Map over object preserving keys

Question

The map function in underscore.js, if called with a javascript object, returns an array of values mapped from the object\'s values.

_.map({one:          


        

Answer 1

With Underscore

Underscore provides a function _.mapObject to map the values and preserve the keys.

_.mapObject({ one: 1, two: 2, three: 3 }, function (v) { return v * 3; });

// => { one: 3, two: 6, three: 9 }

DEMO

---

With Lodash

Lodash provides a function _.mapValues to map the values and preserve the keys.

_.mapValues({ one: 1, two: 2, three: 3 }, function (v) { return v * 3; });

// => { one: 3, two: 6, three: 9 }

DEMO

Answer 2

var mapped = _.reduce({ one: 1, two: 2, three: 3 }, function(obj, val, key) {
    obj[key] = val*3;
    return obj;
}, {});

console.log(mapped);

<script src="http://underscorejs.org/underscore-min.js"></script>
<script src="https://getfirebug.com/firebug-lite-debug.js"></script>

Answer 3

const mapObject = (obj = {}, mapper) =>
  Object.entries(obj).reduce(
    (acc, [key, val]) => ({ ...acc, [key]: mapper(val) }),
    {},
  );

Answer 4

You can use _.mapValues(users, function(o) { return o.age; }); in Lodash and _.mapObject({ one: 1, two: 2, three: 3 }, function (v) { return v * 3; }); in Underscore.

Check out the cross-documentation here: http://jonathanpchen.com/underdash-api/#mapvalues-object-iteratee-identity

Answer 5

I managed to find the required function in lodash, a utility library similar to underscore.

http://lodash.com/docs#mapValues

_.mapValues(object, [callback=identity], [thisArg])

Creates an object with the same keys as object and values generated by running each own enumerable property of object through the callback. The callback is bound to thisArg and invoked with three arguments; (value, key, object).

Answer 6

How about this version in plain JS (ES6 / ES2015)?

let newObj = Object.assign(...Object.keys(obj).map(k => ({[k]: obj[k] * 3})));

jsbin

If you want to map over an object recursively (map nested obj), it can be done like this:

const mapObjRecursive = (obj) => {
  Object.keys(obj).forEach(key => {
    if (typeof obj[key] === 'object') obj[key] = mapObjRecursive(obj[key]);
    else obj[key] = obj[key] * 3;
  });
  return obj;
};

jsbin

Since ES7 / ES2016 you can use Object.entries instead of Object.keys like this:

let newObj = Object.assign(...Object.entries(obj).map([k, v] => ({[k]: v * 3})));