类型缩小

假设我们有一个名为 padLeft 的函数。

function padLeft(padding: number | string, input: string): string {
  throw new Error("Not implemented yet!");
}

如果 padding 是 number，它会将其视为我们想要添加到 input 的空格数。如果 padding 是 string，它应该只是将 padding 前置到 input。让我们尝试实现当 padLeft 为 padding 传递 number 时的逻辑。

function padLeft(padding: number | string, input: string) {
  return " ".repeat(padding) + input;
}

哦，我们在 padding 上遇到错误。TypeScript 警告我们，将 number | string 添加到 number 可能不会给我们想要的东西，这是正确的。换句话说，我们没有先明确检查 padding 是否是 number，也没有处理它是 string 的情况，所以让我们这样做。

function padLeft(padding: number | string, input: string) {
  if (typeof padding === "number") {
    return " ".repeat(padding) + input;
  }
  return padding + input;
}

如果这看起来像是无趣的 JavaScript 代码，那就是重点。除了我们放置的注释之外，这个 TypeScript 代码看起来像 JavaScript。这个想法是 TypeScript 的类型系统旨在使编写典型的 JavaScript 代码尽可能容易，而无需向后兼容以获得类型安全。

虽然它可能看起来不多，但实际上这里有很多东西。就像 TypeScript 如何使用静态类型分析运行时值一样，它在 JavaScript 的运行时控制流结构（如 if/else、条件三元组、循环、真实性检查等）上进行类型分析，这些都会影响这些类型。

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function padLeft(padding: number | string, input: string) {
  if (typeof padding === "number") {
    return " ".repeat(padding) + input;
  }
  return padding + input;
}

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typeof 类型保护

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function printAll(strs: string | string[] | null) {
  if (typeof strs === "object") {
    for (const s of strs) {
      console.log(s);
    }
  } else if (typeof strs === "string") {
    console.log(strs);
  } else {
    // do nothing
  }
}

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真实性缩小

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function getUsersOnlineMessage(numUsersOnline: number) {
  if (numUsersOnline) {
    return `There are ${numUsersOnline} online now!`;
  }
  return "Nobody's here. :(";
}

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// both of these result in 'true'
Boolean("hello"); // type: boolean, value: true
!!"world"; // type: true,    value: true

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function printAll(strs: string | string[] | null) {
  if (strs && typeof strs === "object") {
    for (const s of strs) {
      console.log(s);
    }
  } else if (typeof strs === "string") {
    console.log(strs);
  }
}

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TypeError: null is not iterable

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function printAll(strs: string | string[] | null) {
  // !!!!!!!!!!!!!!!!
  //  DON'T DO THIS!
  //   KEEP READING
  // !!!!!!!!!!!!!!!!
  if (strs) {
    if (typeof strs === "object") {
      for (const s of strs) {
        console.log(s);
      }
    } else if (typeof strs === "string") {
      console.log(strs);
    }
  }
}

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function multiplyAll(
  values: number[] | undefined,
  factor: number
): number[] | undefined {
  if (!values) {
    return values;
  } else {
    return values.map((x) => x * factor);
  }
}

相等性缩小

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function example(x: string | number, y: string | boolean) {
  if (x === y) {
    // We can now call any 'string' method on 'x' or 'y'.
    x.toUpperCase();
    y.toLowerCase();
  } else {
    console.log(x);
    console.log(y);
  }
}

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function printAll(strs: string | string[] | null) {
  if (strs !== null) {
    if (typeof strs === "object") {
      for (const s of strs) {
        console.log(s);
      }
    } else if (typeof strs === "string") {
      console.log(strs);
    }
  }
}

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interface Container {
  value: number | null | undefined;
}

function multiplyValue(container: Container, factor: number) {
  // Remove both 'null' and 'undefined' from the type.
  if (container.value != null) {
    console.log(container.value);

    // Now we can safely multiply 'container.value'.
    container.value *= factor;
  }
}

in 运算符缩小

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type Fish = { swim: () => void };
type Bird = { fly: () => void };

function move(animal: Fish | Bird) {
  if ("swim" in animal) {
    return animal.swim();
  }

  return animal.fly();
}

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type Fish = { swim: () => void };
type Bird = { fly: () => void };
type Human = { swim?: () => void; fly?: () => void };

function move(animal: Fish | Bird | Human) {
  if ("swim" in animal) {
    animal;
  } else {
    animal;
  }
}

instanceof 缩小

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function logValue(x: Date | string) {
  if (x instanceof Date) {
    console.log(x.toUTCString());
  } else {
    console.log(x.toUpperCase());
  }
}

赋值

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let x = Math.random() < 0.5 ? 10 : "hello world!";
x = 1;

console.log(x);
x = "goodbye!";

console.log(x);

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let x = Math.random() < 0.5 ? 10 : "hello world!";
x = 1;

console.log(x);
x = true;

console.log(x);

控制流分析

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function padLeft(padding: number | string, input: string) {
  if (typeof padding === "number") {
    return " ".repeat(padding) + input;
  }
  return padding + input;
}

