Whether a variable is undefined

Question

How do I find if a variable is undefined?

I currently have:

var page_name = $("#pageToEdit :selected").text();
var table_name = $("#pageToEdit :selected").val();
var optionResult = $("#pageToEditOptions :selected").val();

var string = "?z=z";
if ( page_name != 'undefined' ) { string += "&page_name=" + page_name; }
if ( table_name != 'undefined' ) { string += "&table_name=" + table_name; }
if ( optionResult != 'undefined' ) { string += "&optionResult=" + optionResult; }

undefined is a property of javascript so doesn't need to be in quotes. You're checking to see if the values are actually the string 'undefined'. http://www.w3schools.com/jsref/jsref_undefined.asp — daddywoodland, Sep 28 '09 at 07:44
@daddywoodland: Little advice; you shouldn't be referencing W3Schools. They are known for giving out false information and isn't a good resource to recommend to others. — TheCarver, Feb 28 '13 at 03:19
@PaparazzoKid what is wrong with W3Schools? Do you think the link above is incorrect or do you have other examples? I find the a useful reference, obviously not as authoritative as digging through a W3C document but sometimes you just need to quick reference. — Marc Stober, Feb 05 '14 at 23:49
I like to use this function below so I can quickly and easily check the existence of a variable. `function doesExist(el) { if((typeof el !== "undefined") && (typeof el.val() !== "undefined")){ return true; } else{ return false; } }` Then you can just call it like this `if (doesExist(variable_name)) { // do stuff }` — MistyDawn, Jul 05 '18 at 17:37

ScottyUCSD · Answer 1 · 2009-09-28T15:05:56.300

327

jQuery.val() and .text() will never return 'undefined' for an empty selection. It always returns an empty string (i.e. ""). .html() will return null if the element doesn't exist though.You need to do:

if(page_name != '')

For other variables that don't come from something like jQuery.val() you would do this though:

if(typeof page_name != 'undefined')

You just have to use the typeof operator.

edited Sep 28 '09 at 15:05

answered Sep 28 '09 at 07:38

ScottyUCSD

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if(undefinedVar) will throw an error, but if(someObj.undefinedProperty) will not. In the latter case, you can also skip typeof and use === undefined (or !==), without quotes. – eyelidlessness Sep 28 '09 at 08:56
1

And I gave you +1 for correctly pointing out that jQuery's methods won't return undefined. – eyelidlessness Sep 28 '09 at 08:56
Yeah. You're right about the undefined var part. – ScottyUCSD Sep 28 '09 at 15:05
typeof is helpful to play with self defined variables, we can whether it is defined or not – sun Jul 24 '14 at 09:48
1

if(typeof page_name != 'undefined') should be if(typeof page_name !== 'undefined') – Rashod Chamikara Bandara Jun 09 '19 at 04:04

score 136 · Answer 2 · edited May 27 '21 at 16:04

136

if (myVariable === undefined)

or more precisely

if (typeof myVariable === 'undefined')

Note the === is used

edited May 27 '21 at 16:04

Adil Malik

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answered Oct 31 '11 at 11:13

Roger

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Please use `typeof(var) === 'undefined'` as `undefined` is not a constant in JavaScript. – JonnyReeves Feb 12 '12 at 10:21
26

`typeof` is not a function - please don't use brackets with it. – rjmunro Sep 02 '13 at 13:46
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so is this answer valid or not? confused by the upvotes and the comments – Roy Mar 22 '16 at 14:46
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@Roy You should go for `if (typeof var === 'undefined')` – Sep 26 '16 at 09:22
@JonnyReeves I get the following message when applying `typeof(var)`: _Bynary operation argument type string is not compatible with type undefined_ – Pathros May 04 '17 at 15:43
I upvoted Ascam and was too late to remove my vote :( turns out 'if (var === undefined)' work perfectly I tried using 'typeof' and it did not work. – Orion May 13 '17 at 13:21
`if (typeof var === 'undefined')` vs `if (var === undefined)`... Both should work with very subtle differences ***[citation needed]***. Do note the difference of quotes around `undefined` in both use cases which should address the confusion in comments. – Fr0zenFyr Oct 30 '17 at 06:38

score 3 · Answer 3 · answered Sep 28 '09 at 07:39

3

http://constc.blogspot.com/2008/07/undeclared-undefined-null-in-javascript.html

Depends on how specific you want the test to be. You could maybe get away with

if(page_name){ string += "&page_name=" + page_name; }

answered Sep 28 '09 at 07:39

micmcg

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score 3 · Answer 4 · edited Mar 09 '14 at 10:22

3

function my_url (base, opt)
{
    var retval = ["" + base];
    retval.push( opt.page_name ? "&page_name=" + opt.page_name : "");
    retval.push( opt.table_name ? "&table_name=" + opt.table_name : "");
    retval.push( opt.optionResult ? "&optionResult=" + opt.optionResult : "");
    return retval.join("");
}

my_url("?z=z",  { page_name : "pageX" /* no table_name and optionResult */ } );

/* Returns:
     ?z=z&page_name=pageX
*/

This avoids using typeof whatever === "undefined". (Also, there isn't any string concatenation.)

edited Mar 09 '14 at 10:22

Peter Mortensen

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answered Sep 28 '09 at 08:48

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Why is avoiding `typeof ...` a good thing? – rjmunro Sep 02 '13 at 13:46

score -7 · Answer 5 · answered Sep 28 '09 at 07:38

You can just check the variable directly. If not defined it will return a falsy value.

var string = "?z=z";
if (page_name) { string += "&page_name=" + page_name; }
if (table_name) { string += "&table_name=" + table_name; }
if (optionResult) { string += "&optionResult=" + optionResult; }

Whether a variable is undefined

5 Answers5

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