Javascript 执行上下文，鼠标处理程序内的反直觉范围链

Question

这是一个有趣的范围链案例，在很多文档中都没有解释，我觉得很难理解。如果有人能花时间阅读下面注释良好的代码并解释变量是如何得到解决的，那就太好了

我在一个文档上有两个矩形 (DIV)。我在两者上以及在我注册 mouseup 的 mousedown 侦听器中为 mousedown 注册事件侦听器。 mouseup 的监听器中发生了奇怪的事情。

通过使用不同的参数值两次调用 testfunc 来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);
    }            
}

Answer 1

这是因为引用函数的变量没有用var 声明，所以它们是全局的。

它们将被第二个调用覆盖，该调用在 X 附近关闭，值为 9。

---

编辑： 这就是正在发生的事情。为简单起见，我将所有变量都放在同一个范围内。

   // "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'

...或者让代码更具体一点：

  // 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.