Adding fields to js objects, in functional way

Question

I have an array of objects with a set of fields in node.js. Lets say

let arr = [ {a:1,b:2}, {a:3,b:6}]

I would like to extend all objects and assign to a new variable. My naïve approach is like the following way

let arr2 = arr.map(x => x.c = x.a + x.b )

However, the arrow function returns the x.a+x.b value, instead of the x object, so this won't work as I hope. It also alters the arr in place which is very nonfunctional and confusing.

I'm relatively new to js so maybe I'm thinking wrong from the beginning. What is the idiomatic js expression for this kind of object extension?

Ry- · Accepted Answer · 2017-04-02 07:40:32Z

3

You can write all properties from one object to another with Object.assign:

const arr2 = arr.map(x => Object.assign({c: x.a + x.b}, x));

And maybe at some point in the future with object spread (not yet standardized):

const arr2 = arr.map(x => ({...x, c: x.a + x.b}));

answered Apr 2, 2017 at 7:40

Ry-♦

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1 Comment

LudvigH Over a year ago

Nice! Exactly what I hoped for! :D

Dhiraj · Accepted Answer · 2017-04-02 08:10:25Z

1

you can use this

let arr2 = arr.map(x => {
      x.c = x.a + x.b;
      return x;
});

Please also refer working snippet

let arr = [ {a:1,b:2}, {a:3,b:6}];
let arr2 = arr.map(x => {
  return Object.assign({c : x.a + x.b},x);
});


console.log(arr);
console.log(arr2);
console.log(arr);

edited Apr 2, 2017 at 8:10

answered Apr 2, 2017 at 7:43

Dhiraj

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1 Comment

LudvigH Over a year ago

Sorry. This will overwrite arr, :(

user663031 · Accepted Answer · 2017-04-02 08:33:20Z

1

Although the same general idea as other answers, this might be slightly more readable:

arr.map(({a, b}) => ({a, b, c: a + b}))

answered Apr 2, 2017 at 8:33

user663031

1 Comment

Mulan Over a year ago

mhm i like this one ^_^

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