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

var stuff: { [key: string]: string; } = {};
stuff['a'] = ''; // ok
stuff['a'] = 4;  // error

// ... or, if you're using this a lot and don't want to type so much ...
interface StringMap { [key: string]: string; }
var stuff2: StringMap = { };
// same as above

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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<T> = { [day in DayOfTheWeek]: T };

const chores: DayOfTheWeekMap<string> = {
    "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<boolean> = {
    "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.

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A quick update: since Typescript 2.1 there is a built in type Record<T, K> that acts like a dictionary.

In this case you could declare stuff like so:

var stuff: Record<string, any> = {};

You could also limit/specify potential keys by unioning literal types:

var stuff: Record<'a'|'b'|'c', string|boolean> = {};

Here's a more generic example using the record type from the docs:

// For every properties K of type T, transform it to U
function mapObject<K extends string, T, U>(obj: Record<K, T>, f: (x: T) => U): Record<K, U>

const names = { foo: "hello", bar: "world", baz: "bye" };
const lengths = mapObject(names, s => s.length);  // { foo: number, bar: number, baz: number }

TypeScript 2.1 Documentation on Record<T, K>

The only disadvantage I see to using this over {[key: T]: K} is that you can encode useful info on what sort of key you are using in place of "key" e.g. if your object only had prime keys you could hint at that like so: {[prime: number]: yourType}.

Here's a regex I wrote to help with these conversions. This will only convert cases where the label is "key". To convert other labels simply change the first capturing group:

Find: \{\s*\[(key)\s*(+\s*:\s*(\w+)\s*\]\s*:\s*([^\}]+?)\s*;?\s*\}

Replace: Record<$2, $3>

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You can pass a name to the unknown key and then write your types:

type StuffBody = {
  [key: string]: string;
};

Now you can use it in your type checking:

let stuff: StuffBody = {};

But for FlowType there is no need to have name:

type StuffBody = {
  [string]: string,
};

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Actually there is a built-in utility Record:

    const record: Record<string, string> = {};
    record['a'] = 'b';
    record[1] = 'c'; // leads to typescript error
    record['d'] = 1; // leads to typescript error

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Define interface

interface Settings {
  lang: 'en' | 'da';
  welcome: boolean;
}

Enforce key to be a specific key of Settings interface

private setSettings(key: keyof Settings, value: any) {
   // Update settings key
}

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@Ryan Cavanaugh's answer is totally ok and still valid. Still it worth to add that as of Fall'16 when we can claim that ES6 is supported by the majority of platforms it almost always better to stick to Map whenever you need associate some data with some key.

When we write let a: { [s: string]: string; } we need to remember that after typescript compiled there's not such thing like type data, it's only used for compiling. And { [s: string]: string; } will compile to just {}.

That said, even if you'll write something like:

class TrickyKey  {}

let dict: {[key:TrickyKey]: string} = {}

This just won't compile (even for target es6, you'll get error TS1023: An index signature parameter type must be 'string' or 'number'.

So practically you are limited with string or number as potential key so there's not that much of a sense of enforcing type check here, especially keeping in mind that when js tries to access key by number it converts it to string.

So it is quite safe to assume that best practice is to use Map even if keys are string, so I'd stick with:

let staff: Map<string, string> = new Map();

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Building on @shabunc's answer, this would allow enforcing either the key or the value — or both — to be anything you want to enforce.

type IdentifierKeys = 'my.valid.key.1' | 'my.valid.key.2';
type IdentifierValues = 'my.valid.value.1' | 'my.valid.value.2';

let stuff = new Map<IdentifierKeys, IdentifierValues>();

Should also work using enum instead of a type definition.

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interface AccountSelectParams {
  ...
}
const params = { ... };

const tmpParams: { [key in keyof AccountSelectParams]: any } | undefined = {};
  for (const key of Object.keys(params)) {
    const customKey = (key as keyof typeof params);
    if (key in params && params[customKey] && !this.state[customKey]) {
      tmpParams[customKey] = params[customKey];
    }
  }

please commented if you get the idea of this concept

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type KeyOf<T> = keyof T;

class SomeClass<T, R> {
  onlyTFieldsAllowed = new Map<KeyOf<T>, R>();
}

class A {
  myField = 'myField';
}

const some = new SomeClass<A, any>();

some.onlyTFieldsAllowed.set('myField', 'WORKS');
some.onlyTFieldsAllowed.set('noneField', 'Not Allowed!');

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