Checking JavaScript object for null subtelty

Question

I believe I have found cases where I need to check for both undefined and null for a javascript object as follows:

if (x !== undefined && x != null && x.length > 0) {...}

However, in a recent upgrade of JetBrains tools, it tells me that this is sufficient

if (x != undefined && x.length > 0) {...}

My question is, I'm just wanting to insure that a string "x" has a length of non-zero and is not undefined or null (with the least amount tests).

Thoughts?

If you want a string of length > 0, check that `typeof x === 'string' && x.length > 0`. — Matt, Aug 24 '13 at 18:59
@AurélienOoms, **World Peace**, if you pointed the question to me. ``) — gdoron is supporting Monica, Aug 24 '13 at 19:05
possible duplicate of [In Javascript, is myObject == null a valid way to handle checking for undefined as well?](http://stackoverflow.com/questions/18137996/in-javascript-is-myobject-null-a-valid-way-to-handle-checking-for-undefined) (since the answer is yes, it works the other way round as well) — Bergi, Aug 24 '13 at 19:09

score 5 · Answer 1 · answered Aug 24 '13 at 18:56

5

in javascript

undefined == null // true
undefined === null // false

so checking with == for undefined makes the == check for null redundant.

answered Aug 24 '13 at 18:56

gdoron is supporting Monica

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score 2 · Answer 2 · answered Aug 24 '13 at 18:58

2

Try

if (x && x.length)

as undefined, null and 0 are all falsy values.

EDIT: as you seem to know that x should be a string, you can also just use if (x) as an empty string is also falsy.

answered Aug 24 '13 at 18:58

Dennis

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score 2 · Accepted Answer · edited May 23 '17 at 11:54

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Checking if foo === undefined will trigger the error foo is not defined. See variable === undefined vs. typeof variable === "undefined"

The existential operator in CoffeeScript compiles to

typeof face !== "undefined" && face !== null

Edit:

Matt's comment is better if you only want to check for strings:

typeof x === 'string' && x.length > 0

edited May 23 '17 at 11:54

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answered Aug 24 '13 at 19:12

Wex

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score 2 · Answer 4 · edited Jun 20 '20 at 09:12

2

You can use _.isNull from Underscore a JavaScript library provides a whole mess of useful functional programming helpers.

_.isNull(object)

Returns true if the value of object is null.

_.isNull(null);
=> true
_.isNull(undefined);
=> false

edited Jun 20 '20 at 09:12

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answered Feb 11 '16 at 12:35

Alaeddine

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score 2 · Answer 5 · answered Aug 24 '16 at 17:41

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This is what I use and it's the most concise. It covers: undefined, null, NaN, 0, "" (empty string), or false. We can therefore say that "object" is truthy.

if(object){
    doSomething();
}

answered Aug 24 '16 at 17:41

James Drinkard

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score 0 · Answer 6 · answered Aug 24 '13 at 18:58

0

Try this

 if (!x) {
  // is emtpy
}

answered Aug 24 '13 at 18:58

iJade

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The case I'm almost always looking for and I'd like to have a really good "always use" pattern is Is it a non null, non empty string – Peter Kellner Aug 24 '13 at 19:45

score 0 · Answer 7 · answered Aug 24 '13 at 21:55

To check against null AND undefined AND "empty string" you can just write

if(!x) {
   // will be false for undefined, null and length=0
}

BUT you need to be sure that your variable is defined! Otherwise this will cause an error.

If you are checking for a value in an object (e.g. the window object), you can always use that. E.g. for checking for localStorage support:

var supports = {
    localStorage: !!window.localStorage
}

Checking JavaScript object for null subtelty

7 Answers7