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b98y2YZTJWcg0XiYn0N/+2iyoIGrQ/X2PR7bpQDgjGEqnEXiR2BulTEPzy6xGY2XvRRvjgYcCn/2tjVQEFDsjRmrOe8Yuf5b0JhZO/+T+SaCDqtQ66LR5ZFiF+ftHxcSOLwmVuPQnH2ZMe7/rC4TN6GNLay7MiyI56sIlyk98YAj/AB5UuJgYf+OhDmk3wHr3JIwp7FDP43rMS8G65tJ47KQ2hgS+DsJEqHcKvmxrpbaZsQwjqEAPGb3i24/6VxXOhCb7yIKiJ09Hs6Ybef0/O1HobOUrQYlSxfnWFRK0KvMAeZDADE1FvPGARpkSuWOr9KdG6m8Scw0IvvR8VnfWGhlh/BA8z+jSHv/lWcpMCs5QyK8Jc2oZL+3uYega3Z4jARcTfI4EJPhRfbIQH5QmA==

function example() {
  let x: string | number | boolean;

  x = Math.random() < 0.5;

  console.log(x);

  if (Math.random() < 0.5) {
    x = "hello";
    console.log(x);
  } else {
    x = 100;
    console.log(x);
  }

  return x;
}

使用类型谓词

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type Fish = { swim: () => void };
type Bird = { fly: () => void };
declare function getSmallPet(): Fish | Bird;
function isFish(pet: Fish | Bird): pet is Fish {
  return (pet as Fish).swim !== undefined;
}

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type Fish = { swim: () => void };
type Bird = { fly: () => void };
declare function getSmallPet(): Fish | Bird;
function isFish(pet: Fish | Bird): pet is Fish {
  return (pet as Fish).swim !== undefined;
}
// Both calls to 'swim' and 'fly' are now okay.
let pet = getSmallPet();

if (isFish(pet)) {
  pet.swim();
} else {
  pet.fly();
}

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type Fish = { swim: () => void; name: string };
type Bird = { fly: () => void; name: string };
declare function getSmallPet(): Fish | Bird;
function isFish(pet: Fish | Bird): pet is Fish {
  return (pet as Fish).swim !== undefined;
}
const zoo: (Fish | Bird)[] = [getSmallPet(), getSmallPet(), getSmallPet()];
const underWater1: Fish[] = zoo.filter(isFish);
// or, equivalently
const underWater2: Fish[] = zoo.filter(isFish) as Fish[];

// The predicate may need repeating for more complex examples
const underWater3: Fish[] = zoo.filter((pet): pet is Fish => {
  if (pet.name === "sharkey") return false;
  return isFish(pet);
});

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判别联合

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interface Shape {
  kind: "circle" | "square";
  radius?: number;
  sideLength?: number;
}

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interface Shape {
  kind: "circle" | "square";
  radius?: number;
  sideLength?: number;
}

function handleShape(shape: Shape) {
  // oops!
  if (shape.kind === "rect") {
    // ...
  }
}

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interface Shape {
  kind: "circle" | "square";
  radius?: number;
  sideLength?: number;
}

function getArea(shape: Shape) {
  return Math.PI * shape.radius ** 2;
}

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interface Shape {
  kind: "circle" | "square";
  radius?: number;
  sideLength?: number;
}

function getArea(shape: Shape) {
  if (shape.kind === "circle") {
    return Math.PI * shape.radius ** 2;
  }
}

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interface Shape {
  kind: "circle" | "square";
  radius?: number;
  sideLength?: number;
}

function getArea(shape: Shape) {
  if (shape.kind === "circle") {
    return Math.PI * shape.radius! ** 2;
  }
}

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interface Circle {
  kind: "circle";
  radius: number;
}

interface Square {
  kind: "square";
  sideLength: number;
}

type Shape = Circle | Square;

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interface Circle {
  kind: "circle";
  radius: number;
}

interface Square {
  kind: "square";
  sideLength: number;
}

type Shape = Circle | Square;

function getArea(shape: Shape) {
  return Math.PI * shape.radius ** 2;
}

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interface Circle {
  kind: "circle";
  radius: number;
}

interface Square {
  kind: "square";
  sideLength: number;
}

type Shape = Circle | Square;

function getArea(shape: Shape) {
  if (shape.kind === "circle") {
    return Math.PI * shape.radius ** 2;
  }
}

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interface Circle {
  kind: "circle";
  radius: number;
}

interface Square {
  kind: "square";
  sideLength: number;
}

type Shape = Circle | Square;

function getArea(shape: Shape) {
  switch (shape.kind) {
    case "circle":
      return Math.PI * shape.radius ** 2;
    case "square":
      return shape.sideLength ** 2;
  }
}

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never 类型

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穷举检查

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interface Circle {
  kind: "circle";
  radius: number;
}

interface Square {
  kind: "square";
  sideLength: number;
}
type Shape = Circle | Square;

function getArea(shape: Shape) {
  switch (shape.kind) {
    case "circle":
      return Math.PI * shape.radius ** 2;
    case "square":
      return shape.sideLength ** 2;
    default:
      const _exhaustiveCheck: never = shape;
      return _exhaustiveCheck;
  }
}

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interface Circle {
  kind: "circle";
  radius: number;
}

interface Square {
  kind: "square";
  sideLength: number;
}
interface Triangle {
  kind: "triangle";
  sideLength: number;
}

type Shape = Circle | Square | Triangle;

function getArea(shape: Shape) {
  switch (shape.kind) {
    case "circle":
      return Math.PI * shape.radius ** 2;
    case "square":
      return shape.sideLength ** 2;
    default:
      const _exhaustiveCheck: never = shape;
      return _exhaustiveCheck;
  }
}