CODEWITHSUNDEEP
javascriptarrays
dcheepurapalli
dcheepurapalli

Reputation: 75

How does the Javascript Array Push code work internally

What is the internal code logic for Push and Pop methods in javascript..?? How does Push method stores the values in Array.

Upvotes: 0

Views: 6711

Answers (2)

valerii15298
valerii15298

Reputation: 870

We can try some tests and test behavior:

const arr1 = []
const { push: push1 } = arr

const arr2 = []
const { push: push2 } = arr

console.log(push1 === push2) // true
console.log(push1 === Array.prototype.push) // true

push1(1) // TypeError: Cannot convert undefined or null to object

push1.call(arr1, 1) // arr1: [1], arr2: []

push2.call(arr1, 2) // arr1: [1, 2], arr2: []

push1.bind(arr2)(1) // arr1: [1, 2], arr2: [1]

push.call(arr2, 2)

And we can say that push method uses this under the hood...

Upvotes: 0

Guffa
Guffa

Reputation: 700362

The push and pop methods are intentionally generic, they only rely on the existance of a length property, and that they can add and remove properties.

The push method will read the length property, add a property with that name, and increase the length. Basically:

function push(value) {
  var len = this.length;
  this[len] = value;
  len++;
  this.length = len;
  return len;
}

The pop method will read the length property, decrease it, get the property with that name and remove the property. Basically:

function pop() {
  var len = this.length - 1;
  var value = this[len];
  this.length = len;
  delete this[len];
  return value;
}

The actual implementations are a bit more complex, as they support for example multiple parameters for the push method, and some more error checks. There might also implement special optimised code for when the object is actually an array, but then the generic code is still there for other objects.

The methods are intentionally generic so that they can be used on objects that aren't actually arrays. You can make your own object that supports them by just having a length property:

var o = {
  length: 0,
  push: Array.prototype.push,
  pop: Array.prototype.pop
};

o.push(1);
var one = o.pop();

Demo: http://jsfiddle.net/Guffa/9r4gavzb/

Upvotes: 7

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