How to create a Typescript type based on the keys/values of a dictionary?

Question

Is there a way to automatically generate a type based on a dictionary value? E.g. given this dictionary of constant values:

export const SHIPMETHODS: OptionsMap<UpsShipMethodOption> = {
    'UPS': 'GND',
    'UPS AIR2': '2DA',
    'UPS 3DAY SELECT': '3DS'
}

Is there a way to automatically generate a type like this, without having to duplicate everything?

export type ShipMethodOptionMap = {
    'UPS': 'GND',
    'UPS AIR2': '2DA',
    'UPS 3DAY SELECT': '3DS'
}

Unfortunately keyof doesn't seem to work since it's an object and not a type, so this doesn't work:

export type UpsShipMethodOption = keyof SHIPMETHODS // Doesn't work

I also tried using mapped types with a predeclared union type of the keys, but it doesn't work either:

/* 
  Type 'KeyOptions' is not assignable to type 'string | number | symbol'.
  Type 'KeyOptions' is not assignable to type 'symbol'.ts(2322)
*/
export type OptionsMap<KeyOptions> = {
    readonly [Property in KeyOptions]: string;
}

export const SHIPMETHODS: OptionsMap<UpsShipMethodOption> = {
    'UPS': 'GND',
    'UPS AIR2': '2DA',
    'UPS 3DAY SELECT': '3DS'
}
export type UpsShipMethodOption = 'UPS' | 'UPS AIR2' | 'UPS 3DAY SELECT'

I have a bunch of such dictionaries, and most of them are quite large (30+ keys). Duplicating the dictionary into a type everywhere would make it a pain to maintain in case the values change, so any help with this would be great!

score 3 · Accepted Answer · answered Feb 07 '22 at 19:21

I think you just want the typeof operator. It simply gets the type of a value.

const SHIPMETHODS = {
    'UPS': 'GND',
    'UPS AIR2': '2DA',
    'UPS 3DAY SELECT': '3DS'
} as const // as const is important.

type ShipMethodOptionMap = typeof SHIPMETHODS 
/*
type ShipMethodOptionMap = {
    readonly UPS: "GND";
    readonly 'UPS AIR2': "2DA";
    readonly 'UPS 3DAY SELECT': "3DS";
}
*/

type UpsShipMethodOption = keyof ShipMethodOptionMap
// type UpsShipMethodOption = "UPS" | "UPS AIR2" | "UPS 3DAY SELECT"

Just note the as const on your object which allows typescript to infer the string literals so they can become part of the type.

Playground

Ah! This is perfect, thank you! :) – anonmily Feb 07 '22 at 19:30 — anonmily, Feb 07 '22 at 19:30

How to create a Typescript type based on the keys/values of a dictionary?

1 Answers1