How to replace array elements in place

Question

I want to attach a new method to the Array.prototype:

Array.prototype.uniq = function(){
  return this.filter((val, index) => {
    return this.indexOf(val) === index;
  });
};

var a = [1, 1, 2, 3];
console.log(a.uniq()); // output： [1,2,3]
console.log(a); // output: [1,1,2,3]

The method removes duplicates from an array. The problem I have is that whenever uniq is called, a new array is returned. I want to do something like this:

Array.prototype.uniq = function(){
  this = this.filter((val, index) => {  // "ReferenceError: Invalid left-hand side in assignment
    return this.indexOf(val) === index;
  });
};

so that:

var a = [1, 1, 2, 3];
a.uniq();
console.log(a); // output: [1,2,3]

What should I do?

@Schleis sure that works, but I am just curious how to do it inside prototype — Cheng
– Cheng, Commented May 19, 2016 at 15:37
How to replace elements in array with elements of another array — GingerPlusPlus
– GingerPlusPlus, Commented May 19, 2016 at 15:41

Tushar · Accepted Answer · 2016-05-21 12:18:46Z

4

You can iterate over the array using for loop and use splice if the index are not same.

Array.prototype.uniq = function () {
    // Reverse iterate
    for (var i = this.length - 1; i >= 0; i--) {

        // If duplicate
        if (this.indexOf(this[i]) !== i) {
            // Remove from array
            this.splice(i, 1);
        }
    }

    // Return updated array
    return this;
};

var a = [1, 1, 2, 3];
a.uniq();
console.log(a); // output: [1,2,3]

edited May 21, 2016 at 12:18

answered May 19, 2016 at 15:38

Tushar

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2 Comments

Cheng Over a year ago

Thx! I thought about splice but the index changes as I remove elements from the array. Reverse iterate is very smart :) The return this line is not needed right?

Cheng Over a year ago

Tushar, shouldn't the indexes inside the for loop be i-1 because i = this.length makes this[i] out of the loop.

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