TypeScript:如何根据第一个参数的值制作可选的第二个参数

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问题描述

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假设我有一个接受 2 个参数的函数，并且根据第一个参数的值，可能需要也可能不需要第二个参数.

Say I have a function that takes 2 args, and depending on the value of the first arg, the second arg may or may not be required.

例如:

function calculate(item: 'icon' | 'category', state: IState): void {

    if (arg1 === 'icon') {
        // code in which I don't need to access anything in state
    } 

    if (arg1 === 'category') {
        // code in which I do need to access state
    }

}

如果我按原样运行它，如果我写，我会得到一个错误

If I were to run this as is, I would get an error if I write

calculate('icon') // will cause an error

这也会引发错误，因为我没有为第二个参数传递有效值

This will also throw an error because I am not passing a valid value for the second arg

calculate('icon', null) // will also cause an error

为了不出错，我必须这样称呼它

In order to not get any errors, I have to call it like this

calculate('icon', this.state) // acceptable as long as 'this.state' meets the IState definition

如果第一个参数 = 'icon'，我希望能够在不传递第二个参数的情况下调用该函数.像这样

I want to be able to call the function without passing the second argument if the first argument = 'icon'. Like this

calculate('icon') // should not cause an error

但是，如果我这样调用计算它应该会导致错误

However, if I call calculate like this it should cause an error

calculate('category') // should cause an error

任何帮助将不胜感激！

推荐答案

您可以使用多个重载:

function calculate(item: 'icon'): void
function calculate(item: 'category', state: IState): void
function calculate(item: 'icon' | 'category', state?: IState): void {

    if (item === 'icon') {
        // code in which I don't need to access anything in state
    } 

    if (item === 'category' && state) {
        // code in which I do need to access state
    }

}
calculate("icon")
calculate("category") //error

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