typescript - 为什么 'null' 或 'undefined' "is assignable"到 'string' 类型的参数？

Question

让我们来 the following code in TypeScript :

function f<T>(a: T, b: T) { }

f("a", 1);

TypeScript 预计会失败:

Argument of type 'number' is not assignable to parameter of type 'string'.

然而， when I pass null (or undefined ) instead of 1 , TypeScript 改变泛型参数的类型 <T>至 string | null并且不会失败。
我有两个问题:

为什么null或 undefined被区别对待，例如number ?

以及如何防止扩大(将类型更改为 string | null )？

Answer 1

Why null or undefined is treated differently than e.g. number?

老实说，我不是 100% 确定为什么。正如评论中提到的，它推断 a是 'string' 类型，但允许 `null |不明确的。我做了一些实验，但我不明白为什么要这样做。

And how to prevent the widening (changing type to string | null)?Some examples of what you can do:

// Original
function f<T>(a: T, b: T) { }

f('a', 'b')  // Allowed
f(1, 2) // Allowed
f(true, false) // Allowed
f("a", 1); // Has error
f("a", undefined); // Allowed
f("a", null); // Allowed
f(null, null) // Allowed

// Extending then defining
// Will only allow strings
function g<T extends string = string>(a: T, b: T) { }

g('a', 'b')  // Allowed
g(1, 2) // Has error
g(true, false) // Has error
g("a", 1); // Has error
g("a", undefined); // Has error
g("a", null); // Has error
g(null, null) // Has error

// Using Required
// Does not allow null or undefined but allows other non strings
function x<T>(a: Required<T>, b: Required<T>) { }

x('a', 'b') // Allowed
x(1, 2) // Allowed
x(true, false) // Allowed
x("a", 1); // Has error
x("a", undefined); // Has error
x("a", null); // Has error
x(null, null) // Has error

https://www.typescriptlang.org/play?ts=4.0.2#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