Javascript - map value to keys (reverse object mapping)

Question

I want to reverse the mapping of an object (which might have duplicate values). Example:

const city2country = {
    'Amsterdam': 'Netherlands',
    'Rotterdam': 'Netherlands',
    'Paris': 'France'
};

reverseMapping(city2country) Should output:

{
    'Netherlands': ['Amsterdam', 'Rotterdam'],
    'France': ['Paris']
}

I've come up with the following, naive solution:

const reverseMapping = (obj) => {
    const reversed = {};
    Object.keys(obj).forEach((key) => {
        reversed[obj[key]] = reversed[obj[key]] || [];
        reversed[obj[key]].push(key);
    });
    return reversed;
};

But I'm pretty sure there is a neater, shorter way, preferably prototyped so I could simply do:

const country2cities = city2country.reverse();

Answer 1

You could use Object.assign, while respecting the given array of the inserted values.

const city2country = { Amsterdam: 'Netherlands', Rotterdam: 'Netherlands', Paris: 'France' };
const reverseMapping = o => Object.keys(o).reduce((r, k) =>
        Object.assign(r, { [o[k]]: (r[o[k]] || []).concat(k) }), {})

console.log(reverseMapping(city2country));

Answer 2

There is no such built-in function in JavaScript. Your code looks fine, but given that there are so many edge cases here that could wrong, I'd suggesting using invertBy from lodash, which does exactly what you describe.

Example

var object = { 'a': 1, 'b': 2, 'c': 1 };

_.invertBy(object);
// => { '1': ['a', 'c'], '2': ['b'] }