TypeScript 中的类型断言运算符是什么? [英] What is the type assertion operator for in TypeScript?

问题描述

规范并没有过多说明类型断言运算符在 TypeScript 中的哪些方面可能会有所帮助.我的代码中不需要它.所以我很好奇它应该解决什么样的问题.有什么想法吗?

The spec doesn't say much about where the type assertion operator might be helpful in TypeScript. I didn't need it in my code. So I am curious what sort of problems it is supposed to solve. Any ideas?

推荐答案

它有点像 类型转换 但是因为它没有运行时支持(它只是一个编译时断言)TypeScript 选择调用它是类型断言".考虑这个例子:

It's somewhat like type casting however as it does not come with runtime support (its a compile time assertion only) TypeScript choses to call it 'Type Assertion'. Consider this example :

var element1 = document.getElementById('canvas'); // Determined to be HTMLElement
element1.getContext('2d'); // ERROR as it is HTMLElement 



// Determined to be canvas due to your assertion
var element2 = <HTMLCanvasElement>document.getElementById('canvas'); 
element2.getContext('2d'); // Valid 

每当 typescript 类型推断由于不兼容的推断类型而阻止您分配东西时，您将需要它.

You will need it whenever typescript type inference would prevent you to assign things around due to incompatible inferred types.

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