Ability to specify that T[K] extends a type

[Roaders]

Search Terms

enforce key type of generic type

Suggestion

I would like to specify that a property, defined as a function argument, of a generic type is of a specific type

Use Cases

I want to pass an item and a key to a function and specify that the type of the property specified by the key is a certain type

Examples

function cacheItem<T extends {K: string}, K extends keyof T>(item: T, key: K){
     const key = item[key]; // should be typed as string
}

const myItem = {
    id = "string",
    count = 1234
}

cacheItem(myItem, "id") // works
cacheItem(myItem, "count") // should throw error

Checklist

My suggestion meets these guidelines:

• [x ] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [x ] This wouldn't change the runtime behavior of existing JavaScript code
• [x ] This could be implemented without emitting different JS based on the types of the expressions
• [x ] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• [ x] This feature would agree with the rest of TypeScript's Design Goals.

[jack-williams]

Looks related: #31275

[anion155]

function cacheItem<
  T extends any,
  K extends {
    [K in keyof T]: T[K] extends string ? K : never
  }[keyof T]
>(item: T, key: K) {
  const value: string = item[key]; // is typed as string
  value.replace;
}

[jcalz]

{K: string} is not saying what you intend to say, since K is just a literal key there (e.g., foo.K = "oops"). Instead, you can use a mapped type such as {[P in K]: string} or the equivalent Record<K, string>. (Eventually you may be able to use K directly as an index signature as in {[k: K]: string} but that is not currently supported.)

[RyanCavanaugh]

You don't need new machinery to do this.

function cacheItem<K extends string>(item: Record<K, string>, key: K) {
    const val = item[key]; // should be typed as string (is)
}

const myItem = {
    id: "string",
    count: 1234
}

cacheItem(myItem, "id") // works
cacheItem(myItem, "count"); // Error