Here's the reference solution:

  var url = require('url');
  var querystring = require('querystring');
  
  var addr = prompt();
  var query = url.parse(addr).query;
  var params = querystring.parse(query);
  console.log(url.resolve(addr, params.file));

We get parse the address obtained from `prompt()` then parse out the
querystring with `url.parse()` and parse that string into parameters with
`querystring.parse()`. Finally we use `url.resolve()` to resolve the file
parameter with respect to the address we obtained from `prompt()` earlier.

`path`, `events`, and `stream` are other node builtins that are
particularly useful in the browser.