How to mimic arrays in javascript?

Question

I'd like to understand how Arrays are specified and built.

I'm mixing arrays with extra methods and trying to pass in Array.isArray(). I can pass instanceof and Poly-filled isArray

var protoOfArray = Object.getPrototypeOf([])

var tools4Arrays = Object.defineProperty(
  Object.create(protoOfArray),
  'sum', {
    'value': function sum() {
      return this.reduce((a, b) => a + b, 0)
    }
  }
)
tools4Arrays[Symbol.toStringTag] = 'Array'

var fakeArray = Object.defineProperty(
  Object.create(
    tools4Arrays
  ),
  'length', {
    'value': 0,
    'writable': true
  }
)

console.log(`fakeArray instanceof Array: ${fakeArray instanceof Array}`)
console.log(`polyfill.isArray(fakeArray): ${Object.prototype.toString.call(fakeArray) === '[object Array]'}`)
console.log(`Array.isArray(fakeArray): ${Array.isArray(fakeArray)}`)

If I use: fakeArray = Object.setPrototypeOf([], tools4Arrays) the last log is true too.

What else separates arrays of objects? Can I mock it?

Do you mean what else separates arrays from objects? Or are you asking something specifically regarding arrays containing objects? — GenericUser
– GenericUser, Commented Jan 5, 2019 at 15:15
@joe: It's not about an array of objects. Can I build an array editing from an object? Whats transformations need to construct an array over regulars object? My fakeArray don't pass Array.isArray(). How do I get closer? Are this transitions possible? Or are array irreducible from objects? — itacirgabral
– itacirgabral, Commented Jan 5, 2019 at 19:29

GenericUser · Accepted Answer · 2019-01-05 20:10:15Z

1

In order to mimic an Array you'd have to pass the internals of isArray which has the following checks (defined here):

If Type(arg) is not Object, return false.

If the value of the [[Class]] internal property of arg is "Array", then return true.

Return false.

The Type here appears to be the same as instanceof, which you've been able to pass just fine. However, the second step points at some internal [[Class]] property, which I'm not sure we have write access to.

answered Jan 5, 2019 at 20:10

GenericUser

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2 Comments

itacirgabral Over a year ago

its give me some paths to search:change class Symbol.species

itacirgabral Over a year ago

es9 isArray talks about exotics and proxy arrays I did not even know.

itacirgabral · Accepted Answer · 2019-01-07 13:53:35Z

0

I got what I needed, but I still do not have the answer... A fake array is very slower than a regular array, maybe have many internal optimizations and mimic it will lose that speed.

subclass an array without a class

my use case

answered Jan 7, 2019 at 13:53

itacirgabral

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