v8 - Javascript Arrays created with Object.create - not real Arrays? - Stack Overflow

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It looks like Arrays created with Object.create walk like Arrays and quack like Arrays, but are still not real arrays. At least with v8 / node.js.

> a = []
[]
> b = Object.create(Array.prototype)
{}
> a.constructor
[Function: Array]
> b.constructor
[Function: Array]
> a.__proto__
[]
> b.__proto__
[]
> a instanceof Array
true
> b instanceof Array
true
> Object.prototype.toString.call(a)
'[object Array]'
> Object.prototype.toString.call(b)
'[object Object]'

Can some Javascript guru explain why it is so, and how to make it so that my newly created array is indistinguishable from a real array?

My goal here is to clone data structures, including Arrays which may have custom properties attached. I could, of course, manually attach properties to a newly-created Array using Object.defineProperty, but is there a way to do so using Object.create?

It looks like Arrays created with Object.create walk like Arrays and quack like Arrays, but are still not real arrays. At least with v8 / node.js.

> a = []
[]
> b = Object.create(Array.prototype)
{}
> a.constructor
[Function: Array]
> b.constructor
[Function: Array]
> a.__proto__
[]
> b.__proto__
[]
> a instanceof Array
true
> b instanceof Array
true
> Object.prototype.toString.call(a)
'[object Array]'
> Object.prototype.toString.call(b)
'[object Object]'

Can some Javascript guru explain why it is so, and how to make it so that my newly created array is indistinguishable from a real array?

My goal here is to clone data structures, including Arrays which may have custom properties attached. I could, of course, manually attach properties to a newly-created Array using Object.defineProperty, but is there a way to do so using Object.create?

• javascript
• v8
• object-create

Share Improve this question Follow edited Dec 9, 2015 at 13:12 Marco Bonelli 69.5k2121 gold badges126126 silver badges145145 bronze badges asked Jan 26, 2012 at 9:29 Philippe PlantierPhilippe Plantier 8,07333 gold badges2828 silver badges4040 bronze badges 2

• They're rather indistinguishable for your goal. How to make fully indistingushable -- use b = []; – kirilloid Commented Jan 26, 2012 at 9:55
• @kirilloid - that might be a valid approach for some situations but in situations where say a factory function internally does the plumbing based on a prototypeParam, your suggestion is not an option. eg. in the creation of a monoid, start with: var monad = Object.create(prototypeParam); Here, it would be convenient if prototypeParam could be Array.prototype – arcseldon Commented Oct 12, 2014 at 11:52

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2 Answers 2

Sorted by: Reset to default 5

The short answer is no. This article explains it all in some detail.

No, you cannot. Object.create is all about prototypes, but both [] and Object.create(Array.prototype) inherit from the same prototype object.

What you call "desired Object.prototype.toString behaviour" is the internal [[Class]] of the object, which is impossible to set up with Object.create. Creating "true arrays" (with an Array class and special array behaviour - indexed properties, .length) is only possible by using array literals or calling the Array constructor.

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