example.js

var getData = new JSONPInterface();

function callback(data) {
}

getData("http://example.com/?callback=", callback);

jsonpinterface.js

function JSONPInterface() {
  var script;
  var fn;

  return function(url, callback) {
    if (script) {
      document.body.removeChild(script);
    }

    fn = "__callback_" + Math.random().toString(36).substring(7);
    
    window[fn] = function() {
      callback.apply(this, arguments);
      delete window[fn];
    };

    script = document.createElement("script");
    script.src = url + fn;

    document.body.appendChild(script);
  };
}