Create TypeScript generic type that extends interface with only string keys

Question

I want to make a generic class that requires a template parameter that is an interface with only string keys.

I thought I could do something like

class MyClass<T extends Record<string, object>> {
    sendEventData<TKey extends keyof T>(event: TKey, data: T[TKey]) {
        // ...
    }
}

However, if I instantiate it like

interface MyEvents {
    someEvent: { foo: string }
}

const instanace = new MyClass<MyEvents>();

I get a compilation error:

Type 'MyEvents' does not satisfy the constraint 'Record<string, object>'.
  Index signature is missing in type 'MyEvents'.

If I remove extends Record<string, object entirely, it compiles fine, but it doesn't restrict it to a map of string => object.

SLaks · Accepted Answer · 2019-09-23 21:27:15Z

3

Record<string, > implies that it accepts any key, which you don't want.

Instead, write T extends Record<string&keyof T, object> to restrict it to only keys that exist on the type, and that are also strings.

Demo

answered Sep 23, 2019 at 21:27

SLaks

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1 Comment

Nick Bilyk Over a year ago

That doesn't work, keyof T would not be restricted to string. sendEventData<TKey extends keyof T>(event: TKey, data: T[TKey]) { event.toLowerCase() // error, event is type string | number | symbol }

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