annotated.js

function(
  a, // A DOM element
  b, // an event name such as "click"
  c, // a handler function
  d  // (placeholder)
){
  a = a || document; // use the document by default
  d = a[             // save the current onevent　handler
    b = "on" + b     // prepend the event name with "on"
  ];
  b = a[b] =                 // cache and replace the current handler
    function(e) {            // with a function that
      d = d && d(            // executes/caches the previous handler
        e = e || a.event     // with a cross-browser object,
      );

      return (c = c && b(e)) // and executes/caches the passed function,
        ? b                  // returning the current handler if it rebinds
        : d                  // and the previous handler otherwise.
    };
  a = this // cache the window to fetch IE events
}

index.js

function(a,b,c,d){a=a||document;d=a[b="on"+b];b=a[b]=function(e){d=d&&d(e=e||a.event);return(c=c&&b(e))?b:d};a=this}

LICENSE.txt

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
                    Version 2, December 2004

 Copyright (C) 2011 Jed Schmidt <http://jed.is>

 Everyone is permitted to copy and distribute verbatim or modified
 copies of this license document, and changing it is allowed as long
 as the name is changed.

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  0. You just DO WHAT THE FUCK YOU WANT TO.

package.json

{
  "name": "on",
  "keywords": ["bind", "events", "event", "DOM"]
}