Efficiently merging fields from one array into two other arrays

Question

Let's say you've got three arrays of objects:

let a1 = [
  { id: 1, name: 'foo' },
  { id: 2, name: 'bar' },
  { id: 3, name: 'baz' }
]

let a2 = [
  { name: 'foo' },
  { name: 'bar' }
]

let a3 = [
  { name: 'bar' },
  { name: 'baz' }
]

The goal is to use a1 as a source, and add an id field to the elements of a2 and a3 with corresponding name fields in a1. What is an efficient way of accomplishing this? (Note: 'efficient' here meaning 'something more elegant than loops-within-loops-within-loops'.)

The result should look like this:

a2: [
  { id: 1, name: 'foo' },
  { id: 2, name: 'bar' }
]

a3: [
  { id: 2, name: 'bar' },
  { id: 3, name: 'baz' }
]

Answer 1

You could use a Map for referencing the id of a given name. Then assign.

var a1 = [{ id: 1, name: 'foo' }, { id: 2, name: 'bar' }, { id: 3, name: 'baz' }], 
    a2 = [{ name: 'foo' }, { name: 'bar' }],
    a3 = [{ name: 'bar' }, { name: 'baz' }],
    map = new Map(a1.map(o => [o.name, o.id]));
    
[a2, a3].forEach(a => a.forEach(o => o.id = map.get(o.name)));

console.log(a2);
console.log(a3);

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Answer 2

For an alternative answer, it could be like this.

It doesn't include loops and may be the shortest code in the answers.

const a1 = [{ id: 1, name: 'foo' }, { id: 2, name: 'bar' }, { id: 3, name: 'baz' }];
const a2 = [{ name: 'foo' }, { name: 'bar' }];
const a3 = [{ name: 'bar' }, { name: 'baz' }];

let f = x => a1.filter(a => x.some(y => y.name === a.name));

console.log(f(a2));
console.log(f(a3));

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