The String.Trim(), String.StartWith and String.EndWith methods are very useful methods that are prebuilt into the .NET framework, that need a client-side implementation. Below are the javascript equivalents that I have found to be extremely helpful.

String.prototype.trim = function(){return (this.replace(/^[\s\xA0]+/, "").replace(/[\s\xA0]+$/, ""))}

String.prototype.startsWith = function(str) {return (this.match("^"+str)==str)}

String.prototype.endsWith = function(str)  {return (this.match(str+"$")==str)}

Credit: http://www.tek-tips.com/faqs.cfm?fid=6620

Also, The ResolveUrl() method that I (we) use a lot in ASP.NET development. Here is a Javascript equivalent. Note: This requires some help from ASP.NET to provide the resolution of the "~/" and needs to be inline in the HTML.

function ResolveUrl(url) {
 if (url.match("^~/") == "~/") {
  return url.replace("~/", "<%= ResolveUrl("~/") %>");
 }
 
 return url;
}

I hope this helps you.