How to Convert Array of Objects to Hash Map By Key in JavaScript

Published Mar 12, 2022  ∙  Updated Aug 21, 2022

How can we convert an array of objects to a hash map, where each key is a property of each object?

Suppose we have an array of objects.

const objs = [
  { id: 0, name: "Bob", age: 10 },
  { id: 1, name: "John", age: 12 },
  { id: 2, name: "Alice", age: 14 },
];

Let’s say we want to convert this array into a hashmap, where each object is indexed by its id.

const objsById = {
  0: { id: 0, name: "Bob", age: 10 },
  1: { id: 1, name: "John", age: 12 },
  2: { id: 2, name: "Alice", age: 14 },
}

Convert array of objects to hash map using reduce()

const objsById = objs.reduce((map, obj) => {
  map[obj.id] = obj;
  return map;
}, {});
objsById[0] // { id: 0, name: "Bob", age: 10 }

If we want a simple one-liner, we can take some syntactical shortcuts.

const objsById = objs.reduce((map, obj) => ((map[obj.id] = obj), map), {});
objsById[0] // { id: 0, name: "Bob", age: 10 }

If we’re using TypeScript, we’ll want to add a type definition to the avoid the “Property does not exist” error.

Let’s say we have a User interface for the objects in our array.

const objsById: { [key: number]: User } = ...;

Convert array of objects to hash map using map()

We can achieve the same functionality using map().

const objsById = new Map(objs.map(obj => [obj.id, obj]));

In order to get a value by the key, we’ll have to use get().

objsById.get(0) // { id: 0, name: "Bob", age: 10 }
objsById[0] // undefined

If we’re using TypeScript, we can add types to the Map.

const objsById = new Map<number, User>(objs.map(obj => [obj.id, obj]));

Note that we can map any property to any other property in our object. This means we can map from id to name or from name to age.