JavaScript 错误 : "is not a function"

Question

看起来“$smth 不是一个函数”是 JavaScript 中一个非常常见的问题，但是在查看了相当多的线程之后，我仍然无法理解在我的情况下是什么导致了它。

我有一个自定义对象，定义为:

function Scorm_API_12() {
var Initialized = false;

function LMSInitialize(param) {
    errorCode = "0";
    if (param == "") {
        if (!Initialized) {
            Initialized = true;
            errorCode = "0";
            return "true";
        } else {
            errorCode = "101";
        }
    } else {
        errorCode = "201";
    }
    return "false";
}

// some more functions, omitted.
}

var API = new Scorm_API_12();

然后在另一个脚本中，我尝试按以下方式使用此 API:

var API = null;

function ScormProcessInitialize(){
    var result;

    API = getAPI();

    if (API == null){
        alert("ERROR - Could not establish a connection with the API.");
        return;
    }

    // and here the dreaded error pops up
    result = API.LMSInitialize("");

    // more code, omitted
    initialized = true;
}

getAPI() 的东西，看起来像这样:

var findAPITries = 0;

function findAPI(win)
{
   // Check to see if the window (win) contains the API
   // if the window (win) does not contain the API and
   // the window (win) has a parent window and the parent window
   // is not the same as the window (win)
   while ( (win.API == null) &&
           (win.parent != null) &&
           (win.parent != win) )
   {
      // increment the number of findAPITries
      findAPITries++;

      // Note: 7 is an arbitrary number, but should be more than sufficient
      if (findAPITries > 7)
      {
         alert("Error finding API -- too deeply nested.");
         return null;
      }

      // set the variable that represents the window being
      // being searched to be the parent of the current window
      // then search for the API again
      win = win.parent;
   }
   return win.API;
}

function getAPI()
{
   // start by looking for the API in the current window
   var theAPI = findAPI(window);

   // if the API is null (could not be found in the current window)
   // and the current window has an opener window
   if ( (theAPI == null) &&
        (window.opener != null) &&
        (typeof(window.opener) != "undefined") )
   {
      // try to find the API in the current window�s opener
      theAPI = findAPI(window.opener);
   }
   // if the API has not been found
   if (theAPI == null)
   {
      // Alert the user that the API Adapter could not be found
      alert("Unable to find an API adapter");
   }
   return theAPI;
}

现在，API可能已找到，因为我没有收到“无法找到...”消息，代码继续尝试初始化它。但 firebug 告诉我 API.LMSInitialize 不是一个函数，如果我尝试使用 alert(Object.getOwnPropertyNames(API)); 调试它，它会给我一个空白警报。

我错过了什么？

Answer 1

有关调试此类问题的更多通用建议，MDN 有一篇很好的文章 TypeError: "x" is not a function :

It was attempted to call a value like a function, but the value is not actually a function. Some code expects you to provide a function, but that didn't happen.

Maybe there is a typo in the function name? Maybe the object you are calling the method on does not have this function? For example, JavaScript objects have no map function, but JavaScript Array object do.

基本上，对象(js中的所有函数也是对象)并不存在于你认为存在的地方。这可能有许多原因包括(不是一个广泛的列表):

• 缺少脚本库
• 错别字
• 该函数位于您当前无权访问的范围内，例如:

var x = function(){
   var y = function() {
      alert('fired y');
   }
};
    
//the global scope can't access y because it is closed over in x and not exposed
//y is not a function err triggered
x.y();

• 您的对象/函数没有您调用的函数:

var x = function(){
   var y = function() {
      alert('fired y');
   }
};
    
//z is not a function error (as above) triggered
x.z();