transform array of object to object of array?

Question

I want to change this

const a = [{
 id: 1,
 name: 'a'
 }, {
 id: 2,
 name: 'b'
 }]

to

{
 id: [1,2],
 name: [a, b]
 }

I'm stuck at how to push the id and name into the object

 arr.reduce((accum, val) => {
 let accum = {
 id: val.id,
 name: val.name
 }
 accum.id.push(val.id) //it doesn't work like this
 return accum
 }, {})

Answer 1

You were just missing the proper initial accum value, which would contain the arrays you want.

const a = [{
 id: 1,
 name: 'a'
}, {
 id: 2,
 name: 'b'
}]
let arr = a.reduce((accum, val) => {
 accum.id.push(val.id);
 accum.name.push(val.name);
 return accum
}, {
 id: [],
 name: []
})
console.log(arr)

Answer 2

const a = [{
 id: 1,
 name: 'a'
 }, {
 id: 2,
 name: 'b'
 }]
 const newObj = {};
 
a.forEach(ele => {
 const {id, name} = ele;
 newObj.id.push(id);
 newObj.name.push(name);
})
console.log(newObj);

• Kinglish: You haven't initialized the arrays you're trying to push on, so it's erroring out.

Answer 3

If you want to build a list of values for each key, you could reduce every item; and for each item, reduce their key-value pairs.

In the example below, the keyValues function is decoupled from the sample data.

const keyValues = (...items) =>
 items.reduce((accOut, item) =>
 Object.entries(item).reduce((accIn, [key, value]) =>
 ({ ...accIn, [key]: [...(accIn[key] || []), value] }), accOut), {})
const data = [
 { id: 1, name: 'a' },
 { id: 2, name: 'b' }
];
console.log(keyValues(...data));

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Alternatively, you could zip the data and then bin it.

const zip = (...arr) => arr[0].map((_, c) => arr.map(row => row[c]))
const bin = (...arr) =>
 zip(...arr.map(Object.entries)).reduce((acc, ...values) =>
 ({ ...acc, [values[0][0][0]]: values[0].map(([key, value]) => value) }), {})
const data = [
 { id: 1, name: 'a' },
 { id: 2, name: 'b' }
];
console.log(bin(...data));

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