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Volker Schukai
Volker Schukai

Posted on • Updated on

You are an array

Javascript always has some surprises in store. for example, typeof [] return object and not array.

Non ? Si ! Oh !!!!

This has historical reasons. But this can be very confusing

How can you then properly check if something is an array?


For example, you could check if the constructor is an array.

([]).constructor === Array // true
(new Array).constructor === Array // true
({}).constructor === Array // false
(true).constructor === Array // false
(null).constructor === Array // TypeError
(undefined).constructor === Array // TypeError
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As you can see this has a problem. With null and undefined a TypeError is thrown.

You could catch this for example with a try/catch.

try {
  (undefined).constructor === Array // true
} catch(e) {
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But you don't want that.


There is also this exotic possibility to check if it is an array. You can missuse the toString() method for this.[]).indexOf('Array')!==-1
     // => true{}).indexOf('Array')!==-1
     // => false"").indexOf('Array')!==-1
     // => false'Array')!==-1
   // => false'Array')!==-1
   // => false'Array')!==-1
   // => false
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But it doesn't just look awkward, it is awkward.


Alternatively, the instanceof operator, which is also known from other languages, can of course be used here.

[]  instanceof Array // => true
{} instanceof Array // => false
"" instanceof Array // => false
null instanceof Array // => false
true instanceof Array // => false
10 instanceof Array // => false
undefined instanceof Array // => false
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Already looks pretty good. But there is also a problem here.

All these checks work only if the array was created by the original array constructor in the current environment.

const iframe = document.createElement('iframe');
const iframeArray = window.frames[window.frames.length-1].Array;
const array = new iframeArray(1,2,3); 
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Here the two array instances do not match.

array instanceof Array; // false
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But there is a real solution.


For these reasons, since ECMAScript 5 there is a method Array.isArray().

This also works with different instances.

Array.isArray([])  // => true
Array.isArray(Array.prototype)  // => surprise; true
Array.isArray({}) // => false
Array.isArray("") // => false
Array.isArray(null)  // => false
Array.isArray(true)  // => false
Array.isArray(undefined)  // => false
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If it (isArray) is supported, it is the solution!

hope you enjoy it!


Top comments (2)

bias profile image
Tobias Nickel

Yes, 'Array.isArray' is the right choice.
but not only for the sake if convenience. I heared in some podcast, it is about workers and iframes. While Arrays in each frame or worker are a class from the same code, at each runtime they are a "new class". with 'isArray' it works, even dough an array was taken from a other related js scope.

volker_schukai profile image
Volker Schukai

Yes you are right. I try to express that with my iframe example above.