How to declare a typed object with arbitrary keys?

Question

In a .d.ts file, how do I define an object as having string keys and T values?

e.g.

declare var Game: {
    creeps: Object<string, Creep>; // not sure what syntax to use here
};

declare class Creep {
   // definition...
}

Game.creeps is an object, but I don't know what properties/keys it will have (they're defined at run time -- I'm using it like a dictionary), however, I do know that all its values will be Creeps.

My IDE says "Object is not generic" so I guess that syntax isn't quite right.

basarat · Accepted Answer · 2016-04-13 06:33:26Z

58

Use an index signature:

declare var Game: {
    creeps: {[key:string]: Creep}
};

answered Apr 13, 2016 at 6:33

basarat

278k61 gold badges475 silver badges528 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

mpen Over a year ago

Perfect, thank you! Didn't know what to search for. Will accept in ~5min

mpen Over a year ago

This still errors in VSCode. Is that a bug in VS Code or is there another way to say "any property is OK"?

basarat Over a year ago

You need to access it as a dictionary i.e. creeps['key'] instead of creeps.key. If you know the key name then you should add it as a standard prop instead of using an index signagure

mpen Over a year ago

Aha! I didn't expect those behave differently. Thanks again :-)

ocket8888 · Accepted Answer · 2020-12-09 22:51:25Z

13

basrat's answer still works fine but, in Typescript 2.1 or higher, there is a typing that looks more similar to what you originally thought might work:

declare var Game: {
    creeps: Record<string, Creep>
};

There's no real difference, though I'd add that Typescript assumes that for any K value a Record<K, V> can produce a corresponding V value. For example:

type Creep = {
    id: number;
}

function printIt(c: Creep) {
    console.log(c.id);
}

let game: Record<string, Creep> = {"test": {id: 0}};
printIt(thing["quest"]);

will compile - even with strictNullChecks - although it will definitely result in a run-time error. So you need to check index access yourself, and/or use the --noUncheckedIndexedAccess flag as @mpen suggested.

edited Dec 9, 2020 at 22:51

answered Dec 8, 2020 at 21:02

ocket8888

1,1801 gold badge15 silver badges36 bronze badges

1 Comment

mpen Over a year ago

There's a new flag in TS 4.1 that will treat all index signatures as optional if you want to get a warning for that. --noUncheckedIndexedAccess

Collectives™ on Stack Overflow

How to declare a typed object with arbitrary keys?

2 Answers 2

4 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related