accessing arguments in callback array.protoype

Question

I am implementing the map function. To access the array I am mapping over I am using this based on a question I had earlier. Now I am wondering how to access different arguments passed to the callback. So in the official map method you can pass the index. I am trying to do this, but unsure how to access this in my custom method.

Array.prototype.mapz = function(callback) {
  const arr = [];
  for (let i = 0; i < this.length; i++) {
    arr.push(callback(this[i]))
  }
  return arr;
};

let x = [1, 12, 3].mapz((item, index) => {
  return item * 2;
})
console.log(x);

When you want to get the index in the parameter, you must pass it as an argument! — Bergi
– Bergi, Commented Feb 1, 2019 at 12:47

adiga · Accepted Answer · 2019-02-01 14:11:35Z

3

You need to hand over the index as second parameter for the callback

Array.prototype.mapz = function(callback) {
  const arr = [];
  for (let i = 0; i < this.length; i++) {
    arr.push(callback(this[i], i));
  }
  return arr;
};

let x = [1, 12, 3].mapz((item, index) => {
  console.log(index, item);
  return item * 2;
})
console.log(x);

edited Feb 1, 2019 at 14:11

adiga

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answered Feb 1, 2019 at 12:45

Nina Scholz

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1 Comment

peter flanagan Over a year ago

thanks again, all makes sense. Just need it spelled out to me at the start :-D

Melchia · Accepted Answer · 2019-02-01 12:45:00Z

1

You just need to add another argument to your callback function which is your index.

Array.prototype.mapz = function(callback) {
  const arr = [];
  for (let i = 0; i < this.length; i++) {
    arr.push(callback(this[i], i))
  }
  return arr;
};

let x = [1, 12, 3].mapz((item, index) => {
console.log(index);
  return item * 2;
})
console.log(x);

answered Feb 1, 2019 at 12:45

Melchia

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