Lookup Types

keyof Type

TIP

获取类型的属性

官网keyof关键字

interface Person {
  name: string;
  age: number;
  location: string;
}
type K1 = keyof Person; // "name" | "age" | "location"
type K2 = keyof Person[]; // "length" | "push" | "pop" | "concat" | ...
type K3 = keyof { [x: string]: Person }; // string

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与extends结合使用 Pick (ghaiklor.github.io)

type MyPick<T, K extends keyof T> = {
  [key in K]: T[key];
};

interface Todo {
  title: string;
  description: string;
  completed: boolean;
}

type TodoPreview = MyPick<Todo, "title" | "completed">;

const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
};

Indexed Type

number

TIP

If we access the T[number], TypeScript will return a union of all elements from T. E.g. if you have a T = [1, 2, 3], accessing it via T[number] will return 1 | 2 | 3.

But, there is an error “Type ‘number’ cannot be used to index type ‘T’”. It is because we don’t have a ❤️constraint ❤️over T. We need to tell TypeScript that T is an array.

来自--Includes (ghaiklor.github.io)

type Includes<T extends readonly any[], U> = U extends T[number] ? true : false;

const MyArray = [
  { name: "Alice", age: 15 },
  { name: "Bob", age: 23 },
  { name: "Eve", age: 38 },
];
// constraint array
type Person = typeof MyArray[number];

TypeScript: Documentation - Indexed Access Types (typescriptlang.org)

using number to get the type of an array’s elements. We can combine this with typeof to conveniently capture the element type of an array literal:

const MyArray = [
  { name: "Alice", age: 15 },
  { name: "Bob", age: 23 },
  { name: "Eve", age: 38 },
];
 
type Person = typeof MyArray[number];

type TupleToObject<T extends readonly (string | number)[]> = {
  [k in T[number]]: k;
};

const tupleMix = [1, "2", 3, "4"] as const;

TupleToIbject(typeof tupleMix)