【TypeScript】：functions、types

目录
1. React 是怎么定义 useReducer 的？
2. TypeScript 必备知识
  2.1. Functions
    2.1.1. Typing the function
    2.1.2. Writing the function type
    2.1.3. Optional and Default Parameters
    2.1.4. Rest Parameters
    2.1.5. Overloads
  2.2. Unions and Intersection Types
    2.2.1. Union Types
  2.3. Conditional Types
  2.4. Type inference in conditional types    
3. useReducer 定义解析

// this technically does accept a second argument, but it's already under a deprecation warning
// and it's not even released so probably better to not define it.
type Dispatch<A> = (value: A) => void;
// Since action _can_ be undefined, dispatch may be called without any parameters.
type DispatchWithoutAction = () => void;
// Unlike redux, the actions _can_ be anything
type Reducer<S, A> = (prevState: S, action: A) => S;
// If useReducer accepts a reducer without action, dispatch may be called without any parameters.
type ReducerWithoutAction<S> = (prevState: S) => S;
// types used to try and prevent the compiler from reducing S
// to a supertype common with the second argument to useReducer()
type ReducerState<R extends Reducer<any, any>> = R extends Reducer<infer S, any> ? S : never;
type ReducerAction<R extends Reducer<any, any>> = R extends Reducer<any, infer A> ? A : never;
// The identity check is done with the SameValue algorithm (Object.is), which is stricter than ===
type ReducerStateWithoutAction<R extends ReducerWithoutAction<any>> =
    R extends ReducerWithoutAction<infer S> ? S : never;

/**
 * An alternative to `useState`.
 *
 * `useReducer` is usually preferable to `useState` when you have complex state logic that involves
 * multiple sub-values. It also lets you optimize performance for components that trigger deep
 * updates because you can pass `dispatch` down instead of callbacks.
 *
 * @version 16.8.0
 * @see https://reactjs.org/docs/hooks-reference.html#usereducer
 */
// overload where dispatch could accept 0 arguments.
function useReducer<R extends ReducerWithoutAction<any>, I>(
    reducer: R,
    initializerArg: I,
    initializer: (arg: I) => ReducerStateWithoutAction<R>
): [ReducerStateWithoutAction<R>, DispatchWithoutAction];
/**
 * An alternative to `useState`.
 *
 * `useReducer` is usually preferable to `useState` when you have complex state logic that involves
 * multiple sub-values. It also lets you optimize performance for components that trigger deep
 * updates because you can pass `dispatch` down instead of callbacks.
 *
 * @version 16.8.0
 * @see https://reactjs.org/docs/hooks-reference.html#usereducer
 */
// overload where dispatch could accept 0 arguments.
function useReducer<R extends ReducerWithoutAction<any>>(
    reducer: R,
    initializerArg: ReducerStateWithoutAction<R>,
    initializer?: undefined
): [ReducerStateWithoutAction<R>, DispatchWithoutAction];
/**
 * An alternative to `useState`.
 *
 * `useReducer` is usually preferable to `useState` when you have complex state logic that involves
 * multiple sub-values. It also lets you optimize performance for components that trigger deep
 * updates because you can pass `dispatch` down instead of callbacks.
 *
 * @version 16.8.0
 * @see https://reactjs.org/docs/hooks-reference.html#usereducer
 */
// overload where "I" may be a subset of ReducerState<R>; used to provide autocompletion.
// If "I" matches ReducerState<R> exactly then the last overload will allow initializer to be omitted.
// the last overload effectively behaves as if the identity function (x => x) is the initializer.
function useReducer<R extends Reducer<any, any>, I>(
    reducer: R,
    initializerArg: I & ReducerState<R>,
    initializer: (arg: I & ReducerState<R>) => ReducerState<R>
): [ReducerState<R>, Dispatch<ReducerAction<R>>];
/**
 * An alternative to `useState`.
 *
 * `useReducer` is usually preferable to `useState` when you have complex state logic that involves
 * multiple sub-values. It also lets you optimize performance for components that trigger deep
 * updates because you can pass `dispatch` down instead of callbacks.
 *
 * @version 16.8.0
 * @see https://reactjs.org/docs/hooks-reference.html#usereducer
 */
// overload for free "I"; all goes as long as initializer converts it into "ReducerState<R>".
function useReducer<R extends Reducer<any, any>, I>(
    reducer: R,
    initializerArg: I,
    initializer: (arg: I) => ReducerState<R>
): [ReducerState<R>, Dispatch<ReducerAction<R>>];
/**
 * An alternative to `useState`.
 *
 * `useReducer` is usually preferable to `useState` when you have complex state logic that involves
 * multiple sub-values. It also lets you optimize performance for components that trigger deep
 * updates because you can pass `dispatch` down instead of callbacks.
 *
 * @version 16.8.0
 * @see https://reactjs.org/docs/hooks-reference.html#usereducer
 */

