typescript 打字稿类型转换不起作用
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Typescript type casting not working
提问by Kuan
I am pretty new to TypeScript. I started with a book called Typescript Revealed (Pub Feb.2013). In Chapter 2 there is a section called "Casts" that has the following example:
我对打字稿很陌生。我从一本名为 Typescript Revealed（2013 年 2 月出版）的书开始。在第 2 章中有一个名为“Casts”的部分，其中包含以下示例：
var a : int = <int>SomeNumberAsAString;
var a : int = <int>SomeNumberAsAString;
I tried to apply the example, as follows:
我尝试应用该示例，如下所示：
var SomeNumberAsAString = "1000";
var a: int = <int>SomeNumberAsAString;
But compiler gave me an error:
但是编译器给了我一个错误：
hello.ts(2,8): error TS2304: Cannot find name 'int'.
hello.ts(2,15): error TS2304: Cannot find name 'int'.
hello.ts(2,8): 错误 TS2304: 找不到名称“int”。
hello.ts(2,15): 错误 TS2304: 找不到名称“int”。
I'm wondering how to do this cast, or has the specification of Typescript changed?
我想知道如何进行此转换，或者 Typescript 的规范是否已更改？
回答by basarat
(Pub Feb.2013)
（2013 年 2 月发布）
That book is old. Its called numbernow.
那本书是旧的。它number现在叫。
var SomeNumberAsAString = "1000";
var a: number = <number><any>SomeNumberAsAString;
Also this assertion is very unsafe and I would not do this in production code. But it gets the point across :)
此外，这个断言非常不安全，我不会在生产代码中这样做。但它得到了重点:)
More
更多的
A more up to date book chapter on assertions : https://basarat.gitbooks.io/typescript/content/docs/types/type-assertion.html
关于断言的最新书籍章节：https: //basarat.gitbooks.io/typescript/content/docs/types/type-assertion.html
回答by shabunc
I've read @basarat's answer and decided to post my own since I strongly believe that there's still some room for explanation.
我已阅读@basarat 的回答并决定发布我自己的回答，因为我坚信仍有一些解释空间。
Be warned, <number><any>casting won't generate a number. In fact it will allow your code to be compiled (thus you'll pass all static typing checks) but it won't be a number in javascript. Consider this snippet of code:
请注意，<number><any>投射不会生成数字。事实上，它允许编译您的代码（因此您将通过所有静态类型检查），但它不会是 javascript 中的数字。考虑这个代码片段：
let str = "1000";
let a: number = <number><any>str;
let b: number = parseInt(str); //or just let b = parseInt(str)
console.log(typeof a); // string
console.log(typeof b); // number
I hardly can imagine cases when a-case is beneficial compared to b-case. I'd go with just parseInt or parseFloat or Number, whatever fits more. <T><any>casting looks smart but you must be 100% sure what you are supposed to achieve by that idiom.
我很难想象a-case 比 -case 更有利的b情况。我只会使用 parseInt 或 parseFloat 或 Number，无论哪种更适合。<T><any>铸造看起来很聪明，但你必须 100% 确定你应该通过那个习语实现什么。
And in most cases you probably don't want to achieve that )
在大多数情况下，您可能不想实现这一点）
回答by Lonli-Lokli
I prefer this variant
我更喜欢这个变种
let SomeNumberAsAString = '1000';
let a = +SomeNumberAsAString;
console.log(a);回答by Ash Blue
Here is the cleanestway to do it.
这是最干净的方法。
const numberString = '1000';
const a: int = numberString as any;
回答by Alex Chaliy
const a: number = <number> +SomeNumberAsAString;
+SomeNumberAsAStringconverts the string value to the number.
+SomeNumberAsAString将字符串值转换为数字。
<number>before +SomeNumberAsAStringsays to Typescript compiler that we need to cast the type of the value to type number.
<number>之前+SomeNumberAsAString对 Typescript 编译器说我们需要将值的类型转换为 type number。
