Typescript: How to convert an array of keys into a union type of objects?

Question

1. Array of Strings -> Union of Strings (It works)

I saw a solution creates a union of string literals from an array of strings.

const keys = ["cat", "dog"] as const;

type Keys = typeof keys[number]; // "name" | "age"

2. Array of Strings -> Union of Objects (Is it possible?)

My question is if it's possible to create a union of objects from an array of strings? The code below works, I just want to shorten the SET_PROPERTY_PAYLOAD with something like keyof IObject and generate a union of object types from there.

interface IObject = {
  id: string;
  name: string;
  age: number;
}

// Is it possible to shorten this type using typescript's primitives
type SET_PROPERTY_PAYLOAD = 
| {
    id: string;
    propertyName: "id"; // keyof IObject at index 0
    value: string; // IObject["id"]
  }
| {
    id: string;
    propertyName: "name"; // keyof IObject at index 1
    value: string; // IObject["name"]
  }
| {
    id: string;
    propertyName: "age"; // keyof IObject at index 2
    value: number; // IObject["age"]
  };

My goal for this is to narrow the type of value by inferring from the current value of propertyName.

Answer 1

You want a mapped type.

interface IObject {
  id: string;
  name: string;
  age: number;
}

type SET_PROPERTY_PAYLOAD = {
    [K in keyof IObject]: {
        id: string,
        propertyName: K,
        value: IObject[K]
    }
}[keyof IObject]

This type maps over the keys in IObject and create a new object type for each key.

SET_PROPERTY_PAYLOAD should then be exactly equivalent to your long form version above.

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