Question

我试图在我的函数中做一个参数可选性, 输入标注正丢弃一个错误。

示例代码:

showDialog(title: string, value: string, callback: Function = null) {
       
}

标注错误 :

Type  null  is not assignable to type  Function .ts(2322)

回调是可选的。我如何避免此错误?

Answer 1

s 在 < a href=>" https://www.typescriptlang.org/docs/handbook/2/conflices.html#optional-paraters" rel=“不跟随 nofolverer” > optional 参数和可能为 null 的参数之间有区别。

对于一个隐含输入为 T\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\可以\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\


showDialog(title: string, value: string, callback?: Function) {}

可选参数必须放在参数列表的末尾,也就是说,不能随附所需的参数。
用于 null  的参数,您将使用 < a href="https://www. typescriptlan.org/docs/handbook/2/ everyday-types.html#union-types" rel=“nofollown noreferrer” > union type :
showDialog(title: string, value: string, callback: Function | null) {}

您也可以给出 null  的默认值:
showDialog(title: string, value: string, callback: Function | null=null) {}

Answer 2

您不需要使用 Null 来定义可选参数。参数是可选参数, 而不是用于定义任何变量的“ ” 标注。

示例1:使用可选箭头函数。

function showDialog(title: string, value: string, callback?: ()=>{}) {
       
}

示例2:函数的返回类型

function showDialog(title: string, value: string, callback?: () => void): void {
 
}

实例3:使用功能关键字

function showDialog(title: string, value: string, callback?: Function) {

}

友情链接