How to create generic Map interface in TypeScript

Question

I would like to create a Map interface in TypeScript but I cannot seem to figure out how to constrain the property accessor to please the compiler

Desired Interface

export interface IMap<I extends string | number, T> {
  [property: I]: T;
}

Error:

An index signature type must be 'string' or 'number'

indeed you can only do this with string or number as of today — CoderPi
– CoderPi, Commented Jan 8, 2016 at 21:19

Seamus · Accepted Answer · 2016-01-08 22:28:18Z

36

You are allowed to define both a string and numeric index signature.

From the spec:

An object type can contain at most one string index signature and one numeric index signature.

So you can do this:

interface IMap<T> {
    [index: string]: T;
    [index: number]: T;
}

Is that what you were after?

Also, when you define only a string index signature:

Specifically, in a type with a string index signature of type T, all properties and numeric index signatures must have types that are assignable to T.

And so:

class Foo {
    [index: string]: number;
}

let f = new Foo();

f[1] = 1; //OK

f[6] = "hi"; //ERROR: Type 'string' is not assignable to type 'number'

answered Jan 8, 2016 at 22:28

Seamus

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3 Comments

Chic Over a year ago

Thanks, however I was looking to be able to use the interface as IMap<number, string> or IMap<string, string> defining the index type per usage. It appears this is not possible.

Seamus Over a year ago

Ah, OK. Yeah, you are right. You could create two separate interfaces, but that sort of defeats the purpose.

Chic Over a year ago

Yeah, there's only two types it could be. If TypeScript allowed indexing via Enums there'd be more need for a generic.

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