类型定义中的感叹号 [英] Exclamation Mark in type definition

问题描述

目前，我偶然发现了Angular的 ContentChildren装饰器.在第一个代码示例中，使用以下语法:

Currently I stumbled upon the ContentChildren decorator of Angular. In the first code example the following syntax is used:

import {AfterContentInit, ContentChildren, Directive, QueryList} from '@angular/core';

@Directive({selector: 'child-directive'})
class ChildDirective {
}

@Directive({selector: 'someDir'})
class SomeDir implements AfterContentInit {
  @ContentChildren(ChildDirective) contentChildren !: QueryList<ChildDirective>;  // this line is relevant

  ngAfterContentInit() {
    // contentChildren is set
  }
}

请注意在@ContentChildren(ChildDirective) contentChildren变量定义之后的感叹号和冒号.在此StackOverflow线程中，我发现了在访问变量或对象属性时，可以将该语法用作非null断言运算符".

Note the exclamation mark followed by a colon right after the @ContentChildren(ChildDirective) contentChildren variable definition. In this StackOverflow thread I discovered that this syntax can be used as a "non-null assertion operator" when accessing a variable or object property.

我的问题是，类型定义之前的感叹号是否具有与正常情况下完全相同的含义.它只是简单地说TypeScript编译器:好吧，不用担心null或未定义"，还是在此上下文中此语法还有其他特定含义?

My question is now whether the exclamation mark before a type definition has exactly the same meaning like in a normal context. Does it simply say the TypeScript compiler: "Okay, don't worry about null or undefined", or does this syntax have another specific meaning in this context?

推荐答案

我的问题是，类型定义之前的感叹号是否具有与正常情况下完全相同的含义.

My question is now whether the exclamation mark before a type definition has exactly the same meaning like in a normal context.

不，这实际上不是一回事，在这种情况下，它所做的事情有所不同.通常，当您声明成员(类型中不包含undefined)时，必须直接或在构造函数中对其进行初始化.如果在名称后添加!，TypeScript将忽略此名称，并且如果您不立即对其进行初始化，则不会显示错误:

No that's actually not the same thing, in this context it does something different. Normally when you declare a member (which doesnt' include undefined in it's type) it has to be initialized directly or in the constructor. If you add ! after the name, TypeScript will ignore this and not show an error if you don't immediately initialize it:

class Foo {
  foo: string; // error: Property 'foo' has no initializer and is not definitely assigned in the constructor.
  bar!: string; // no error
}

同样的事情实际上也适用于局部变量:

The same thing actually applies to local variables as well:

let foo: string;
let bar!: string;

console.log(foo); // error: Variable 'foo' is used before being assigned.
console.log(bar); // no error

游乐场

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