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Typescript does not detect property being changed by method (1 answer)

Closed 14 hours ago.

体积有以下文字:

const x = [1];

if (x.length !== 1)
throw "error";

x.pop();

if (x.length === 0)
throw "error";

compliler Weirdly在if(x.length=0)和上提出申诉。这一比较看来是无意的,因为第一类和第二类没有重叠。


似乎在第一个条件之后,它总是假定时间为1,而阵列修改并没有改变。 我不理解理由,也不知道我们如何解决这一问题?
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Answer 1

This is one of these cases when TypeScript s type narrowing isn t the smartest. TypeScript doesn t follow .length changes through methods, so its way of narrowing the .length type to a union often does not make much sense.

不幸的是,你需要工作。其中一项是确定新期限,而不是偷窃:

if (x.length > 0) { // This is needed, as an error would be thrown for an empty array
    x.length -= 1; // works like x.pop() minus returning the last item
}

Answer 2

The following code

if (x.length !== 1) throw "error";

narrows down the type of x.length to be 1.

就第二个问题而言,只在Java的运行时间控制流动结构上才有缩小类型,如:

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