JavaScript if fails with [""] (an array containing an empty string)

Question

Possible Duplicate:
JavaScript compare arrays

var x = [""]
if (x === [""]) { alert("True!") } 
else { alert("False!") }

For some reason, this alerts False. I cannot seem to figure out why. What can I do to make this alert True?

Two objects are equal only if they refer to the same object. Even `[] != []`. One simple way to compare them would be to compare their JSON representations. — Blender, Jan 03 '13 at 16:51
Why should it alert true? You compare two different objects. The happen to have the identical values, but they are still two distinct objects. — Mithrandir, Jan 03 '13 at 16:53
https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/Comparison_Operators — j08691, Jan 03 '13 at 16:54
@Blender those are arrays, not objects, but your statements still hold for arrays. — jbabey, Jan 03 '13 at 17:07

score 4 · Answer 1 · edited May 23 '17 at 12:04

4

Two objects are equal if they refer to the exact same Object. In your example x is one Object and [""] is another. You cant compare Objects this way. This link maybe useful.

edited May 23 '17 at 12:04

Community

1
1

answered Jan 03 '13 at 16:58

Peter Gerasimenko

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Robert Koritnik · Accepted Answer · 2013-01-04T16:32:25.520

Compare values not whole arrays

...as they're objects and you're working with implicit references here. One object is stored in your x valiable which you're trying to compare (by reference) with an in-place created object (array with an empty string element). These are two objects each having it's own reference hence not equal.

I've changed your example to do what you're after while also providing the possibility to have an arbitrary number of empty strings in an array:

if (x.length && x.join && x.join("") === "")
{
    alert("True!")
}
else
{
    alert("False!")
}

This will return True! for any arrays like:

var x = [];
var x = [""];
var x = [null];
var x = [undefined];
var x = ["", null, undefined];
...

score 1 · Answer 3 · answered Jan 03 '13 at 16:54

1

Arrays cannot be reliably compared this way unless they reference the same object. Do this instead:

if ( x[0] == "" )

or if you want it to be an array:

if ( x.length && x[0] == "" )

answered Jan 03 '13 at 16:54

David G

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`instanceof Array` is not reliable when you're dealing with cross-document objects. – John Dvorak Jan 03 '13 at 16:55

score 0 · Answer 4 · answered Jan 03 '13 at 16:54

0

You could use toString in this case. Careful, there are some outliers where this will not work.

var x = [""]
alert(x.toString() == [""].toString()) // true

answered Jan 03 '13 at 16:54

jhummel

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2

`JSON.stringify` is a vastly safer option that could be used in the same way. – apsillers Jan 03 '13 at 16:56
`[","].toString() === ["",""].toString(); // true` – the system Jan 03 '13 at 17:12

Blender · Answer 5 · 2013-01-03T17:01:49.803

0

In JavaScript, two objects are equal only if they refer to the same object. Even [] == [] is false.

A probably slow but generic solution would be to compare the string representations of the two objects:

JSON.stringify(a1) == JSON.stringify(a2)

edited Jan 03 '13 at 17:01

answered Jan 03 '13 at 16:56

Blender

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It's much simpler to join the two arrays and check whether values are the same than iterating over all elements. – Robert Koritnik Jan 03 '13 at 16:58

JavaScript if fails with [""] (an array containing an empty string)

5 Answers5

Compare values not whole arrays