声明一个在 TypeScript 中抛出错误的函数

在 TypeScript 中声明一个抛出错误的函数

Declare a function that throws an Error in TypeScript

要声明抛出错误的函数，请将其返回类型设置为never. 该never类型用于从不返回值的函数，换句话说，就是抛出异常或终止程序执行的函数。

索引.ts


// 👇️ function throwErr(): never
function throwErr(): never {
  throw new Error('Something went wrong');
}

never
类型在 TypeScript 中很少使用。

当函数永远不会到达返回语句时使用它，这主要有两个原因：

函数抛出错误
函数无限循环

如果我们没有显式键入函数的返回值，TypeScript 会将其推断为void.

索引.ts


// 👇️ function throwErr(): void
function throwErr() {
  throw new Error('Something went wrong');
}

never和
void之间的区别在于它void用于不返回任何内容（或 return
undefined）的函数。

如果我们不小心从一个返回类型为的函数返回一个值
void，我们就会得到一个错误。

然而，具有返回类型的函数never永远不会返回任何东西。

很少需要将函数的返回类型设置为never. 例如，如果函数仅在某些时候抛出错误，则不应将其返回类型设置为never.

索引.ts


function sometimesThrow(): number {
  if (Math.random() > 0.5) {
    return 100;
  }

  throw new Error('Something went wrong');
}

// 👇️ const result: number
const result = sometimesThrow();

console.log(result.toFixed());

The function in the example above returns a number some of the time, and
throws an error on some invocations.

You could use a
union type
to set its value to number or never.

index.ts


function sometimesThrow(): number | never {
  if (Math.random() > 0.5) {
    return 100;
  }

  throw new Error('Something went wrong');
}

// 👇️ const result: number
const result = sometimesThrow();

console.log(result.toFixed());

But this is only useful to inform your colleagues that the function might throw
an error.

Notice that the result variable is still typed as a number even though we set the function’s return type to number | never.

This is because every other type absorbs never in a union type.

index.ts


// 👇️ type T = number
type T = number | never;

Notice that number | never is simplified to number. This is the case when
using never with any other type in a union.

You could be explicit and set the function’s return type to number | never, because that would indicate to your colleagues that the function might throw an unhandled error and they might have to wrap the invocation in a try/catch statement.

However, this doesn’t technically influence the function’s return type.

It should also be noted that there isn’t a way for you to distinguish the type
of error a function throws.

For example, if you have a class CustomError that extends from Error,
there’s no way for you to tell TypeScript that your function throws a
CustomError instead of an Error.

If you set the function’s return type to CustomError, this means that the
function will return (and not throw) a value typed as CustomError.

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