从泛型类型推断返回类型？

Question

The following works but .getData() has a return type of any which isn't ideal as this is a consumer face API call.

In reality the return type here isn't any thing, but is strictly dependant on the type of object held in the items array.

Is there anyway I can provide a return type to consumer of this method?

class Group<T> {
  items: Array<T> = [];

  addItem(item: T) {
      this.items.push(item);
  }

  getData(itemIndex: number) {
    // Any is T
    return (this.items[itemIndex] as any).data;
  }
}

class NumberItem {
  data: number;

  constructor(data: number) {
      this.data = data;
  }
}

class StringItem {
  data: String = "";

  constructor(data: String) {
      this.data = data;
  }
}

let n1 = new NumberItem(3);
let n2 = new NumberItem(4);

let g1 = new Group<NumberItem>();
g1.addItem(n1);
g1.addItem(n2);

console.log(g1.getData(1));

Playground Link

Answer 1

Give T an upper bound so that it has a data property, then the return type is T['data'] . This way there is also no need for a type assertion inside the getData method.

class Group<T extends { data: any }> {
  items: Array<T> = [];

  addItem(item: T) {
    this.items.push(item);
  }

  getData(itemIndex: number): T['data'] {
    return this.items[itemIndex].data;
  }
}

Usage:

let n1 = new NumberItem(3);
let n2 = new NumberItem(4);

let g1 = new Group<NumberItem>();
g1.addItem(n1);
g1.addItem(n2);

// result: number
const result = g1.getData(1);