With PHP

echo urlencode("http://www.image.com/?username=unknown&password=unknown");

Result

http%3A%2F%2Fwww.image.com%2F%3Fusername%3Dunknown%26password%3Dunknown

With Javascript:

var myUrl = "http://www.image.com/?username=unknown&password=unknown";
var encodedURL= "http://www.foobar.com/foo?imageurl=" + encodeURIComponent(myUrl);

DEMO: http://jsfiddle.net/Lpv53/

You can use one of these:

• https://developer.mozilla.org/en-US/docs/Web/API/URL
• https://developer.mozilla.org/en/docs/Web/API/URLSearchParams

Example:

var url = new URL("http://foo.bar/?x=1&y=2");

// If your expected result is "http://foo.bar/?x=1&y=2&x=42"
url.searchParams.append('x', 42);

// If your expected result is "http://foo.bar/?x=42&y=2"
url.searchParams.set('x', 42);

You can use url.href or url.toString() to get the full URL

URLSearchParams has increasing browser support.

const data = {
  var1: 'value1',
  var2: 'value2'
};

const searchParams = new URLSearchParams(data);

// searchParams.toString() === 'var1=value1&var2=value2'

Node.js offers the querystring module.

const querystring = require('querystring');

const data = {
  var1: 'value1',
  var2: 'value2'
};

const searchParams = querystring.stringify(data);

// searchParams === 'var1=value1&var2=value2'

encodeURIComponent() should work. For example,

'&url=' + encodeURIComponent("http://a.com/?q=query&n=10")

produces

"&url=http%3A%2F%2Fa.com%2F%3Fq%3Dquery%26n%3D10"

(which doesn't have any & or ? in the value). When your server gets this url, it should be able to decode that to get the original:

param["url"] = "http://a.com/?q=query&n=10"

I'm not sure what server you're using (e.g. Rails, Django, ...) but that should work "out of the box" on any normal system.

2.5 years after the question was asked you can safely use Array.forEach. As @ricosrealm suggests, decodeURIComponent was used in this function.

function getJsonFromUrl(url) {
  if(!url) url = location.search;
  var query = url.substr(1);
  var result = {};
  query.split("&").forEach(function(part) {
    var item = part.split("=");
    result[item[0]] = decodeURIComponent(item[1]);
  });
  return result;
}

actually it's not that simple, see the peer-review in the comments, especially:

• hash based routing (@cmfolio)
• array parameters (@user2368055)
• proper use of decodeURIComponent and non-encoded = (@AndrewF)
• non-encoded + (added by me)

For further details, see MDN article and RFC 3986.

Maybe this should go to codereview SE, but here is safer and regexp-free code:

function getSearchOrHashBased(url) {
  if(!url) url = location.href;
  var question = url.indexOf("?");
  var hash = url.indexOf("#");
  if(hash==-1 && question==-1) return "";
  if(hash==-1) hash = url.length;
  return question==-1 || hash==question+1
    ? url.substring(hash)
    : url.substring(question+1, hash);
}

// use query = getSearchOrHashBased(location.href)
// or query = location.search.substring(1)
function getJsonFromUrl(query) {
  var result = {};
  query.split("&").forEach(function(part) {
    if(!part) return;
    part = part.replaceAll("+", " ");
    var eq = part.indexOf("=");
    var key = eq>-1 ? part.substring(0,eq) : part;
    var val = eq>-1 ? decodeURIComponent(part.substring(eq+1)) : "";
    var from = key.indexOf("[");
    if(from==-1) result[decodeURIComponent(key)] = val;
    else {
      var to = key.indexOf("]",from);
      var index = decodeURIComponent(key.substring(from+1,to));
      key = decodeURIComponent(key.substring(0,from));
      if(!result[key]) result[key] = [];
      if(!index) result[key].push(val);
      else result[key][index] = val;
    }
  });
  return result;
}

This function can parse even URLs like

var url = "foo%20e[]=a%20a&foo+e[%5Bx%5D]=b&foo e[]=c";
// {"foo e": ["a a",  "c",  "[x]":"b"]}

var obj = getJsonFromUrl(url)["foo e"];
for(var key in obj) { // Array.forEach would skip string keys here
  console.log(key,":",obj[key]);
}
/*
  0 : a a
  1 : c
  [x] : b
*/

7 years after the question was asked the functionality was standardized as URLSearchParams and 4 more years after, the access can be further simplified by Proxy as explained in this answer, however that new one can not parse the sample url above.