how to return object with some of its values modified lodash

Question

const fruits = {
    apple: 2,
    orange: 3,
    grape: 4,
    banana: 5
}

I would like to modify some of the values of fruits where I also have access to its current value.

Something like this

const premiumFrutis = _.doSomething(fruits, apple + 2, banana + 3)

// premiumFrutis = {
//     apple: 4,
//     orange: 3,
//     grape: 4,
//     banana: 8
// }

Answer 1

This is the job of _.assignWith or _.assignInWith. The difference is that assignIn will go through the prototype chain but when dealing with plain objects, then both will work the same.

In both cases, a customiser option can be provided which will be used when the values of the objects are combined. A simple implementation is

const combineValues = (srcValue, objValue) => 
      srcValue + objValue;

which will sum values for the same key:

const fruits = {
    apple: 2,
    orange: 3,
    grape: 4,
    banana: 5
}

const combineValues = (srcValue, objValue) => 
  srcValue + objValue;

const modification = {apple: 2, banana: 3};

const premiumFrutis = _.assignWith(fruits, modification, combineValues);
console.log(premiumFrutis);
// {
//     apple: 4,
//     orange: 3,
//     grape: 4,
//     banana: 8
// }

<script src="https://cdn.jsdelivr.net/npm/[email protected]/lodash.min.js"></script>

However, it is not even needed to implement this, since Lodash already provides _.add and it can be directly used:

const fruits = {
    apple: 2,
    orange: 3,
    grape: 4,
    banana: 5
}

const modification = {apple: 2, banana: 3};

const premiumFrutis = _.assignWith(fruits, modification, _.add);
console.log(premiumFrutis);
// {
//     apple: 4,
//     orange: 3,
//     grape: 4,
//     banana: 8
// }

<script src="https://cdn.jsdelivr.net/npm/[email protected]/lodash.min.js"></script>

This will also do subtraction if you pass negative values:

const fruits = {
    apple: 2,
    orange: 3,
    grape: 4,
    banana: 5
}

const modification = {apple: 2, banana: -1};

const premiumFrutis = _.assignWith(fruits, modification, _.add);
console.log(premiumFrutis);
// {
//     apple: 4,
//     orange: 3,
//     grape: 4,
//     banana: 4
// }

<script src="https://cdn.jsdelivr.net/npm/[email protected]/lodash.min.js"></script>

---

The only potential problem is more modifications need to be passed than there are keys in the original objects:

const fruits = {
    apple: 2,
    orange: 3,
    grape: 4,
    banana: 5
}

const modification = {apple: 2, banana: 3, lemon: 42};

const premiumFrutis = _.assignWith(fruits, modification, _.add);
console.log(premiumFrutis);
// {
//     apple: 4,
//     orange: 3,
//     grape: 4,
//     banana: 8,
//     lemon: 42
// }

<script src="https://cdn.jsdelivr.net/npm/[email protected]/lodash.min.js"></script>

If no new items should be added, then _.pickBy can be used to remove any modifications that are not in the initial object:

const fruits = {
    apple: 2,
    orange: 3,
    grape: 4,
    banana: 5
}

const modification = {apple: 2, banana: 3, lemon: 42};
const onlyExisting = _.pickBy(modification, (value, key) => key in fruits);

const premiumFrutis = _.assignWith(fruits, onlyExisting, _.add);
console.log(premiumFrutis);
// {
//     apple: 4,
//     orange: 3,
//     grape: 4,
//     banana: 8
// }

<script src="https://cdn.jsdelivr.net/npm/[email protected]/lodash.min.js"></script>