Why does my if condition prevent either clause from executing?

I'm trying to re-write the URLs of a set of links that I select using a jQuery class selector. However, I only wish to re-write the links that don't alr开发者_如何转开发eady have a href attribute specified, so I put in an if/else construct to check for this... However, it's not working. It does work without the if else statement so I'm pretty sure that is where I screwed up. I'm new to both JavaScript and jQuery so sorry if my question is elementary and/or overly obvious.

   var url = window.location;
   var barTwitter = $("a.shareTwitter").attr('href');
   if (barTwitter).val() == "null") {
   $("a.barTwitter").attr('href','http://www.twitter.com/home?status='+ url +'');
     } else {
   $("a.barTwitter").attr('href',barTwitter);
   }

if (barTwitter).val() == "null") {

This is syntactically invalid (count the parentheses!). You rather want to do:

if (barTwitter.val() == "null") {

Further, the val() function only works on input elements which are wrapped by jQuery, not on element attribute values which are at end just normal variables. You rather want to compare normal variables against the literal null:

if (barTwitter == null) {

There are actually a few problems with your code... BalusC correctly describes the first one - syntax errors in your if condition - but you should probably consider some of the rest...

I'll start with your code corrected according to BalusC's answer, with comments added to describe what's happening:

var url = window.location; // obtain the URL of the current document
// select the href attribute of the first <a> element with a shareTwitter class
var barTwitter = $("a.shareTwitter").attr('href');
if (barTwitter == null) { // if that attribute was not specified,
   // set the attribute of every matching element to a combination of a fixed URL
   // and the window location 
   $("a.barTwitter").attr('href','http://www.twitter.com/home?status='+ url +'');
} else {
   // set the attribute of every matching element to that of the first 
   // matching element
   $("a.barTwitter").attr('href',barTwitter);
}

Other issues with your code

Ok... now the problems:

1. jQuery matches sets - a single selector can potentially match multiple elements. So if there are multiple links on the page with the shareTwitter class, you'll be pulling the href attribute for the first one, but changing all of them. That's probably not what you want, although if there is only a single link with that class then you don't care.
2. In the else clause, you're not actually modifying the href at all... Unless you have multiple matching links, in which case you'll change all of them such that they have the href of the first one. Again, probably not what you want, although irrelevant if there is only one link... So, in the best-case scenario, the else clause is pointless and could be omitted.
3. You can actually omit the if/else construct entirely: jQuery allows you to test for the existence of attributes in the selector itself!
4. You're including the URL of the current page in the querystring of your new, custom URL - however, you're not properly escaping that URL... This could cause problems, as full URLs generally contain characters that are not strictly valid as part of URL querystrings.

Notes on working with JavaScript

A quick aside: if you plan on doing any development using JavaScript, you should obtain some tools. At minimum, install Firebug and familiarize yourself with the use of that and JSLint. The former will inform you of errors when the browser fails to parse or execute your code (in addition to many, many other useful debugging and development tasks), and the latter will check your code for syntax and common style errors: in this case, both tools would have quickly informed you of the initial problems with your code. Instructing you in the proper use of these tools is beyond the scope of this answer, but trust me - you owe it to yourself to take at least a few hours to read up on and play with them.

Toward safer code

Ok, back to the task at hand... Here's how I would re-write your code:

var url = window.location; // obtain the URL of the current document
// escape URL for use in a querystring
url = encodeURIComponent(url);
// select all <a> elements with a shareTwitter class and no href attribute
var twitterLinks = $("a.shareTwitter:not([href])");
// update each selected link with a new, custom link
twitterLinks.attr('href', 'http://www.twitter.com/home?status='+ url +'');

Note that even though this new code accomplishes the same task, it does so while avoiding several potential problems and remaining concise. This is the beauty of jQuery...

firs of all your syntax is screwed up: if (barTwitter).val() == "null") should be if (barTwitter.val() == "null") or if ((barTwitter).val() == "null")

Secondly barTwitter is either going to be a string or null so you cant call val which is a jQuery Object method specific to input elements.

Lastly you probably dont want to compare to null because it possible the value will be an empty string. Thus its better to use length property or some other method. A sample with lenght is below.. but im not sure what attr returns if if ther eis no value... check the docs.

var url = window.location;
   var barTwitter = $("a.shareTwitter").attr('href');
   if (barTwitter.length < 1) {
   $("a.barTwitter").attr('href','http://www.twitter.com/home?status='+ url +'');
     } else {
   $("a.barTwitter").attr('href',barTwitter);
   }