Typescript: Type 'string' cannot be used to index type

Question

In the following code, TypeScript seems to flippantly decide when a type is indexable by a string:

type Model = { [k: string]: any };

export class MyClass<TModel extends Model> {

    get(target: TModel, key: string) {
        return target[key];  //passes
    }

    set(target: TModel, key: string, value: any) {
        target[key] = value; //ts(2536) Type 'string' cannot be used to index type 'TModel'
    } 

}

TypeScript version: 4.7.2.

I've been bumping into this issue a lot lately and I'm wondering if anyone knows why.

Faust · Accepted Answer · 2022-10-22 18:04:46Z

1

Not sure of the exact explanation of why that doesn't work -- but I can fix the problem for you:

Type your "key" arguments as keyof TModel rather than string. E.g.:

set(target: TModel, key: keyof TModel, value: any) {

Perhaps because TModel extends Model, the checker thinks there could be a non-string prop key that would be referenced by a string.

Another solution might be to simply remove the generic parameter and type the target arguments as Model. You're not really using that generic type anyway. Anything that extends Model will still be accepted as those first arguments.

This code compiles fine with either approach:

const sym = Symbol("whatevs");
const my = new MyClass();
const model = { some: "thing", [sym]: "what?" };
my.set(model, "some", "other thing");

edited Oct 22, 2022 at 18:04

answered Oct 22, 2022 at 17:36

Faust

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3 Comments

RTD Over a year ago

It works to fix that single case, but the key needs to be re-usable with different objects. Sometimes their type isn't defined so this becomes complicated. It seems like the ideal would be to define the type as indexable by string.

Faust Over a year ago

You might not need the generic parameter in this case -- just added to my answer.

Faust Over a year ago

Either approach i've suggested here should work in all cases.

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