泛型类型（T）与打字稿中的任何类型有什么区别 [英] What are the difference between generic Type(T) vs any in typescript

问题描述

function identity(arg: any): any {
    return arg;
}

功能2

Function 2

function identity<T>(arg: T): T {
    return arg;
}

功能3

Function 3

function identity<T>(arg: T[]): T[] {
    return arg;
}

---

---

功能1&如果我们传递任何类型的数据类型，则接受3，但如果我们传递数组，则函数2不接受。 泛型类型在编译时接受所有类型的数据类型。但这里为什么不接受？

Function 1 & 3 is accepted if we passing any kind of data type, But the function 2 does not accept if we pass an array. generic type is accepting all kind of data type on compile time. but here why it does not accept?

哪个功能有利于提高性能（功能1或功能3）？

Also which function is good for better performance ( function 1 or function 3)?

推荐答案

如果这是一个只返回一个参数且没有类型限制的身份函数，则没有区别：

There is no difference if this is identity function that just returns an argument and used without type restrictions:

const foo: any = fn(['whatever']);

类型代码有所不同：

const foo: string = fn('ok');
const bar: string = fn([{ not: 'ok' }]);

此外，泛型类型的使用提供了语义。此签名表明该函数是无类型的并返回任何内容：

Also, the usage of generic type provides semantics. This signature suggests that the function is untyped and returns anything:

function fn(arg: any): any { ... }

此签名表明该函数返回与其参数相同的类型：

This signature suggests that the function returns the same type as its argument:

function fn<T>(arg: T): T { ... }

实际函数通常比 return arg 示例更有意义。通用类型可以受益于类型限制（而任何显然不能）：

Real functions are usually more meaningful than just return arg example. Generic type can benefit from type restrictions (while any obviously can't):

function fn<T>(arg: T[]): T[] {
  return arg.map((v, i) => arg[i - 1]);
}

但是当函数与其他泛型一起使用时，好处会变得更加明显类和泛型函数（如果涉及非泛型函数则被删除）：

But the benefits become more obvious when the function is used in conjunction with other generic classes and generic functions (and eliminated if non-generics are involved):

function fn<T>(arg: T[]): T[] {
  return Array.from(new Set<T>(arg));
}

这样可以始终如一地维持 T 输入（参数）和输出（返回值）之间的类型：

This allows to consistently maintain T type between input (argument) and output (returned value):

const foo: string[] = fn(['ok']);
const bar: string[] = fn([{ not: 'ok' }]);

性能不会有任何差异，因为TypeScript类型仅在设计时存在。

There cannot be any difference in performance because TypeScript types exist only on design time.

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