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Volker Schukai
Volker Schukai

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You are an array

#javascript #array #browser #webdev

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?

Constructor

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.

Exotic

There is also this exotic possibility to check if it is an array. You can missuse the toString() method for this.

Object.prototype.toString.call([]).indexOf('Array')!==-1 // => true Object.prototype.toString.call({}).indexOf('Array')!==-1 // => false Object.prototype.toString.call("").indexOf('Array')!==-1 // => false Object.prototype.toString.call(null).indexOf('Array')!==-1 // => false Object.prototype.toString.call(true).indexOf('Array')!==-1 // => false Object.prototype.toString.call(undefined).indexOf('Array')!==-1 // => false
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But it doesn't just look awkward, it is awkward.

instanceOf

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'); document.body.appendChild(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.

Array.isArray

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!

References

Top comments (2)

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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.

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volker_schukai profile image
Volker Schukai •

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