Getting an array of all DOM events possible

All DOM events start with on. You can loop through any Element instance, and list all properties which start with on.

Example. Copy-paste the following code in the console (Firefox, using Array comprehensions ;)):

[i for(i in document)].filter(function(i){return i.substring(0,2)=='on'&&(document[i]==null||typeof document[i]=='function');})

Another method to get the events is by looking at the specification, which reveals:

  // event handler IDL attributes
  [TreatNonCallableAsNull] attribute Function? onabort;
  [TreatNonCallableAsNull] attribute Function? onblur;
  [TreatNonCallableAsNull] attribute Function? oncanplay;
  [TreatNonCallableAsNull] attribute Function? oncanplaythrough;
  [TreatNonCallableAsNull] attribute Function? onchange;
  [TreatNonCallableAsNull] attribute Function? onclick;
  [TreatNonCallableAsNull] attribute Function? oncontextmenu;
  [TreatNonCallableAsNull] attribute Function? oncuechange;
  [TreatNonCallableAsNull] attribute Function? ondblclick;
  [TreatNonCallableAsNull] attribute Function? ondrag;
  [TreatNonCallableAsNull] attribute Function? ondragend;
  [TreatNonCallableAsNull] attribute Function? ondragenter;
  [TreatNonCallableAsNull] attribute Function? ondragleave;
  [TreatNonCallableAsNull] attribute Function? ondragover;
  [TreatNonCallableAsNull] attribute Function? ondragstart;
  [TreatNonCallableAsNull] attribute Function? ondrop;
  [TreatNonCallableAsNull] attribute Function? ondurationchange;
  [TreatNonCallableAsNull] attribute Function? onemptied;
  [TreatNonCallableAsNull] attribute Function? onended;
  [TreatNonCallableAsNull] attribute Function? onerror;
  [TreatNonCallableAsNull] attribute Function? onfocus;
  [TreatNonCallableAsNull] attribute Function? oninput;
  [TreatNonCallableAsNull] attribute Function? oninvalid;
  [TreatNonCallableAsNull] attribute Function? onkeydown;
  [TreatNonCallableAsNull] attribute Function? onkeypress;
  [TreatNonCallableAsNull] attribute Function? onkeyup;
  [TreatNonCallableAsNull] attribute Function? onload;
  [TreatNonCallableAsNull] attribute Function? onloadeddata;
  [TreatNonCallableAsNull] attribute Function? onloadedmetadata;
  [TreatNonCallableAsNull] attribute Function? onloadstart;
  [TreatNonCallableAsNull] attribute Function? onmousedown;
  [TreatNonCallableAsNull] attribute Function? onmousemove;
  [TreatNonCallableAsNull] attribute Function? onmouseout;
  [TreatNonCallableAsNull] attribute Function? onmouseover;
  [TreatNonCallableAsNull] attribute Function? onmouseup;
  [TreatNonCallableAsNull] attribute Function? onmousewheel;
  [TreatNonCallableAsNull] attribute Function? onpause;
  [TreatNonCallableAsNull] attribute Function? onplay;
  [TreatNonCallableAsNull] attribute Function? onplaying;
  [TreatNonCallableAsNull] attribute Function? onprogress;
  [TreatNonCallableAsNull] attribute Function? onratechange;
  [TreatNonCallableAsNull] attribute Function? onreset;
  [TreatNonCallableAsNull] attribute Function? onscroll;
  [TreatNonCallableAsNull] attribute Function? onseeked;
  [TreatNonCallableAsNull] attribute Function? onseeking;
  [TreatNonCallableAsNull] attribute Function? onselect;
  [TreatNonCallableAsNull] attribute Function? onshow;
  [TreatNonCallableAsNull] attribute Function? onstalled;
  [TreatNonCallableAsNull] attribute Function? onsubmit;
  [TreatNonCallableAsNull] attribute Function? onsuspend;
  [TreatNonCallableAsNull] attribute Function? ontimeupdate;
  [TreatNonCallableAsNull] attribute Function? onvolumechange;
  [TreatNonCallableAsNull] attribute Function? onwaiting;

  // special event handler IDL attributes that only apply to Document objects
  [TreatNonCallableAsNull,LenientThis] attribute Function? onreadystatechange;

Here is a version that works in Chrome, Safari and FF.

Object.getOwnPropertyNames(document).concat(Object.getOwnPropertyNames(Object.getPrototypeOf(Object.getPrototypeOf(document)))).filter(function(i){return !i.indexOf('on')&&(document[i]==null||typeof document[i]=='function');})

UPD 1:

And here is the version that works in IE9+, Chrome, Safari and FF.

Object.getOwnPropertyNames(document).concat(Object.getOwnPropertyNames(Object.getPrototypeOf(Object.getPrototypeOf(document)))).concat(Object.getOwnPropertyNames(Object.getPrototypeOf(window))).filter(function(i){return !i.indexOf('on')&&(document[i]==null||typeof document[i]=='function');}).filter(function(elem, pos, self){return self.indexOf(elem) == pos;})

UPD 2:

And here's a version using newer JavaScript features (the [...new Set(...)] is to filter out duplicates, replacing the filter approach).

[...new Set([
 ...Object.getOwnPropertyNames(document),
 ...Object.getOwnPropertyNames(Object.getPrototypeOf(Object.getPrototypeOf(document))),
 ...Object.getOwnPropertyNames(Object.getPrototypeOf(window)),
].filter(k => k.startsWith("on") && (document[k] == null || typeof document[k] == "function")))];

PS: the result is an array of events name like ["onwebkitpointerlockerror", "onwebkitpointerlockchange", "onwebkitfullscreenerror", "onwebkitfullscreenchange", "onselectionchange", "onselectstart", "onsearch", "onreset", "onpaste", "onbeforepaste", "oncopy"] ... ect.