如何使泛型和默认值一起工作

Question

如何将此代码修改为运行

function say<T = boolean>(arg:T = false) {
//                        ^^^^^^^^^^^^^ Could be instantiated with an arbitrary type which could be unrelated to
    if(arg) return ['hello']
    return 'hello'
}

const a = say() //type is string
const b = say(true) //type is string[]

Answer 1

您可以使用 conditional return type with assertions，像这样：

TS Playground

function say <T extends boolean = false>(value = false as T) {
  return (value ? ['hello'] : 'hello') as T extends true ? string[] : string;
}

或者一个重载的函数签名，像这样：

TS Playground

function say (value?: false): string;
function say (value: true): string[];
function say (value = false) {
  return (value ? ['hello'] : 'hello') as unknown;
}

在这两种情况下，使用 false 或隐式 undefined 调用函数将导致 string，使用 true 调用将导致 string[] :

say(); // string
say(false); // string
say(true); // string[]

如何使泛型和默认值一起工作

How to make generics and default value work together

generics

function

typescript