英文:
typescript number returns string as typeof
问题
在代码中，我创建了一个名为 numericQuantity 的变量，它是一个被强制转换为数字类型的字符串 quantity。有没有解释为什么在使用 typeof 运算符时，即使变量的类型为 :number，它返回的结果仍然是 string？
let quantity: string = '10';
let numericQuantity: number = quantity as unknown as number;
console.log(typeof quantity); // string
console.log(typeof numericQuantity); //string
我在 TypeScript Playground 上尝试了这段代码，得到了相同的结果。
英文:
I created a numericQuantity variable which is a casted string quantity, is there an explanation why logging the typeof returns string even when the variable has a :number type?
let quantity: string = '10';
let numericQuantity: number = quantity as unknown as number;
console.log(typeof quantity); // string
console.log(typeof numericQuantity); //string
I tried this on the typescript playground and got the same result.
答案1
得分: 2
不要使用 quantity as unknown as number，这只有在你知道代码是正确且有效的情况下才能满足 TypeScript 编译器，但类型系统不允许时才有帮助。
typeof 不是 TypeScript 运算符，它返回声明的类型。它是 JavaScript 运算符，返回运行时值的类型。而你的 numericQuantity 变量保存的是一个字符串。
如果你实际上想编写一个程序，将字符串值转换为数字值，而不是将 numericQuantity = quantity，但要断言编译器“我知道得比你更好，这实际上是一个数字”，那么请这样写：
const quantity = '10';
const numericQuantity = Number(quantity);
console.log(typeof quantity); // string
console.log(typeof numericQuantity); // number
英文:
Don't use quantity as unknown as number, that only helps to satisfy the typescript compiler when you know that code is correct and working but the type system won't allow it.
typeof is not a typescript operator that returns the declared type. It's a javascript operator that returns the type of the runtime value. And your numericQuantity variable holds a string.
If you actually want to write a program that converts the string value to a number value, instead of assigning numericQuantity = quantity but asserting the compiler "I know better than you, this is actually a number", then write
const quantity = '10';
const numericQuantity = Number(quantity);
console.log(typeof quantity); // string
console.log(typeof numericQuantity); // number
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