user input to URL

You could build out manual query string parsers and constructors, an example would be like:

function parseQuery(qstr){ var query = {}; var a = qstr.split('&'); //take the passed query string and split it on &, creating an array of each value for (var i in a) { //iterate the array of values var b = a[i].split('='); //separate the key and value pair query[decodeURIComponent(b[0])] = decodeURIComponent(b[1]); //call decodeURIComponent to sanitize the query string } return query; //returned the parsed query string object } function buildQuery(obj){ var str = []; for(var p in obj) //iterate the query object if (obj.hasOwnProperty(p)) { //check if the object has the propery name we're iterating str.push(encodeURIComponent(p) + "=" + encodeURIComponent(obj[p])); //push the encoded key value pair from the object into the string array } return str.join("&"); //take the array of key value pairs and join them on & } 

Then below we take the string that you gave, for example:

var $str = 'createimage&t1=1412757517&t2=1412843917&assumeinitialstates=yes&assumestatesduringnotrunning=yes&initialassumedhoststate=0&initialassumedservicestate=0&assumestateretention=yes&includesoftstates=no&host=SCP-3&service=MODIFICATION+TIME+EDR+FILES&backtrack=4&zoom=4'; 

Now we call the parseQuery function on our string.

var obj = parseQuery($str); 

Then we iterate the object which was produced from our parseQuery function

Object.keys(obj).forEach(function(k, i) { switch(k){ case 't1': obj[k] = 'replacedt1'; break; case 'service': obj[k] = 'replacedServices'; break; case 'host': obj[k] = 'replacedHost'; } }); 

Now the obj variable has the newly updated values. We can rebuild the query using our buildQuery function by passing the object in.

console.log(buildQuery(obj)); 

Which will produce something like:

createimage=undefined&t1=replacedt1&t2=1412843917&assumeinitialstates=yes&assumestatesduringnotrunning=yes&initialassumedhoststate=0&initialassumedservicestate=0&assumestateretention=yes&includesoftstates=no&host=replacedHost&service=replacedServices&backtrack=4&zoom=4 

As usual, the jsFiddle

You can use the new URL object (for older browsers, there is a polyfill) :

var url = new URL("http://some-url/trends.cgi?createimage&t1=1412757517&t2=1412843917&assumeinitialstates=yes&assumestatesduringnotrunning=yes&initialassumedhoststate=0&initialassumedservicestate=0&assumestateretention=yes&includesoftstates=no&host=SCP-3&service=MODIFICATION+TIME+EDR+FILES&backtrack=4&zoom=4"); url.searchParams.set("t1", "someNewT1"); url.searchParams.set("t2", "someNewT2"); url.searchParams.set("host", "someNewHost"); url.searchParams.set("service", "someNewService"); alert(url.href); /* http://some-url/trends.cgi?host=someNewHost&assumestateretention=yes&initialassumedservicestate=0&t2=someNewT2&initialassumedhoststate=0&assumeinitialstates=yes&zoom=4&backtrack=4&createimage=&assumestatesduringnotrunning=yes&includesoftstates=no&service=someNewService&t1=someNewT1 */ 

Played with JavaScript a bit, I believe this solves your problem: http://jsfiddle.net/dk48vwz7/

var linktext = "http://site/some-url/trends.cgi?createimage&t1=1412757517&t2=1412843917&assumeinitialstates=yes&assumestatesduringnotrunning=yes&initialassumedhoststate=0&initialassumedservicestate=0&assumestateretention=yes&includesoftstates=no&host=SCP-3&service=MODIFICATION+TIME+EDR+FILES&backtrack=4&zoom=4"; //we'll use an in-memory "hyperlink" object for basic parsing var anchor = document.createElement("A"); anchor.href=linktext; //the query string starts with ?, we remove it. //then, split it by & symbol var queryvars = anchor.search.replace(/^\?/, '').split('&'); //now looping through all parts of query string, creating an object in form key->value var querycontent = {}; for( i = 0; i < queryvars.length; i++ ) { var queryvar = queryvars[i].split('='); querycontent[queryvar[0]] = queryvar[1]; } //this allows us to reference parts of the query as properties of "querycontent" variable querycontent.service = "AnotherService" //TODO: change the properties you actually need //and now putting it all back together var querymerged = []; var g = ""; for (var key in querycontent){ var fragment = key; if (querycontent[key]) { fragment += "=" + querycontent[key]; } querymerged.push(fragment); } anchor.search = querymerged.join("&") //finally, access the `href` property of anchor to get the link you need document.getElementById("test").innerText=anchor.href;