打字稿编译日期类型错误

Question

使用typescript 1.4我在以下代码行中遇到了有趣的错误:

var dateFrom:Date;
var dateTo:Date;
if(typeof discount.dateFrom ===  "string"){
    dateFrom = new Date(discount.dateFrom); // Line 362
} else {
    dateFrom = discount.dateFrom;
}

if(typeof discount.dateTo ===  "string"){
    dateTo = new Date(<string>discount.dateTo); // Line 368
} else {
    dateTo = discount.dateTo;
}

转换器返回以下内容:

[FULL_PATH]/Quotation.ts(362,37): error TS2345: Argument of type 'Date' is not assignable to parameter of type 'string'.
[FULL_PATH]/Quotation.ts(368,35): error TS2352: Neither type 'Date' nor type 'string' is assignable to the other.

与线362和368的区别在于我试图解决问题的两种情况.

我在代码中的其他地方使用了这个gimmic,它运行正常.

我在lib.d.ts中包含了Date构造函数的定义以供参考:

new (): Date;
new (value: number): Date;
new (value: string): Date;
new (year: number, month: number, date?: number, hours?: number, minutes?: number, seconds?: number, ms?: number): Date;
(): string;

Answer 1

假设discount.dateFrom是联合类型，例如string|Date，看起来您正在尝试在对象的属性上使用类型保护，而不是在普通的局部变量上。这是不支持的：

if (typeof discount.dateFrom === "string") {
    // This doesn't change the type of discount.dateFrom
}

但是，如果你写：

var dateFromProp = discount.dateFrom;
if (typeof dateFromProp === "string") {
    // dateFromProp is a string in this scope
}

然后应该可以工作。