Something

function createMyMouseDownHandler(stuffINeed) {
  // Notice that stuffINeed isn't passed into the event handler. It's captured in a closure: this is called currying.
  var somethingElseINeed = getSomethingElseFromStuff(stuffINeed);

  return function (evt) {
    doSomethingWithEventTargetAndStuffINeed(stuffINeed, somethingElseINeed, evt.target);
  };
}

function bindMouseDownToMyItem () {
  var stuffINeedInAnEventHandler = foo;
  myItem.mousedown(createMyMouseDownHandler());
}