how to get the value from a callback function

问题:

I am relatively new to javascript and I am facing some difficulty.I have two java script files as I have shown below. I am having trouble getting the value of the variable entry_title inside the getRss function and storing it inside the variables Rss1_title and Rss2_title . Creating a global variable and assigning it to entry_title will make things worse as I will not be able to know from which Rss url the title came from. Is there a easy way to get the value of the callback functions ?

<script type="text/javascript" src="jsRss.js"></script> <script type="text/javascript" src="notification.js"></script> 

My notification.js file

    function get_rss1_feeds(){         var Rss1_title = getRss("http://yofreesamples.com/category/free-coupons/feed/?type=rss");     }      function get_rss2_feeds(){         var Rss2_title = getRss("http://yofreesamples.com/category/real-freebies/feed/?type=rss");       } setTimeout('get_rss1_feeds()',8000); setTimeout('get_rss2_feeds()',7000); 

My jsRss.js file:

function getRss(url){      if(url == null) return false;      google.load("feeds", "1");      // Our callback function, for when a feed is loaded.     function feedLoaded(result) {         if (!result.error) {              var entry = result.feed.entries[0];                          var entry_title = entry.title;      // need to get this value                    }      }            function Load() {         // Create a feed instance that will grab feed.         var feed = new google.feeds.Feed(url);         // Calling load sends the request off.  It requires a callback function.         feed.load(feedLoaded);                   }     google.setOnLoadCallback(Load);              } 

Errors :

When the setTimeout(get_rss1_feeds, 8000); method is called I get a blank screen. I get a error in my console saying octal literals and octal escape sequences are deprecated and it is pointing to the 6th line in this script. Is it because I am using google-api for parsing my Rss?

if (window['google'] != undefined && window['google']['loader'] != undefined) { if (!window['google']['feeds']) { window['google']['feeds'] = {}; google.feeds.Version = '1.0'; google.feeds.JSHash = '8992c0a2cdf258e5bd0f517c78243cd6'; google.feeds.LoadArgs = 'file\75feeds\46v\0751'; } google.loader.writeLoadTag("css", google.loader.ServiceBase + "/api/feeds/1.0/8992c0a2cdf258e5bd0f517c78243cd6/default+en.css", false); google.loader.writeLoadTag("script", google.loader.ServiceBase + "/api/feeds/1.0/8992c0a2cdf258e5bd0f517c78243cd6/default+en.I.js", false); }  

回答1:

Seeing as it's a different scope, you can either return it in a callback, or provide it in another way such as exporting it to a higher scope that is visible to your desired location. In this case, it's the global scope, so I'd advise against that.

function getRss(url, callback) { //... function feedLoaded(result) {     if (!result.error) {         var entry = result.feed.entries[0];         var entry_title = entry.title; // need to get this value         callback && callback(entry_title);             } } 

and call it like so,

function get_rss1_feeds() {     var Rss1_title = getRss("http://yofreesamples.com/category/free-coupons/feed/?type=rss", function(entry_title) {         // This scope has access to entry_title     }); } 

As an aside, use your setTimeout like so:

setTimeout(get_rss1_feeds, 8000); 

rather than

setTimeout("get_rss1_feeds()", 8000); 

as the latter uses eval, whereas the former passes a reference to the function.

回答2:

Eventhough it will make your code a mess, you can append the variables to the window object. For example:

function a() {  window.testStr = "test";  }  function b() {  alert(window.testStr); } 

Or even create your own object, instead of using window, as such:

var MyRSSReader = {  TitleOne : '',  TitleTwo : ''  }   MyRSSReader.TitleOne = "My title"; 

Wikipedia has a nice article about global variables, and why they are bad.

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