const indexByKey = key => arr =>
  arr.reduce((acc, cur) => {
    acc[cur[key]] = cur
    return acc
  }, {})

Use this function to make look ups based on a key for an array of items O(1). So long as your list doesn’t change, you should only have to run this once.

Here’s an example:

const indexById = indexByKey('id')

const members = [
  { id: 1, name: 'Kyle' },
  { id: 2, name: 'Krios' }, // my cat
  { id: 3, name: 'Tali' }, //  my other cat

const membersById = indexById(members)
    '1': { id: 1, name: 'Kyle' },
    '2': { id: 2, name: 'Krios' },
    '3': { id: 3, name: 'Tali' }

Make sure the key you’re using is unique to each object, otherwise you’ll have an incorrect index due to key collisions.

Liked the post?
Give the author a dopamine boost with a few "beard strokes". Click the beard up to 50 times to show your appreciation.

Kyle Shevlin's face, which is mostly a beard with eyes

Kyle Shevlin is the founder & lead software engineer of Agathist, a software development firm with a mission to build good software with good people.

Good software by good people.
Visit to learn more
Sign up for my newsletter
Let's chat some more about TypeScript, React, and frontend web development. Unsubscribe at any time.
Logo for Just Enough Functional Programming
Just Enough Functional Programming
Check out my courses!
If you enjoy my posts, you might enjoy my courses, too. Click the button to view the course or go to Courses for more information.