How to infer the type of object keys from a string array

Question

I have the following function:

    const infer = (...params: string[]): Record<string, string> => {
      const obj: Record<string, string> = {};
      // Put all params as keys with a random value to obj
      // [...]
      return obj;
    }

This function will take n strings and return an object containing exactly those strings as keys, with random values.

So infer("abc", "def") might return {"abc": "1337", "def":"1338"}.

Is there any way to infer the return type to get complete type-safety from this function? The code of the function guarantees that each key will be present in the returned object and that each value will be a string.

ford04 · Accepted Answer · 2019-09-20 12:40:00Z

5

You could declare it like this:

const infer = <T extends string[]>(...params: T): Record<T[number], string> => {
  const obj: Record<string, string> = {};
  // Put all params as keys with a random value to obj
  // [...]
  return obj;
};

const t = infer("a", "b", "c") // const t: Record<"a" | "b" | "c", string>

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answered Sep 20, 2019 at 12:40

ford04

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8 Comments

Philip Feldmann Over a year ago

Amazing, can you explain what the T[number] part does exactly?

ford04 Over a year ago

Sure. If you have type MyTuple = ["a", "b", "c"], you can use the lookup/index access operator to access item types by their numeric index value, e.g. MyTuple[0] has type "a". number in MyTuple[number] simply stands for all possible numeric index values, so MyTuple[number] is the union of all possible item types, which are your desired string values.

etarrowo Over a year ago

What if I pass the keys in as a single list? How can I get the same output?

ford04 Over a year ago

@etarrowo You can drop the rest parameter from infer and let T extend string, like here

etarrowo Over a year ago

awesome thank you @ford04 this one has been puzzling me for a bit

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