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'] }

Answer 3

You can use something like this to get raid of duplicates first :

function removeDuplicates(arr, key) {
   if (!(arr instanceof Array) || key && typeof key !== 'string') {
    return false;
   }

  if (key && typeof key === 'string') {
    return arr.filter((obj, index, arr) => {
        return arr.map(mapObj => mapObj[key]).indexOf(obj[key]) === index;
    });

  } else {
    return arr.filter(function(item, index, arr) {
        return arr.indexOf(item) == index;
    });
  }
} 

and then use this to make it reverse :

function reverseMapping(obj){
  var ret = {};
  for(var key in obj){
  ret[obj[key]] = key;
  }
    return ret;
  }

Answer 4

You could use reduce to save the declaration line reduce .

Abusing && to check if the map[object[key]] is defined first before using Array.concat .

It's shorter, but is it simpler? Probably not, but a bit of fun ;)

const reverseMapping = (object) => 
    Object.keys(object).reduce((map, key) => {
        map[object[key]] = map[object[key]] && map[object[key]].concat(key) || [key]
     return map;
    }, {});

Answer 5

You could try getting an array of values and an array of keys from the current object, and setup a new object to hold the result. Then, as you loop through the array of values -

• if the object already has this value as the key, like Netherlands , you create a new array, fetch the already existing value (ex: Rotterdam ), and add this and the new value ( Amsterdam ) to the array, and set up this array as the new value for the Netherlands key.
• if the current value doesn't exist in the object, set it up as a new string, ex: France is the key and Paris is the value.

Code -

 const city2country = { 'Amsterdam': 'Netherlands', 'Rotterdam': 'Netherlands', 'Paris': 'France', }; function reverseMapping(obj) { let values = Object.values(obj); let keys = Object.keys(obj); let result = {} values.forEach((value, index) => { if(!result.hasOwnProperty(value)) { // create new entry result[value] = keys[index]; } else { // duplicate property, create array let temp = []; // get first value temp.push(result[value]); // add second value temp.push(keys[index]); // set value result[value] = temp; } }); console.log(result); return result; } reverseMapping(city2country)

The benefit here is - it adjusts to the structure of your current object - Netherlands being the repeated values, gets an array as it's value in the new object, while France gets a string value Paris as it's property. Of course, it should be very easy to change this.

Note - Object.values() might not be supported across older browsers.

Answer 6

@Nina Scholz answer works well for this exact question. :thumbsup:

But if you don't need to keep both values for the Netherlands key ( "Netherlands": ["Amsterdam", "Rotterdam"] ), then this is a little bit shorter and simpler to read:

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

console.log(
  Object.entries(city2country).reduce((obj, item) => (obj[item[1]] = item[0]) && obj, {})
);
// outputs `{Netherlands: "Rotterdam", France: "Paris"}`