！对象方法后在打字稿中使用运算符 [英] ! operator in typescript after object method

问题描述

我有一个对象X，它带有方法getY()，并用打字稿返回了带有方法a()的对象Y. 这是什么意思?

I have an object X with a method getY() returning an object Y with a method a(), in typescript. What does it mean an expression like this one:

X.getY()!.a()

我猜想!运算符用于检查null，但是它具体如何工作?语言在哪里定义?

I guess the ! operator is used to check against null, but how does it work concretely? Where is defined in the language?

推荐答案

它称为非null断言运算符"，它告诉编译器x.getY()不为null.

It's called the "Non-null assertion operator" and it tells the compiler that x.getY() is not null.

这是TypeScript 2.0的一项新功能，您可以在新页面，内容如下:

It's a new typescript 2.0 feature and you can read about it in the what's new page, here's what it says:

一个新的！修复后表达式运算符可用于断言其 在以下类型的上下文中，操作数是非null和undefined的: 检查者无法得出结论.具体来说，操作 X！产生x类型的值，其中排除了null和undefined. 类似于形式为x和x的类型断言，即T！ 非null断言运算符被简单地删除在发出 JavaScript代码.

A new ! post-fix expression operator may be used to assert that its operand is non-null and non-undefined in contexts where the type checker is unable to conclude that fact. Specifically, the operation x! produces a value of the type of x with null and undefined excluded. Similar to type assertions of the forms x and x as T, the ! non-null assertion operator is simply removed in the emitted JavaScript code.

// Compiled with --strictNullChecks
function validateEntity(e?: Entity) {
    // Throw exception if e is null or invalid entity
}

function processEntity(e?: Entity) {
    validateEntity(e);
    let s = e!.name;  // Assert that e is non-null and access name
}

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编辑

记录此功能有一个问题:记录非空断言运算符(！)

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Edit

There's an issue for documenting this feature: Document non-null assertion operator (!)

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