如何立即调用IIFE防止其污染全球范围？ [英] How does an IIFE's being called immediately prevent it from polluting global scope?

问题描述

在关于立即调用的函数表达式的Udacity课程中（关于提供的代码片段），它说：

In a Udacity lesson on immediately invoked function expressions (regarding the provided code snippet) it says:

正在返回的函数结束（即捕获）
hi变量。这允许myFunction维护一个私有的，可变的
状态，该状态无法在函数外部访问！更重要的是：
因为表达的函数被立即调用，IIFE很好地将
包含在代码中，这样我们就不会污染全局范围。

我很难理解调用匿名函数会立即做什么来阻止变量 hi 从污染全球范围开始，由于 hi 已在函数中定义，是不是已经在本地/私有范围内了？

I'm strugggling to understand what calling the anonymous function immediately has to do with prevent the variable hi from "polluting the global scope," and since hi is already defined in a function, isn't it already in local/private scope?

const myFunction = (
  function () {
    const hi = 'Hi!';
    return function () {
      console.log(hi);
    }
  }
)();

推荐答案

在现代JavaScript中，你有让并阻止范围（我非常确定 const 也有块范围），所以你可以这样做：

In modern JavaScript, you have let and block scope (and I'm pretty sure const has block scope, too), so you could just do this:

let myFunction;
{
    let hi = 'Hi!';
    myFunction = function () {
        console.log(hi);
    };
}

这会创建 myFunction 没有将 hi 泄漏到周围的范围内。

This creates myFunction without leaking hi into the surrounding scope.

在传统的JavaScript中，只有 var 和函数范围，你可以这样做：

In traditional JavaScript, where you only have var and function scope, you could do this:

var myFunction;
function a_private_scope() {
    var hi = 'Hi!';
    myFunction = function () {
        console.log(hi);
    };
}
a_private_scope();

a_private_scope 限制<$ c的范围$ c> hi ，但是（它是一个函数声明）需要显式调用它，我们仍然会向周围的范围泄漏一个名称（这次它是 a_private_scope ，作为范围的函数的名称。。

a_private_scope limits the scope of hi, but (it being a function declaration) it needs to be called explicitly, and we still leak a name to the surrounding scope (this time it's a_private_scope, the name of the function-serving-as-a-scope).

通过使用函数表达式并立即呼吁它，我们避免这个名字污染：

By using a function expression and immediately calling it, we avoid this second name pollution:

var myFunction;
(function () {
    var hi = 'Hi!';
    myFunction = function () {
        console.log(hi);
    };
})();

现在外部范围内唯一定义的是 myFunction 。用作 hi 范围的匿名函数没有名称可能会污染周围的范围。

Now the only thing defined in the outer scope is myFunction. The anonymous function that serves as a scope for hi has no name it could pollute the surrounding scope with.

最后我们可以通过使用返回值来清理它，所以我们不必提及 myFunction 两次：

Finally we can clean it up a bit by using return values, so we don't have to mention myFunction twice:

var myFunction = function () {
    var hi = 'Hi!';
    return function () {
        console.log(hi);
    };
}();

（这也为我们节省了一对（ ）因为函数关键字不再出现在语句的开头。）

(This also saves us a pair of ( ) because the function keyword doesn't appear at the beginning of a statement anymore.)

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