How to encode a query string so that it is the value of another query string in javascript?

encodeURIComponent will work. (You may or may not want the leading ‘?’, depending on what the script is expecting.)

var c="d e"
var query= '?a=b&c="+encodeURIComponent(c);
var uri= "http://www.example.com/script?query='+encodeURIComponent(query);
window.location= uri;

Takes me to:

http://www.example.com/script?query=%3Fa%3Db%26c%3Dd%2520e

When you hover over that it may appear once-decoded in the browser’s status bar, but you will end up in the right place.

escape/unescape() is the wrong thing for encoding query parameters, it gets Unicode characters and pluses wrong. There is almost never a case where escape() is what you really need.