Can I constrain an objects keys to an enums values using typescript?

Question

Essentially, I want a way to ensure that an options argument has keys which are the values of a specific enum:

//enum Mode { Foo, Bar };
interface Mode { Foo: number, Bar: number }

interface View { 
    text: string;
};

class FooView implements View {
    text = 'foo';
 }

class BarView implements View { 
    text = 'bar';
}

function initialize(options: { mode: {[P in keyof Mode]?: View} }) {
    let mode: View = options.mode.Foo;
}

initialize({ mode: { Bar: new FooView() } });

It works perfectly if I use an interface/class instead of enum, but this is truly an enum (conceptually)...

See this playground

Community · Accepted Answer · 2017-05-23 11:53:14Z

0

Keys have to be either strings or numbers. You can kind of do this, but you have to use the bracket syntax and set your object's key as a number:

enum Mode { Foo, Bar };

function initialize(options: { mode: {[key:number]: View} }) {
    let mode: View = options.mode[Mode.Foo];
}

initialize({ mode: { [Mode.Bar]: new FooView() } });

The idea for this answer comes from Sohnee's answer to a similar question.

Obviously the caveat here is that someone could just as easily do something like

initialize({ mode: { [999]: new FooView() } });

Less than optimal. The best you could do there is throw an error at run time if the value is not a valid Mode:

if (!Object.keys(options.mode).every((key) => key in Mode)) {
    throw new Error("Nice try");
}

edited May 23, 2017 at 11:53

CommunityBot

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answered Feb 10, 2017 at 22:10

Heretic Monkey

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1 Comment

Lucas Over a year ago

I was thinking along those lines... Definitely less than optimal. Thanks for the info!

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