如何在 JavaScript 中创建 CoffeeScript 风格的存在运算符?

问题描述

CoffeeScript 将 user?.id 变成

CoffeeScript turns user?.id into

if (typeof user !== "undefined" && user !== null) {
   user.id;
}

是否可以创建一个 JavaScript 函数 exists 来做类似的事情?即

Is it possible to create a JavaScript function exists that would do something similar? i.e.

exists(user).id

将导致 user.id 或 null

如果一个函数接受另一个参数，即 exists(user, 'id') 会更容易，但这看起来不太好.

It would be easier if a function accepts another parameter, i.e. exists(user, 'id'), but that wouldn't look as nice.

推荐答案

不，你不能产生这样的功能.问题在于:

No, you can't produce such a function. The problem is that this:

any_function(undeclared_variable)

如果 undeclared_variable 没有在任何地方声明，

将产生一个 ReferenceError.例如，如果您运行此独立代码:

will produce a ReferenceError if undeclared_variable was not declared anywhere. For example, if you run this stand alone code:

function f() { }
f(pancakes);

你会得到一个 ReferenceError 因为 pancakes 没有在任何地方声明.演示:http://jsfiddle.net/ambiguous/wSZaL/

you'll get a ReferenceError because pancakes was not declared anywhere. Demo: http://jsfiddle.net/ambiguous/wSZaL/

但是，typeof operator 可用于尚未声明的内容，因此:

However, the typeof operator can be used on something that has not been declared so this:

console.log(typeof pancakes);

将简单地在控制台中记录一个 undefined.演示:http://jsfiddle.net/ambiguous/et2Nv/

will simply log an undefined in the console. Demo: http://jsfiddle.net/ambiguous/et2Nv/

如果您不介意可能的 ReferenceErrors，那么您的问题中已经有了必要的功能:

If you don't mind possible ReferenceErrors then you already have the necessary function in your question:

function exists(obj, key) {
    if (typeof obj !== "undefined" && obj !== null)
        return obj[key];
    return null; // Maybe you'd want undefined instead
}

或者，由于您不需要能够在此处对未声明的变量使用 typeof，您可以将其简化为:

or, since you don't need to be able to use typeof on undeclared variables here, you can simplify it down to:

function exists(obj, key) {
    if(obj != null)
      return obj[key];
    return null;
}

注意对 != 的更改，undefined == null 为真，即使 undefined === null 不是.

Note the change to !=, undefined == null is true even though undefined === null is not.

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