Faking ‘onpaste’ in Firefox

When trying to find a solution for cleaning Rich Text pasting into a textarea, we needed to find a way to detect pastes and trigger an event based on said action. Internet Explorer and Safari both have an onpaste event that allows you to hook javascript into a paste event, but Firefox does not allow this.

After a little Googling, I didn’t really come across much of a solution so I decided to roll my own.

  function checkForPaste(event) {     var e = event.element();     if ((e.previousValue && e.value.length > e.previousValue.length + 1) ||         (!e.previousValue && e.value.length > 1)) {        if (e.onpaste) {         e.onpaste(e)       } else if (e.readAttribute("onpaste")) {         eval(e.readAttribute("onpaste"));       }     }       e.previousValue = e.value;   }        function firefoxOnPaste() {     $$('textarea').each(function(e) {        if (e.onpaste || e.readAttribute("onpaste")) {         Event.observe(e,'input',checkForPaste);       }     });   }      if (Prototype.Browser.Gecko) {     document.observe('dom:loaded', firefoxOnPaste);   }

This snippet of code will automatically detect if an onpaste has been either added to a textarea’s attribute list (e.g. <textarea onpaste='alert("Pasted!")/>) or set programmatically. It will then automatically simulate paste detection using the oninput event and trigger the onpaste code when it believes a paste has been made.

The snippet will detect correctly for all pasting I’ve tried, including selecting a chunk and pasting a replacement. The only major caveat I’ve seen thus far is that the first input change after the page load will register as a paste if the textarea’s value has already been set. In any case, I thought it was a relatively straightforward way to solve the problem.