How to dynamically create an object based on a readonly tuple in TypeScript?

Issue

In TypeScript, I can define the properties of a type based on a readonly tuple with the following construct:

const keys = ['A', 'B', 'C'] as const;

type T = { [key in typeof keys[number]]: boolean; };

This behaves as expected:

const t1: T = {A: true, B: true, C: true}; // OK
const t2: T = {A: true, B: true}; // Type error

Now, let's say that I want to dynamically create an object of type T based on the keys array, e.g. using reduce() or ES2019's Object.fromEntries():

const t1: T = keys.reduce((a, v) => ({ ...a, [v]: true }), {}); // Type error
const t2: T = Object.fromEntries(keys.map(s => [s, true])); // Type error

In both cases, I need an explicit type assertion to stop the compiler from complaining:

const t1: T = keys.reduce((a, v) => ({ ...a, [v]: true }), {}) as T; // OK
const t2: T = Object.fromEntries(keys.map(s => [s, true])) as T; // OK

Can anyone suggest a way to declare and initialize an object of type T based on the keys array, without having to resort to an object literal or explicit type assertion?

Solution

The simplest solution is to use Object.create(null) instead of {} to initialise the object. This is cheating a bit, because Object.create has return type any which means you don't get a type error simply because the type isn't checked.

function makeObject<K extends PropertyKey, V>(keys: K[], value: V): Record<K, V> {
  const obj: Record<K, V> = Object.create(null);
  for(const k of keys) {
    obj[k] = value;
  }
  return obj;
}

The upside is that because the object has null as its prototype, it doesn't inherit properties like 'toString' which might mess up code which expects dynamically-accessed properties to definitely have type V if they exist. If you explicitly do want to inherit those properties, you can use Object.create({}) instead.

Answered By - kaya3