Array to object reduce

Question

Hello I want to transform array to object

input = [1,2,3]

expected output = {1:1,2:2,3:3}

I tried this

const arrToObj = (arr) => {
  arr.reduce((acc, curr, i, arr)=> {
    acc[curr]=arr[i]
  },{})
}
console.log(arrToObj([1,2,3,4,5,6,7,8,9]))

but it throws errror in second iteration. What is wrong and how to make it work?

Possible duplicate of [Convert Array to Object](https://stackoverflow.com/questions/4215737/convert-array-to-object) — Heretic Monkey, Mar 24 '18 at 23:24

Fecosos · Answer 1 · 2018-03-24T20:59:20.867

0

something like this?

const arr = [1,2,3,4,5,6,7,8,9]
const obj = arr.reduce((accumulator, currentValue, index) => {
  accumulator[index+1] = currentValue;
  return accumulator;
}, {});

console.log(obj)

edited Mar 24 '18 at 20:59

answered Mar 24 '18 at 20:49

Fecosos

944
7
17

Yup, it works but I want to achieve that using reduce – Luke Mar 24 '18 at 20:53
Thank you. I noticed that i didn't return acc in my code and that was a reason of error – Luke Mar 24 '18 at 21:08

Aluan Haddad · Accepted Answer · 2023-05-16T06:00:44.160

What you have tried so far has two problems

The first is serious:

You forgot to return from your accumulating function, so it returns undefined.

It throws the second time around because Array.prototype.reduce passes the specified seed as the first argument on the first call to the accumulating function but, on the second call, it passes the result of the first call.

So initially, reduce calls your function like this

result = accumulatorFunction(seed, arr[0], 0, arr);

which expands to

result = accumulatorFunction({}, 1, 0, [1, 2, 3]);

The subsequent call, however, looks like this

result = accumulatorFunction(result, arr[1], 1 arr);

which expands to

result = accumulatorFunction(undefined, 2, 1, [1, 2, 3]);

So you need to write something like

arr.reduce((acc, curr, i, arr) => {
  acc[curr] = arr[i];
  return acc;
}, {});

Another issue with your code, and more one of style, is that you needlessly provide and use the fourth argument passed to the accumulating function. This argument is just the array itself anyway. Using it is awkward and makes your code more complex, more prone to mistakes, and harder to understand.

Instead write

arr.reduce((acc, curr) => {
  acc[curr] = curr;
  return acc;
}, {});

"use strict";

const input = [1, 2, 3];

const output = input.reduce((acc, curr) => {
  acc[curr] = curr;
  return acc;
}, {});

console.log(output);

score 0 · Answer 3 · answered Mar 24 '18 at 21:25

0

const arrToObj = a => a.reduce((r, v) => (r[v] = v, r), {})

console.log( arrToObj( [1, 2, 3] ) )

If the values match the indexes, it can be even simpler with ECMAScript 2018 Spread syntax :

console.log( { ...[, 1, 2, 3] } )

answered Mar 24 '18 at 21:25

Slai

22,144
5
45
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ECMAScript 2018 syntax looks cool in this example. Thanks for answer! – Luke Mar 25 '18 at 20:45
This is an elegant approach but you linked to the wrong documentation. Array spread is not the same as object spread – Aluan Haddad Mar 26 '18 at 03:11
@AluanHaddad it's generic for the spread syntax, so it covers function call, array, and object. – Slai Mar 26 '18 at 03:23

Array to object reduce

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