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辨析联合类型

当类中含有字面量成员时,我们可以用该类的属性来辨析联合类型。

做为一个例子,考虑 SquareRectangle 的联合类型 ShapeSquareRectangle有共同成员 kind,因此 kind 存在于 Shape 中。

interface Square {
  kind: 'square';
  size: number;
}

interface Rectangle {
  kind: 'rectangle';
  width: number;
  height: number;
}

type Shape = Square | Rectangle;

如果你使用类型保护风格的检查(=====!=!==)或者使用具有判断性的属性(在这里是 kind),TypeScript 将会认为你会使用的对象类型一定是拥有特殊字面量的,并且它会为你自动把类型范围变小:

function area(s: Shape) {
  if (s.kind === 'square') {
    // 现在 TypeScript 知道 s 的类型是 Square
    // 所以你现在能安全使用它
    return s.size * s.size;
  } else {
    // 不是一个 square ?因此 TypeScript 将会推算出 s 一定是 Rectangle
    return s.width * s.height;
  }
}

详细的检查

通常,联合类型的成员有一些自己的行为(代码):

interface Square {
  kind: 'square';
  size: number;
}

interface Rectangle {
  kind: 'rectangle';
  width: number;
  height: number;
}

// 有人仅仅是添加了 `Circle` 类型
// 我们可能希望 TypeScript 能在任何被需要的地方抛出错误
interface Circle {
  kind: 'circle';
  radius: number;
}

type Shape = Square | Rectangle | Circle;

一个可能会让你的代码变差的例子:

function area(s: Shape) {
  if (s.kind === 'square') {
    return s.size * s.size;
  } else if (s.kind === 'rectangle') {
    return s.width * s.height;
  }

  // 如果你能让 TypeScript 给你一个错误,这是不是很棒?
}

你可以通过一个简单的向下思想,来确保块中的类型被推断为与 never 类型兼容的类型。例如,你可以添加一个更详细的检查来捕获错误:

function area(s: Shape) {
  if (s.kind === 'square') {
    return s.size * s.size;
  } else if (s.kind === 'rectangle') {
    return s.width * s.height;
  } else {
    // Error: 'Circle' 不能被赋值给 'never'
    const _exhaustiveCheck: never = s;
  }
}

它将强制你添加一种新的条件:

function area(s: Shape) {
  if (s.kind === 'square') {
    return s.size * s.size;
  } else if (s.kind === 'rectangle') {
    return s.width * s.height;
  } else if (s.kind === 'circle') {
    return Math.PI * s.radius ** 2;
  } else {
    // ok
    const _exhaustiveCheck: never = s;
  }
}

Switch

TIP

你可以通过 switch 来实现以上例子。

function area(s: Shape) {
  switch (s.kind) {
    case 'square':
      return s.size * s.size;
    case 'rectangle':
      return s.width * s.height;
    case 'circle':
      return Math.PI * s.radius ** 2;
    default:
      const _exhaustiveCheck: never = s;
  }
}

strictNullChecks

如果你使用 strictNullChecks 选项来做详细的检查,你应该返回 _exhaustiveCheck 变量(类型是 never),否则 TypeScript 可能会推断返回值为 undefined

function area(s: Shape) {
  switch (s.kind) {
    case 'square':
      return s.size * s.size;
    case 'rectangle':
      return s.width * s.height;
    case 'circle':
      return Math.PI * s.radius ** 2;
    default:
      const _exhaustiveCheck: never = s;
      return _exhaustiveCheck;
  }
}

Redux

Redux 库正是使用的上述例子。

以下是添加了 TypeScript 类型注解的redux 要点

import { createStore } from 'redux';

type Action =
  | {
      type: 'INCREMENT';
    }
  | {
      type: 'DECREMENT';
    };

/**
 * This is a reducer, a pure function with (state, action) => state signature.
 * It describes how an action transforms the state into the next state.
 *
 * The shape of the state is up to you: it can be a primitive, an array, an object,
 * or even an Immutable.js data structure. The only important part is that you should
 * not mutate the state object, but return a new object if the state changes.
 *
 * In this example, we use a `switch` statement and strings, but you can use a helper that
 * follows a different convention (such as function maps) if it makes sense for your
 * project.
 */
function counter(state = 0, action: Action) {
  switch (action.type) {
    case 'INCREMENT':
      return state + 1;
    case 'DECREMENT':
      return state - 1;
    default:
      return state;
  }
}

// Create a Redux store holding the state of your app.
// Its API is { subscribe, dispatch, getState }.
let store = createStore(counter);

// You can use subscribe() to update the UI in response to state changes.
// Normally you'd use a view binding library (e.g. React Redux) rather than subscribe() directly.
// However it can also be handy to persist the current state in the localStorage.

store.subscribe(() => console.log(store.getState()));

// The only way to mutate the internal state is to dispatch an action.
// The actions can be serialized, logged or stored and later replayed.
store.dispatch({ type: 'INCREMENT' });
// 1
store.dispatch({ type: 'INCREMENT' });
// 2
store.dispatch({ type: 'DECREMENT' });
// 1

与 TypeScript 一起使用可以有效的防止拼写错误,并且能提高重构和书写文档化代码的能力。