typescript - "Cannot invoke an object which is possibly ' 未定义 '"即使在确保它之后!== 未定义

Question

为什么我会收到 Cannot invoke an object which is possibly 'undefined'即使在我检查了 func 之后，Typescript 也会出错引用不是未定义的？

type Hoge = {
    func?: (str: string) => boolean
}

const myFunc = (obj: Hoge) => {
    const data = ['AAA', 'BBB', 'CCC']

    if(obj.func !== undefined) {
        data.filter(obj.func) // ok
        data.filter(v => obj.func(v)) // ng Cannot invoke an object which is possibly 'undefined'.
    }
}

Answer 1

嗯你知道回调到data.filter立即执行但 typescript 应该如何知道？
考虑这个例子:

type Hoge = {
    func?: (str: string) => boolean
}

const myFunc = (obj: Hoge) => {
    if(obj.func !== undefined) {
        setTimeout(() => obj.func!('a'), 100); // Force assume func is not undefined
    }
    obj.func = undefined; // TS will allow this since func can be undefined, but this is a problem
}
myFunc({
    func: (str: string) => { 
        console.log(str); return true; 
    }
})

因为在您的情况下，您确实知道该函数是在 if 块中调用的，您应该使用:

data.filter(v => obj.func!(v))

这会让 TS 知道您知道该函数在那时不是未定义的
Playground link