Javascript执行上下文，鼠标处理程序中的反直观作用域链

这是一个有趣的范围链案例，在我发现很难理解的很多文档中都没有解释。如果有人能花时间阅读下面注释良好的代码，并解释如何解析变量，那将非常好

我在一个文档上有两个矩形（div）。我在mousedown上注册mousedown的事件侦听器，并在mousedown侦听器中注册mouseup。在穆塞普的听者中发生了奇怪的事情

通过使用不同的参数值调用testfunc两次，可以创建两个执行上下文：

window.onload = function() {
     test_func("horizontal"); // First Execution context
     test_func("vertical");  // Second Execution Context
}

在第一个矩形（水平）的mouseup侦听器中，正在使用第二个执行上下文（垂直），这是违反直觉的：

function test_func(dir) {
    var X = 9; // variable which helps to track the execution contexts
    if(dir === "horizontal")
       X = 1; // leave at 9 if vertical

    mouseup_vert = function() {}
    mouseup_horiz = function() {
        // Here the value of X I am getting is 9 whereas I am expecting 11
        // QUESTION: Why I am getting the second execution context??
    }
    mousedown_vert = function() {
        // As expected the value of X here is 9
        X=99; 
        // set X to 99 to check if during mouseup same exec context is picked
        document.addEventListener("mouseup", mouseup_vert, false);
    }
    mousedown_horiz = function() {
        // As expected value of X is 1, so using first execution context
        X=11;
        // set this to check if during mouseup I get a value of 11
        document.addEventListener("mouseup", mouseup_horiz, false);
    }

    if (dir === "horizontal") {
        e = document.getElementById("horiz");           
        e.addEventListener("mousedown", mousedown_horiz, false);
    } else {
        e = document.getElementById("vert");            
        e.addEventListener("mousedown", mousedown_vert, false);
    }            
}

---

这是因为引用函数的变量不是用

var

声明的，所以它们是全局变量

它们将被第二个调用覆盖，该调用将在

X

附近关闭，值为

9

---

编辑：以下是实际发生的情况。为简单起见，我在同一范围内创建所有变量

…或使代码更具体一点：

---

确切地那些“mouseup_foo”变量都是全局变量。它们必须用

var

关键字声明！！另外，从事件处理程序内部设置事件处理程序将是一个大问题。如果它是全局的，那么在mousedown_horiz中X将被设置为11，这是我应该在mouseup_horiz中看到的值，而不是我看到的值，因此我的question@Raks符号“mouseup_vert”、“mousedown_vert”、“mouseup_horiz”和“mousedown_horiz“它们是全球性的。毫无疑问。@Raks：它没有显示更新的值的原因是，在下一次调用覆盖该函数实例之前，您正在引用该函数实例。

mouseup\u-vert/\u-horiz

函数在mousedown事件发生之前不会被引用，因此它们会获得全局变量的最新版本，该变量现在引用第二次

test\u-func

调用中的函数……换句话说，您立即引用了存储在

mousedown\u-horiz/\u-vert

中的函数，因此，在它们有机会被覆盖之前，它们是安全的。由于您对存储在

mouseup\u vert/\u horiz

中的函数的引用被延迟，因此在引用它们之前，它们有可能被覆盖。

   // "a" is referencing a function.
var a = function() { alert( 'a' ); };

   // "b" is referencing the same function that "a" is referencing.
var b = a;

   // The reference "a" holds to the function is being overwritten with a 
   //     reference to a new function.
a = function() { alert( "I'm a new function" ); };

   // "c" is referencing the same function that "a" is referencing (the new one).
var c = a;

   // So what is "b" referencing? It is still the original function, because it
   //    picked up that reference *before* the original reference "a" held was
   //    overwritten.

b(); // alerts 'a'

  // reference a function
var mouseup_vert = function() { alert( "I'm a mouseup function." ); };


  // reference a function
var mousedown_vert = function() {
                    // When this function is invoked, add a listener using
                    //    whatever function is referenced by mouseup_vert
                 document.addEventListener( "mouseup", mouseup_vert, false);
};


  // The function currently referenced by "mousedown_vert" is passed here
  //    as the event handler.
document.addEventListener("mousedown", mousedown_vert, false);


  // Overwrite "mouseup_vert" just for kicks.
mouseup_vert = function() { alert( "I'm a completely different function." ); };


  // NOW trigger a "mousedown" event. What happens? The function passed as the
  //    handler is invoked. 

  // What does that function do? It adds a "mouseup" handler. 

  // Which variable is referenced? "mouseup_vert".

  // What is the current value of "mouseup_vert"? The new function, since we
  //    overwrote the reference to the original.