I have this
var date = $('#Date').val();
this get the value in the textbox what would look like this
12/31/2009
Now I do this on it
var id = 'c_' + date.replace("/", '');
and the result is
c_1231/2009
It misses the last '/' I don't understand why though.
You need to set the g flag to replace globally:
date.replace(new RegExp("/", "g"), '')
// or
date.replace(/\//g, '')
Otherwise only the first occurrence will be replaced.
Unlike the C#/.NET class library (and most other sensible languages), when you pass a String in as the string-to-match argument to the string.replace method, it doesn't do a string replace. It converts the string to a RegExp and does a regex substitution. As Gumbo explains, a regex substitution requires the global flag, which is not on by default, to replace all matches in one go.
If you want a real string-based replace — for example because the match-string is dynamic and might contain characters that have a special meaning in regexen — the JavaScript idiom for that is:
var id= 'c_'+date.split('/').join('');