I've been loving this series because it gives me the opportunity to really dive into some of these new features.

Today, we're looking at Object.fromEntries!

Let's start with Object.entries

The first thing to know is that in a previous version of ECMAScript, we were introduced to Object.entries. This was a nifty function that allowed us to iterate through the keys and values in an object by turning it into an array.

At its most basic it transformed an object like this.

const obj = {a: 1, b: 2, c: 3}
const entries = Object.entries(obj)
// [['a', 1], ['b', 2], ['c', 3]]

But a more common use case was to iterate through that result.

const obj = {a: 1, b: 2, c: 3}
const entries = Object.entries(obj)

for(const [key, value] of entries) {
    // do something with key and value here
}

However, when you used Object.entries you'd be stuck with your data in an array. Then along comes Object.fromEntries.

Enter Object.fromEntries

As it turns out, Object.fromEntries is just the inverse of Object.entries. Take this example.

const obj = {a: 1, b: 2, c: 3}
const entries = Object.entries(obj)
const newObj = Object.fromEntries(entries)
// {a: 1, b: 2, c: 3}

This example doesn't do anything other than change the data structure back and forth. But with so many helper functions available for arrays, it's easy to see the benefits of being able to do this.

Why we want this

We have so many wonderful functions that allow us to transform arrays. Things like map, reduce, filter, flat and flatMap. Object.entries gave us the ability to use them for objects too. If we transformed our Object into an Array they were available for use.

const obj = {a: 1, b: 2, c: 3}
const result = Object.entries(obj).map(([key, value]) => [key, value * 2]) 
// [['a', 2], ['b', 4], ['c', 6]]

But without Object.fromEntries we're stuck with our transformation in an Array structure. With its addition, we can do this instead!

const obj = {a: 1, b: 2, c: 3}
const result = Object.fromEntries(
         Object.entries(obj).map(
            ([key, value]) => [key, value * 2]
         ))
// {a: 2, b: 4, c: 6}

Not just objects

One of the great things about this function is that it works on all iterables. That means you can turn an Array into an Object, but you can also turn a Map into an Object.

const map = new Map([ ['a', 1], ['b', 2], ['c', 3] ]);
const obj = Object.fromEntries(map);
// {a: 1, b: 2, c: 3}

One thing to watch out for

There is a difference between an Object and an Array in that the latter does not require unique keys. This means Object.fromEntries can cause you to drop information.

const arr = [['a', 1], ['a', 2], ['c', 3]]
const entries = Object.fromEntries(arr)
// {a: 2, c: 3}

In this example, we've lost the value 1.

And that's that

And there we have it! Being able to use all of the array manipulation functions for objects is incredibly useful! And having Object.fromEntries closes the loop that Object.entries created.

Hope you've enjoyed our ES2019 fun!