Encoding spaces to + in javascript

Use the data parameters of jQuery's AJAX methods and you won't need to escape anything - it'll all be done for you by jQuery.

Furthermore, this query is probably supposed to be a GET, rather than a POST. When you assign to location.href you cause a page reload of the new URL, rather than a true AJAX request.

var result = $("select#model option:selected").html();
$.get("advanced_search.php", {
     keywords: result
}, function() { ... });

IMHO there is almost never a good reason to create POST or GET parameters by hand - it's far simpler to use an object of key / value pairs.

You want encodeURIComponent which not only replaces spaces but anything else that may cause issues.

..."keywords=" + encodeURIComponent( result )

location.href="advanced_search.php?" + "keywords=" + result

should probably be

location.href="advanced_search.php?keywords=" + encodeURIComponent(result)

since if result contains a # then that will become part of the fragment instead of part of a query parameter. encodeURIComponent will properly escape a string so that your server returns result when you ask for the value of the "keywords" GET parameter.

As Mozilla's docs explain

To avoid unexpected requests to the server, you should call encodeURIComponent on any user-entered parameters that will be passed as part of a URI. For example, a user could type "Thyme &time=again" for a variable comment. Not using encodeURIComponent on this variable will give comment=Thyme%20&time=again. Note that the ampersand and the equal sign mark a new key and value pair.

If you have an already constructed URL, then

encodeURI('/advanced_search.php?keywords=2001-2004 Honda Civic TypeR')

== '/advanced_search.php?keywords=2001-2004%20Honda%20Civic%20TypeR'

The builtin encodeURI function takes a string and replaces all characters that can't appear in a URL with the %-encoded equivalent.

Encodes a Uniform Resource Identifier (URI) by replacing each instance of certain characters by one, two, three, or four escape sequences representing the UTF-8 encoding of the character (will only be four escape sequences for characters composed of two "surrogate" characters).

Any server should treat %20 in a URL as equivalent to + except that %20 works outside the path part of the URL.

If for some reason, you really need +, then after encodeURI it should be safe to replace %20 with + thus.

var myUrl = '/advanced_search.php?keywords=2001-2004 Honda Civic TypeR';
encodeURI(myUrl).replace(/%20/g, '+')

I would suggest you revisit your code just slightly, as POSTing data but just sending the information over the QueryString (everything after the ? in the URL) is missing the point just a little.

You should be sending your keywords in an object, like so:

var result = $("select#model option:selected").html();
$.post("advanced_search.php", { keywords: result }, function(data){
    // success callback code goes here
});

If your PHP looks for the $_POST['keywords'], you should find your data in there when the $post is executed by jQuery.

Sending it across the QueryString is not undoable, but in that case you really should be doing something along the lines of:

var result = $("select#model option:selected").html();
$.get("advanced_search.php", 
    { keywords: result }, 
    function(data){
        // success callback code goes here
    });

However, you should get used to POSTing data properly, as sending data over the querystring is both unsecure and prone to issues due to encoding and URL length limits.

Also, this begs the question: Why are you encoding the HTML from within the tag? Why are you not simply putting a simple key in your , like this:

<select id="model">
    <option value="key1">2001-2004 Honda Civic TypeR</option>
    <option value="key2">2005-2008 Honda Civic TypeR</option>
    <option value="key3">2009-2012 Honda Civic TypeR</option>
    <option value="key4">2013-2015 Honda Civic TypeR</option>
</select>

var result = $("select#model option:selected").val();
$.post("advanced_search.php", { keywords: result }, function(data){
    // success callback code goes here
});

Should be simpler to run the query, as well, considering you can have your values shortened and not require any mucking around with encoding/decoding values, unless you don't have keys for each select option, in which case, I guess I understand the approach you're taking.