indexBy

Creates an object from an array, using a function to map each value to a key.

• Use Array.prototype.reduce() to create an object from arr.
• Apply fn to each value of arr to produce a key and add the key-value pair to the object.

const indexBy = (arr, fn) =>
  arr.reduce((obj, v, i) => {
    obj[fn(v, i, arr)] = v;
    return obj;
  }, {});

indexBy([
  { id: 10, name: 'apple' },
  { id: 20, name: 'orange' }
], x => x.id);
// { '10': { id: 10, name: 'apple' }, '20': { id: 20, name: 'orange' } }