javascript - 为什么我必须在闭包中声明一个函数来访问闭包中定义的变量？

Question

为什么我必须在闭包中声明一个函数才能访问闭包中的变量？我期望能够在闭包之外定义函数，但在闭包周围定义该函数，提供所需的变量，但除非该函数实际上在闭包内定义，否则这些变量不可用。

http://jsfiddle.net/c5oba93a/1/

//reusable function defined outside of closure, I want to swap out myMethod within this function.
var reusableFunction = function () {
    //common code
    try {
        console.log(myVar);
        myMethod.call(this);
    } catch (e) {
        console.log(e);
    }
    //common code
};


var functOneForClosure = function () {
    console.log('functOne');
};
var functTwoForClosure = function () {
    console.log('functTwo');
};

//myMethod and myVar are undefined in reusableFunction
(function () {
    var myMethod = functOneForClosure;
    var myVar = 'variable in closure with functOne';
    return reusableFunction;
})()();
(function (myMethodIn) {
    var myMethod = myMethodIn;
    var myVar = 'variable in closure with functTwo';
    return reusableFunction;
})(functOneForClosure)();

//if the function is defined within the closure it behaves as expected.
var getReusableVersion =(function (myMethod) {
    var myVar = 'defining function within closure makes the difference';
    return function () {
        //common code
        try {
            console.log(myVar);
            myMethod.call(this);
        } catch (e) {
            console.log(e);
        }
        //common code
    };
});

getReusableVersion(functOneForClosure)();

Answer 1

@pherris 根据您之前的评论:这没有帮助。 IIFE 内的 this 引用了 window 对象，现在您可以使用其他属性来垃圾邮件它。但是reusableFunction中的myVar和myMethod仍然是未定义的，你必须使用this.myVar和this.myMethod来代替。比向窗口发送垃圾邮件更好的方法是将变量作为参数传递:

var reusableFunction = function (myVar, myMethod) {
    // no try/catch needed anymore
    console.log(myVar);
    if (typeof myMethod == 'function') myMethod.call(this);
    else console.log(myMethod);
};

现在有两种方法可以通过 IIFE 调用它。

第一个从 IIFE 返回函数并绑定(bind)参数，然后执行它:

(function () {
    var myMethod = functOneForClosure;
    var myVar = 'variable in closure with functOne';

    // first argument of .bind() appears in the function as 'this',
    // the others appear as fixed arguments of the function
    return reusableFunction.bind(window, myVar, myMethod);
})()();

第二种方式在 IIFE 内部调用它，因此后面的调用被删除:

(function () {
    var myMethod = functOneForClosure;
    var myVar = 'variable in closure with functOne';

    return reusableFunction(myVar, myMethod);

    // of course its possible to call on 'this', but that doesn't change anything:
    /* return reusableFunction.call(this, myVar, myMethod); */

})();