Spread syntax in object key set not working

Question

I have an array of strings, and I want to turn it into an object where each key is a word. I'm doing this:

let arr = ['a', 'b', 'c'];
let obj = {};
obj[...arr] = true;

However, it seems like this doesn't work :(

Does anyone know what I'm doing wrong?

Answer 1

You can use Array.prototype.reduce() in a slightly more succinct way:

const fill = v => (o, k) => ({ ...o, [k]: v })
let arr = ['a', 'b', 'c']
let obj = arr.reduce(fill(true), {})

console.log(obj)

If you really don't like copying the accumulator o each iteration, you can use Object.assign() instead:

const fill = v => (o, k) => Object.assign(o, { [k]: v })
let arr = ['a', 'b', 'c']
let obj = arr.reduce(fill(true), {})

console.log(obj)

Or even just

const fill = v => k => ({ [k]: v })
let arr = ['a', 'b', 'c']
let obj = Object.assign(...arr.map(fill(true)))

console.log(obj)

Answer 2

Using forEach is shorter.

let arr = ['a', 'b', 'c'];

var obj = {}

arr.forEach(e=>obj[e]=true)

console.log(obj)

But you could use Array.prototype.reduce() that is probably faster.

let arr = ['a', 'b', 'c'];

var obj = arr.reduce((acc,c,i)=>{
    acc[c] = true; return acc;
},{})

console.log(obj)

To do the opposite, you could use spread syntax:

var obj = {
    "a": true,
    "b": true,
    "c": true
  }
var arr = [...Object.keys(obj)];

console.log(arr)

Spread syntax needs an iterable such as an array, but could be an object, see this about iteration protocols. In the case above, we are using Object.keys(obj) that returns an array.

Answer 3

You can use reduce function to fill the obj with true values

const arr = ['a', 'b', 'c'];
const obj = arr.reduce((acc, val) => {
   acc[val] = true;
   return acc;
 }, {});
    
console.log(obj);