ts笔记: Everyday Types

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https://www.typescriptlang.org/docs/handbook/2/everyday-types.html

字面量类型推断

数字和字符串的字面量会被推断为 number 和string类型，所以常见的报错：

const req = { url: "https://example.com", method: "GET" };
handleRequest(req.url, req.method);

> Argument of type 'string' is not assignable to parameter of type '"GET" | "POST"'.

的解决方法

// Change 1:
const req = { url: "https://example.com", method: "GET" as "GET" };
// Change 2
handleRequest(req.url, req.method as "GET");

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类型断言

ts 不允许 ‘不可能’的类型断言。比如

const x = "hello" as number;

> Conversion of type 'string' to type 'number' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.

这种情况下的解决方法就是断言2次，先转化为any/unknown， 再断言为其他类型

const a = (expr as any) as T;

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Type 和 Interface的区别

Type 创建后不可二次修改， Interface 始终可扩展编辑的

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联合类型使用

type U= A｜B

如果使用的是A 跟B都有的属性，那么ts不会报错，
如果使用的是A或者 B 独有的属性，那么使用前需要先判断具体是哪一个类型，ts才不会报错。

function printId(id: number | string) {
  if (typeof id === "string") {
    // In this branch, id is of type 'string'
    console.log(id.toUpperCase());
  } else {
    // Here, id is of type 'number'
    console.log(id);
  }
}

https://jkchao.github.io/typescript-book-chinese/typings/discrominatedUnion.html#%E8%AF%A6%E7%BB%86%E7%9A%84%E6%A3%80%E6%9F%A5

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Null 和 Undefined

项目建议开启 strictNullChecks

如果你知道变量一定存在，则可以使用：非空断言操作符

function liveDangerously(x?: number | null) {
  // No error
  console.log(x!.toFixed());
}