefined'"?

Question

I coded the following:

showTitles = (typeof showTitles !== 'undefined') ? showTitles : 'Y';
showSelectGroup = (typeof showSelectGroup !== 'undefined') ? showSelectGroup : 'Y';

But JSLint is saying:

Warning 3 JS Lint: Unexpected 'typeof'. Use '===' to compare directly with undefined.

How should I change my code?

I don't know anything about jslint, but I think it's telling you to do this: `(showtitles === undefined)`. — Travesty3, Sep 27 '12 at 18:45
It says it in the message: `Use '===' to compare directly with undefined`. — Felix Kling, Sep 27 '12 at 18:45
You're better off writing `var showTitles = showTitles || 'Y';` — CaffGeek, Sep 27 '12 at 18:51

CaffGeek · Answer 1 · 2012-09-27T18:50:42.970

9

Probably by using

showTitles = (showTitles === undefined) ? 'Y' : showTitles;
showSelectGroup = (showSelectGroup === undefined) ? 'Y' : showSelectGroup;

jslint has no issues with that (assuming showTitles and showSelectGroup are declared with var)

However, I'd write it as

var showTitles = showTitles || 'Y';
var showSelectGroup = showSelectGroup || 'Y';

edited Sep 27 '12 at 18:50

answered Sep 27 '12 at 18:45

CaffGeek

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score 8 · Accepted Answer · answered Sep 27 '12 at 18:45

8

Note that whether this is best practice in general is debatable, but if you want to make it work with JSLint, you could do this

showTitles = (showTitles !== undefined) ? showTitles : 'Y';

answered Sep 27 '12 at 18:45

Alex Turpin

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@Marilou not if you're using strict comparison. That being said, I would encourage doing research on that and deciding whether you want to do stuff that way for yourself and not just blindly follow JSLint's "advice". – Alex Turpin Sep 27 '12 at 18:50
Have you initialized your variable beforehand? [This code](http://jsfiddle.net/ASfgD/1/) passes JSLint and doesn't have any errors. – Alex Turpin Apr 09 '13 at 14:30
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But if you are trying to test whether or not a variable is defined, you have to use typeof. Yet JSLint still complains about it. Very annoying. – mattacular May 29 '13 at 15:59
@mattacular Do you have a use case for which comparing to `undefined` does not work properly? – Alex Turpin May 29 '13 at 19:04
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Yeah... when you are trying to test whether or not a variable is defined. Most of the time, you can refactor your code so that it isn't necessary (someVariable || false) but there are edge cases where you need to test for that and a typeof is the only way I'm aware of to do it safely. For example "aVariableThatMightBeUndefined === undefined" will throw a ReferenceError – mattacular May 31 '13 at 13:56
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@mattacular you can simply use context for that. `this.aVariableThatMightBeUndefined === undefined`. – Alex Turpin Jun 03 '13 at 20:54
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If you need to test of the existence of a global var you must use `typeof`. – Rico Sonntag Jun 21 '13 at 07:32
@RicoSonntag no such thing as a "global var" in JavaScript. They all have context you can use. – Alex Turpin Jun 21 '13 at 18:45
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@Alex Turpin so would the correct way be window['variableWhichImTesting'] === undefined – szydan Apr 11 '14 at 15:02
@szydan yep, you got it! – mattacular May 12 '14 at 16:04

score 8 · Answer 3 · answered Dec 01 '15 at 03:22

This message reflects the latest best practices. As of ES5 strict mode, the global value of undefined can no longer be changed and a direct comparison is simpler code and faster. In short, JSLint is aware of all this, and is giving you good advice.

In this case, change typeof showTitles !== 'undefined' to showTitles === undefined.

Why does JSLint tell me to use "=== undefined" instead of "typeof ... === 'undefined'"?

3 Answers3

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