Javascript prototype and modify original object

Question

How can we update passed object in prototype? I have created similar prototype as Array.reverse, but how I can modify original object?

Array.prototype.myReverse = function() {
  let arr = [];
  for (let i = 0; i < this.length; i++) {
    arr.unshift(this[i]);
  }
  return arr;
}

let a = [9, 0, 3, 4];
console.log("Before ", a); // [9, 0, 3, 4]
console.log("reverse - ", a.myReverse()); // [4, 3, 0, 9]
//not modifying original object , how to modify original object
console.log("After ", a); // [9, 0, 3, 4]

I have check few examples but I didnt get how to update orginal object inside prototype How can we crate a prototype which will update original object (Careful: reverse is destructive -- it changes the original array.) If not possible for predefined Array then how we can create similar MyArray to write a prototype for updating original object.

@Pointy , Thaks. I am getting error while reassigning this object with equal operator.. I tried removing and addeing all element. I will update in answer — Amit
– Amit, Commented Jan 6, 2020 at 14:04
I said modify the properties of this. You cannot assign a value to this. — Pointy
– Pointy, Commented Jan 6, 2020 at 14:05

Sheraff · Accepted Answer · 2020-01-06 14:03:52Z

2

You cannot assign this directly, but you can still change its properties. So keeping in the style of the code you posted, you could do something along the lines of:

Array.prototype.myReverse = function() {
  let arr = [...this]
  for (let i = 0; i < this.length; i++) {
    this[i] = arr.pop()
  }
}

answered Jan 6, 2020 at 14:03

Sheraff

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Mr. Polywhirl · Accepted Answer · 2020-01-06 14:43:01Z

If you want to reverse the array in-place (and return it if you would like), you can create a temporary stack by popping the head of the array until it is empty and then pushing the temporary elements as if they were in a queue.

To temp:
- ARR→POP ⇒ TMP→PUSH (LILO)
- ARR→SHIFT ⇒ TMP→UNSHIFT (FIFO)
From temp:
- TMP→POP ⇒ ARR→UNSHIFT (LOFI)
- TMP→SHIFT ⇒ ARR→PUSH (FOLI)

Where ARR is the self-reference array.

if (Array.prototype.reverseItems === undefined) {
  Array.prototype.reverseItems = function() {
    let tmp = []
    while (this.length > 0) tmp.push(this.pop())    // or `tmp.unshift(this.shift()`
    while (tmp.length  > 0) this.unshift(tmp.pop()) // or `this.push(tmp.shift())`
    return this
  }
}

let original = [ 9, 0, 3, 4 ]
original.reverseItems() // in-place
console.log('Reversed:', original.join(','))

Amit · Accepted Answer · 2020-01-06 17:36:45Z

-1

@pointy Thanks for suggestion..

I have modified properties of this and updated original object

Array.prototype.myReverse = function () {
 let arr = this.slice(); // creating a copy of array
  this.splice(0,this.length); // removing all elements from array
  for(let i = 0; i<arr.length;i++){
    this.unshift(arr[i]);
  }
  return this;
}

let a = [9,0,3,4];

console.log("Before ",a);
console.log("reverse - ", a.myReverse());
console.log("After ", a);

Few other links for native imlimatation of exsiting array prototypes

https://medium.com/@ofirrifo/naive-implementation-of-js-array-methods-a56319cad6b8

https://gist.github.com/alexhawkins/28aaf610a3e76d8b8264

Node.js change Number object value inside prototype

edited Jan 6, 2020 at 17:36

answered Jan 6, 2020 at 14:09

Amit

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