How does Trello access the user's clipboard?


When you hover over a card in Trello and press Ctrl+C, the URL of this card is copied to the clipboard. How do they do this?

As far as I can tell, there is no Flash movie involved. I've got Flashblock installed, and the Firefox network tab shows no Flash movie loaded. (That's the usual method, for example, by ZeroClipboard.)

How do they achieve this magic?

(Right at this moment I think I had an epiphany: You cannot select text on the page, so I assume they have an invisible element, where they create a text selection via JavaScript code, and Ctrl+C triggers the browser's default behaviour, copying that invisible node's text value.)

This question is tagged with javascript coffeescript clipboard trello

~ Asked on 2013-07-08 13:26:56

The Best Answer is


Disclosure: I wrote the code that Trello uses; the code below is the actual source code Trello uses to accomplish the clipboard trick.

We don't actually "access the user's clipboard", instead we help the user out a bit by selecting something useful when they press Ctrl+C.

Sounds like you've figured it out; we take advantage of the fact that when you want to hit Ctrl+C, you have to hit the Ctrl key first. When the Ctrl key is pressed, we pop in a textarea that contains the text we want to end up on the clipboard, and select all the text in it, so the selection is all set when the C key is hit. (Then we hide the textarea when the Ctrl key comes up.)

Specifically, Trello does this:

TrelloClipboard = new class
  constructor: ->
    @value = ""

    $(document).keydown (e) =>
      # Only do this if there's something to be put on the clipboard, and it
      # looks like they're starting a copy shortcut
      if [email protected] || !(e.ctrlKey || e.metaKey)

      if $("input:visible,textarea:visible")

      # Abort if it looks like they've selected some text (maybe they're trying
      # to copy out a bit of the description or something)
      if window.getSelection?()?.toString()

      if document.selection?.createRange().text

      _.defer =>
        $clipboardContainer = $("#clipboard-container")
        $("<textarea id='clipboard'></textarea>")

    $(document).keyup (e) ->
      if $("#clipboard")

  set: (@value) ->

In the DOM we've got:

<div id="clipboard-container"><textarea id="clipboard"></textarea></div>

CSS for the clipboard stuff:

#clipboard-container {
  position: fixed;
  left: 0px;
  top: 0px;
  width: 0px;
  height: 0px;
  z-index: 100;
  display: none;
  opacity: 0;
#clipboard {
  width: 1px;
  height: 1px;
  padding: 0px;

... and the CSS makes it so you can't actually see the textarea when it pops in ... but it's "visible" enough to copy from.

When you hover over a card, it calls


... so then the clipboard helper knows what to select when the Ctrl key is pressed.

~ Answered on 2013-07-08 14:00:57


I actually built a Chrome extension that does exactly this, and for all web pages. The source code is on GitHub.

I find three bugs with Trello's approach, which I know because I've faced them myself :)

The copy doesn't work in these scenarios:

  1. If you already have Ctrl pressed and then hover a link and hit C, the copy doesn't work.
  2. If your cursor is in some other text field in the page, the copy doesn't work.
  3. If your cursor is in the address bar, the copy doesn't work.

I solved #1 by always having a hidden span, rather than creating one when user hits Ctrl/Cmd.

I solved #2 by temporarily clearing the zero-length selection, saving the caret position, doing the copy and restoring the caret position.

I haven't found a fix for #3 yet :) (For information, check the open issue in my GitHub project).

~ Answered on 2013-08-03 23:01:10

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