Invert object

Inverts the key-value pairs of an object, without mutating it.

• Use Object.keys() and Array.prototype.reduce() to invert the key-value pairs of an object and apply the function provided (if any).
• Omit the second argument, fn, to get the inverted keys without applying a function to them.
• The corresponding inverted value of each inverted key is an array of keys responsible for generating the inverted value. If a function is supplied, it is applied to each inverted key.

const invertKeyValues = (obj, fn) =>
  Object.keys(obj).reduce((acc, key) => {
    const val = fn ? fn(obj[key]) : obj[key];
    acc[val] = acc[val] || [];
    acc[val].push(key);
    return acc;
  }, {});

invertKeyValues({ a: 1, b: 2, c: 1 }); // { 1: [ 'a', 'c' ], 2: [ 'b' ] }
invertKeyValues({ a: 1, b: 2, c: 1 }, value => 'group' + value);
// { group1: [ 'a', 'c' ], group2: [ 'b' ] }