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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?

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  • 12
    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. Commented Jan 3, 2013 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. Commented Jan 3, 2013 at 16:53
  • 1
    developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/… Commented Jan 3, 2013 at 16:54
  • @Blender those are arrays, not objects, but your statements still hold for arrays. Commented Jan 3, 2013 at 17:07
  • 1
    @jbabey: Arrays are objects. Commented Jan 3, 2013 at 17:07

5 Answers 5

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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.

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2

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];
...
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Comments

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] == "" )
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1 Comment

instanceof Array is not reliable when you're dealing with cross-document objects.
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
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2 Comments

JSON.stringify is a vastly safer option that could be used in the same way.
[","].toString() === ["",""].toString(); // true
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)
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1 Comment

It's much simpler to join the two arrays and check whether values are the same than iterating over all elements.

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