How to get Type of Array in Typescript generics

Question

I have a method like this:

public select(fieldName: keyof TType)

Where TType could be an array type. In case of an array type, fieldName will correctly offer me all the propertynames of type Array.

In case I call this method with a type of User[] I want to get to the properties of User instead of the properties of Array.

Is there any way of how this could be done?

Extra question: Is there any way to restrict TType to be of an array type?

Answer 1

You can definitely make a conditional type function that unwraps an array type up to one level deep, and then use keyof on the result of that. For example:

// unwrap up to one level
type Unarray<T> = T extends Array<infer U> ? U : T;

// your class maybe
declare class Thingy<T> {
  constructor(t: T);
  public select(fieldName: keyof Unarray<T>): void;
}
// your interface maybe
interface User {
  name: string,
  age: number
}

declare const u1: User;
declare const u2: User;
const x = new Thingy(u1);
x.select("name"); // okay
const y = new Thingy([u1, u2]);
y.select("age"); // okay
y.select("push"); // error

That should work as you want, for the typings, I think. Obviously you also need to have an implementation which works (and note that conditional types in implementations usually require some type assertions or overloads to make the compiler happy... but you seem to be asking about the typings, not the implementation).

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As for your extra question, yes, you can restrict T to just array types, as follows:

// your class maybe
declare class Thingy<T extends Array<any>> {
  constructor(t: T);
  public select(fieldName: keyof (T[number])): void;
}
// your interface maybe
interface User {
  name: string,
  age: number
}

declare const u1: User;
declare const u2: User;
const x = new Thingy(u1); // error
const y = new Thingy([u1, u2]);
y.select("age"); // okay

Note that I did away with the conditional types altogether here because it's more straightforward.

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Hope that helps; good luck!