How to set value for all the objects in array for javascript?

Question

How to set value for all the items inside an array?

For example, I have the array: ["chin","eng","maths"]

I want to set it to {"chin" :true,"eng":true,"maths":true}

And push to firebase as a child.

You mean changing ["chin","eng","maths"] to {"chin" :true,"eng":true,"maths":true} as a structure? Or as a string? The simple fact of changing [] to {} means a different structure. ([] = Array, {} = Object) — qelli
– qelli, Commented May 29, 2019 at 19:02

Shidersz · Accepted Answer · 2019-05-29 18:57:33Z

4

One possible approach is to use Array.reduce() like this:

const input = ["chin", "eng", "maths"];

let obj = input.reduce((acc, item) => (acc[item] = true, acc), {});

console.log(obj);

.as-console {background-color:black !important; color:lime;}
.as-console-wrapper {max-height:100% !important; top:0;}

Or, you can go with spreading, but with a little overhead on performance:

let obj = input.reduce((acc, item) => ({...acc, [item]: true}), {});

answered May 29, 2019 at 18:46

Shidersz

17.2k2 gold badges28 silver badges52 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Does that create a new object each iteration? Seems expensive.

Probably just as expensive as doing what youd already be doing without using reduce

"In case performance" - yes when a solution is shorter, performs better and is just as readable, I guess that should be the primary answer.

@James yeah, you right, I have added a better version without spreading.

adiga · Accepted Answer · 2019-05-29 18:59:42Z

1

The simplest way is to loop through the array using for...of and add each key to an object:

const keys = ["chin", "eng", "maths"],
      output = {};

for (const key of keys) {
  output[key] = true;
}

console.log(output)

Another option is to create a 2D array of key-value pair entries using map. Then use Object.fromEntries() to create the object

const keys = ["chin","eng","maths"]
const output = Object.fromEntries(keys.map(k => [k, true]))

console.log(output)

answered May 29, 2019 at 18:59

adiga

35.4k9 gold badges66 silver badges88 bronze badges