infer
表示在 extends 条件语句中待推断的类型变量。
type ParamType<T> = T extends (...arg: P) ? P : T
// 经典例子 ReturnType
//用于提取函数类型的返回值类型：
type ReturnType<T> = T extends (...args: any[]) => infer P ? P : any;
// 用于提取构造函数中参数（实例）类型：
type ConstructorParameters<T extends new (...arg: any[]) => any> = T extends new (...arg: infer P) => any ? P :never
type ConstructorParameters1<T extends new (...arg: any[]) => any> = T extends new (...arg: infer P) => any ? P :never
class TestClass {
constructor( public name: string, public age: number){
}
}
type params = ConstructorParameters1<typeof TestClass> // [string,number]
const test:params = ['123',123]
is
使用 is 来判定值的类型
function isAxiosError (error: any): error is AxiosError {
return error.isAxiosError
}
// 防止类型报错
if (isAxiosError(err)) {
code = `Axios-${err.code}`
}
