Change array index with array.map()

Question

Is it somehow possible to iterate an array in JS using Array.map() and modify the index values of the resulting array?

// I start with this values:
var arrSource = [
  {id:7, name:"item 1"}, 
  {id:10, name:"item 2"},
  {id:19, name:"item 3"},
];

var arrResult = arrSource.map(function(obj, index) {
    // What to do here?
    return ???; 
});

/*
This is what I want to get as result of the .map() call:

arrResult == [
  7: {id:7, name:"item 1"}, 
  10: {id:10, name:"item 2"},
  19: {id:19, name:"item 3"},
];
*/

Felix Kling · Accepted Answer · 2016-08-16 19:19:29Z

3

No. Array#map performs a 1:1 mapping (so to speak).

You'd have to create a new array and explicitly assign the elements to the specific indexes:

var arrResult = [];
arrSource.forEach(function(value) {
   arrResult[value.id] = value;
});

Of course you can use .reduce for that too.

answered Aug 16, 2016 at 19:19

Felix Kling

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3 Comments

VLAZ Over a year ago

Yes, alternatively map then sort.

Felix Kling Over a year ago

@Vld: Not sure how either of these would help here.

VLAZ Over a year ago

Actually, you're right. I misread the question thinking OP wants to map then re-order.

Redu · Accepted Answer · 2016-08-16 19:59:38Z

0

Of course you can do it as follows;

var arrSource = [
  {id:7, name:"item 1"}, 
  {id:10, name:"item 2"},
  {id:19, name:"item 3"},
];

newArr = arrSource.map((e,i,a) => a[a.length-1-i]);
console.log(newArr)

Yes... always the source array gets mutated if you need some irregularities with Array.prototype.map() such as

var arrSource = [
  {id:7, name:"item 1"}, 
  {id:10, name:"item 2"},
  {id:19, name:"item 3"},
];

newArr = arrSource.map((e,i,a) => a[a.length] = e);
console.log(JSON.stringify(arrSource,null,4));

which is not surprising.

edited Aug 16, 2016 at 19:59

answered Aug 16, 2016 at 19:29

Redu

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8 Comments

Felix Kling Over a year ago

That just seems to reverse the order of elements.

Redu Over a year ago

@Felix Kling Yes.. or whatever order you like depending on your logic.

Felix Kling Over a year ago

But that's not what the OP wants to do. The OP wants to assign arr[0] to arr[7], arr[1] to arr[10], etc.

Redu Over a year ago

@Felix Kling and in this particular snippet we are assigning arr[2] to arr[0]

Felix Kling Over a year ago

Sure. The point is that you cannot assign to arbitrary indexes via .map. You can only assign to indexes that already have a value (i.e. exist). But even then, you cannot really control to which index something is assigned to. You have to think the other way round: What do I want to assign to index i? And since in your example index 7 will never be processed it's not possible to achieve what the OP wants with .map.

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