TypescriptのGenerics

// オブジェクトのキーのunion型を作る
// U extends keyof Tのようにすると、Uの型は、name | ageのunion型になります。
function copy<T extends { name: string }, U extends keyof T>(value: T, key: U): T {
  value[key];
  return value;
}
console.log(copy({ name: 'Quill', age: 20}, 'age'));

// classに対してgenericsを使う方法
class LightDatabase<T extends string | number | boolean> {
  private data: T[] = [];
  add(item: T) {
    this.data.push(item);
  }
  remove(item: T) {
    this.data.splice(this.data.indexOf(item), 1);
  }
  get() {
    return this.data;
  }
}
const stringLightDatabase = new LightDatabase();
stringLightDatabase.add('Apple');
stringLightDatabase.add('Banana');
stringLightDatabase.add('Grape');
stringLightDatabase.remove('Banana');
console.log(stringLightDatabase.get());

// Utility Types(型のlibrary)
interface Todo {
  title: string;
  text: string;
}
// 全部optional型になるUtility class
type Todoable = Partial<Todo>
// Readonly classだったら、全てにreadonly
type ReadTodo = Readonly<Todo>

const fetchData: Promise<string> = new Promise(resolve => {
  setTimeout(() => {
    resolve('hello');
  }, 3000);
})
fetchData.then(data => {
  data.toUpperCase();
})
const vegetables: Array<string> = ['Tomato', 'Broccoli', 'Asparagus'];

interface ResponseData<T extends { message: string } = any> {
  data: T;
  status: number;
}
let genericsTmp2: ResponseData;
interface Vegetables {
  readonly tomato: string;
  pumpkin?: string;
}
let genericsTmp3: keyof Vegetables;
// readonlyの頭に ー を付けると、Vegetablesのreadonlyが全て打ち消す
type MappedTypes = {
  -readonly [P in keyof Vegetables]-?: string
}
// tomato型が、string型に代入できるのであれば、number、できないのであればboolean
type ConditionalTypes = 'tomato' extends string ? number : boolean;
type ConditionalTypesInfer = { tomato: string } extends { tomato: infer R } ? R : boolean;
type DistributiveConditionalTypes<T> = T extends 'tomato' ? number : boolean;
let genericsTmp4: DistributiveConditionalTypes<'tomato' | 'pumpkin'>