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Create an object lookup from an array of JavaScript objects

Surprisingly shorthand JavaScript code for converting an array into an object lookup

March 14, 2021 · 3 min read

Many lodash utility functions come in handy when performing data transformations. But thanks to ESNext, I usually only need one or two helpers from lodash, and I would rather avoid adding it as a dependency. Per-method packages do exist, but these are discouraged. So, I usually write the code myself.

Sometimes the code is short, one-liners which is perfect. Other times it’s longer and I feel less great about having to maintain it myself. Take for instance converting an array of objects into an object lookup. For a long time, I created an empty object and built it up using a for loop. Then when I got a handle of .reduce(), I was able to shorten the code quite a bit:

const teams = [
  { id: 'atl', name: 'Atlanta Hawks', wins: 18, losses: 20 },
  { id: 'bos', name: 'Boston Celtics', wins: 19, losses: 18 },
  { id: 'bkn', name: 'Brooklyn Nets', wins: 26, losses: 13 },
  { id: 'cha', name: 'Charlotte Hornets', wins: 19, losses: 18 },
  { id: 'chi', name: 'Chicago Bulls', wins: 16, losses: 20 },
  // ...remaining 25 teams
]

const teamLookup = teams.reduce(
  (runningLookup, team) => ({
    ...runningLookup,
    [team.id]: team,
  }),
  {},
)

The value of teamLookup would be:

{
  "atl": { "id": "atl", "name": "Atlanta Hawks", "wins": 18, "losses": 20 },
  "bos": { "id": "bos", "name": "Boston Celtics", "wins": 19, "losses": 18 },
  "bkn": { "id": "bkn", "name": "Brooklyn Nets", "wins": 26, "losses": 13 },
  "cha": { "id": "cha", "name": "Charlotte Hornets", "wins": 19, "losses": 18 },
  "chi": { "id": "chi", "name": "Chicago Bulls", "wins": 16, "losses": 20 }
}

This solution worked pretty well. The .reduce() method is so powerful that we can transform an array into basically anything we want. But I feel that .reduce() in general can be tricky to write and understand. It’s such a funky API. And a problem with this solution is that it creates a new object with each loop iteration. As a result, we are throwing away lots of in-progress objects.

A more “optimal” approach could be:

const teamLookup = teams.reduce((runningLookup, team) => {
  runningLookup[team.id] = team

  return runningLookup
}, {})

This approach is more or less the equivalent of my original for loop solution. But I’m not a fan of it as much. Even though we are only creating a single object, the assignment within the .reduce() doesn’t feel very “functional.” I always try to avoid mutating objects. Plus, this code would likely raise alarms from the no-param-reassign eslint rule.

Recently while reading a PR, I was reminded about Object.fromEntries(), which was officially added to ES2019. With it, we can create the same lookup with one statement, but in a more readable way (in my opinion, of course):

const teamLookup = Object.fromEntries(teams.map((team) => [team.id, team]))

Object.fromEntries() is the reverse of Object.entries() (ES2017). I use Object.entries() all the time to convert an object into an array of [key, value] pairs when I need both the key and the value for .map(), .filter(), .find(), etc. So instead of transforming an object to an array of pairs, Object.fromEntries() transforms an array of pairs to an object.

Therefore, to create our teamLookup, we first use .map() to quickly convert the teams array to an array of [id, team] pairs. Then, because Object.fromEntries() transforms pairs into an object, we get our team lookup! 🎉 Not too bad, right?

I’m now motivated to investigate more single-statement data transformations we can make. I think I’ll do a post on it. Stay tuned. 😉

Keep learning my friends. 🤓

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Hi, I'm Ben Ilegbodu. 👋🏾

I'm a Christian, husband, and father of 3, with 15+ years of professional experience developing user interfaces for the Web. I'm a Principal Frontend Engineer at Stitch Fix, frontend development teacher, Google Developer Expert, and Microsoft MVP. I love helping developers level up their frontend skills.

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