// I'm not sure if I keep this 2-ary or if I make it (2,3)-ary; it's currently (2,3)-ary.
// The Flow types do have an overload for 3-ary invocation with undefined initializer.

// NOTE: without the ReducerState indirection, TypeScript would reduce S to be the most common
// supertype between the reducer's return type and the initialState (or the initializer's return type),
// which would prevent autocompletion from ever working.

// TODO: double-check if this weird overload logic is necessary. It is possible it's either a bug
// in older versions, or a regression in newer versions of the typescript completion service.
function useReducer<R extends Reducer<any, any>>(
    reducer: R,
    initialState: ReducerState<R>,
    initializer?: undefined
): [ReducerState<R>, Dispatch<ReducerAction<R>>];

function add(x: number, y: number): number {
  return x + y;
}

let myAdd = function (x: number, y: number): number {
  return x + y;
};

let myAdd: (x: number, y: number) => number = function (
  x: number,
  y: number
): number {
  return x + y;
};

function buildName(firstName: string, lastName?: string) {
  if (lastName) return firstName + " " + lastName;
  else return firstName;
}

function buildName(firstName: string, lastName = "Smith") {
  // ...
}

function buildName(firstName: string, ...restOfName: string[]) {
  return firstName + " " + restOfName.join(" ");
}

let buildNameFun: (fname: string, ...rest: string[]) => string = buildName;

let suits = ["hearts", "spades", "clubs", "diamonds"];

function pickCard(x: { suit: string; card: number }[]): number;
function pickCard(x: number): { suit: string; card: number };
function pickCard(x: any): any {
  // Check to see if we're working with an object/array
  // if so, they gave us the deck and we'll pick the card
  if (typeof x == "object") {
    let pickedCard = Math.floor(Math.random() * x.length);
    return pickedCard;
  }
  // Otherwise just let them pick the card
  else if (typeof x == "number") {
    let pickedSuit = Math.floor(x / 13);
    return { suit: suits[pickedSuit], card: x % 13 };
  }
}

let myDeck = [
  { suit: "diamonds", card: 2 },
  { suit: "spades", card: 10 },
  { suit: "hearts", card: 4 },
];

let pickedCard1 = myDeck[pickCard(myDeck)];
alert("card: " + pickedCard1.card + " of " + pickedCard1.suit);

let pickedCard2 = pickCard(15);
alert("card: " + pickedCard2.card + " of " + pickedCard2.suit);

/**
 * Takes a string and adds "padding" to the left.
 * If 'padding' is a string, then 'padding' is appended to the left side.
 * If 'padding' is a number, then that number of spaces is added to the left side.
 */
function padLeft(value: string, padding: string | number) {
  // ...
}

interface Bird {
  fly(): void;
  layEggs(): void;
}

interface Fish {
  swim(): void;
  layEggs(): void;
}

declare function getSmallPet(): Fish | Bird;

let pet = getSmallPet();
pet.layEggs();

// Only available in one of the two possible types
pet.swim();
Property 'swim' does not exist on type 'Bird | Fish'.
  Property 'swim' does not exist on type 'Bird'.

interface ErrorHandling {
  success: boolean;
  error?: { message: string };
}

interface ArtworksData {
  artworks: { title: string }[];
}

interface ArtistsData {
  artists: { name: string }[];
}

// These interfaces are composed to have
// consistent error handling, and their own data.

type ArtworksResponse = ArtworksData & ErrorHandling;
type ArtistsResponse = ArtistsData & ErrorHandling;

const handleArtistsResponse = (response: ArtistsResponse) => {
  if (response.error) {
    console.error(response.error.message);
    return;
  }

  console.log(response.artists);
};

T extends U ? X : Y

declare function f<T extends boolean>(x: T): T extends true ? string : number;

type ReturnType<T> = T extends (...args: any[]) => infer R ? R : any;