js map array to dictionary

Converting a JavaScript Array to a Dictionary using the Map method

Have you ever needed to convert a JavaScript array into a dictionary? Well, you've come to the right place! In this post, we'll discuss how to use the map method to convert an array to a dictionary in JavaScript.

The Problem

Imagine you have an array of objects that represent users:

const users = [
  {id: 1, name: 'John'},
  {id: 2, name: 'Jane'},
  {id: 3, name: 'James'}
]

But now you need to access data from this array based on an id value. It would be much more efficient to convert this array into a dictionary where the keys are the ids and the values are the corresponding objects:

const usersDict = {
  1: {id: 1, name: 'John'},
  2: {id: 2, name: 'Jane'},
  3: {id: 3, name: 'James'}
}

The Solution

The map method can be used to convert an array into a dictionary. Here's how:

const usersDict = users.map(user => ({[user.id]: user})).reduce((acc, obj) => Object.assign(acc, obj), {})

Let's break down what's happening here. First, we're using the map method to transform each object in the users array into a new object that has the id as the key and the entire object as the value:

users.map(user => ({[user.id]: user}))

This will return an array of objects that look like this:

[
  {1: {id: 1, name: 'John'}},
  {2: {id: 2, name: 'Jane'}},
  {3: {id: 3, name: 'James'}}
]

Next, we're using the reduce method to combine all of these objects into one dictionary:

.reduce((acc, obj) => Object.assign(acc, obj), {})

This will merge each object into the accumulator object, which is initially an empty object. The end result is a single object that represents the dictionary we want:

{
  1: {id: 1, name: 'John'},
  2: {id: 2, name: 'Jane'},
  3: {id: 3, name: 'James'}
}

Alternative Solution

Another way to accomplish the same thing is to use the reduce method twice:

const usersDict = users.reduce((acc, user) => ({...acc, [user.id]: user}), {})

This accomplishes the same thing as the previous solution, but uses the spread operator to merge each object into the accumulator rather than Object.assign.

Conclusion

So there you have it! Two different ways to convert a JavaScript array into a dictionary using the map method. Hopefully this helps you next time you encounter this problem!