Enforcing the type of the indexed members of a Typescript object?

Question

I would like to store a mapping of string -> string in a Typescript object, and enforce that all of the keys map to strings. For example:

var stuff = {};
st         


        

Answer 1

interface AgeMap {
    [name: string]: number
}

const friendsAges: AgeMap = {
    "Sandy": 34,
    "Joe": 28,
    "Sarah": 30,
    "Michelle": "fifty", // ERROR! Type 'string' is not assignable to type 'number'.
};

Here, the interface AgeMap enforces keys as strings, and values as numbers. The keyword name can be any identifier and should be used to suggest the syntax of your interface/type.

You can use a similar syntax to enforce that an object has a key for every entry in a union type:

type DayOfTheWeek = "sunday" | "monday" | "tuesday" | "wednesday" | "thursday" | "friday" | "saturday";

type ChoresMap = { [day in DayOfTheWeek]: string };

const chores: ChoresMap = { // ERROR! Property 'saturday' is missing in type '...'
    "sunday": "do the dishes",
    "monday": "walk the dog",
    "tuesday": "water the plants",
    "wednesday": "take out the trash",
    "thursday": "clean your room",
    "friday": "mow the lawn",
};

You can, of course, make this a generic type as well!

type DayOfTheWeek = "sunday" | "monday" | "tuesday" | "wednesday" | "thursday" | "friday" | "saturday";

type DayOfTheWeekMap = { [day in DayOfTheWeek]: T };

const chores: DayOfTheWeekMap = {
    "sunday": "do the dishes",
    "monday": "walk the dog",
    "tuesday": "water the plants",
    "wednesday": "take out the trash",
    "thursday": "clean your room",
    "friday": "mow the lawn",
    "saturday": "relax",
};

const workDays: DayOfTheWeekMap = {
    "sunday": false,
    "monday": true,
    "tuesday": true,
    "wednesday": true,
    "thursday": true,
    "friday": true,
    "saturday": false,
};

10.10.2018 update: Check out @dracstaxi's answer below - there's now a built-in type Record which does most of this for you.

1.2.2020 update: I've entirely removed the pre-made mapping interfaces from my answer. @dracstaxi's answer makes them totally irrelevant. If you'd still like to use them, check the edit history.