[javascript] 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.)

Something very similar can be seen on http://goo.gl when you shorten the URL.

There is a readonly input element that gets programmatically focused, with tooltip press CTRL-C to copy.

When you hit that shortcut, the input content effectively gets into the clipboard. Really nice :)

With the help of raincoat's code on GitHub, I managed to get a running version accessing the clipboard with plain JavaScript.

function TrelloClipboard() {
    var me = this;

    var utils = {
        nodeName: function (node, name) {
            return !!(node.nodeName.toLowerCase() === name)
        }
    }
    var textareaId = 'simulate-trello-clipboard',
        containerId = textareaId + '-container',
        container, textarea

    var createTextarea = function () {
        container = document.querySelector('#' + containerId)
        if (!container) {
            container = document.createElement('div')
            container.id = containerId
            container.setAttribute('style', [, 'position: fixed;', 'left: 0px;', 'top: 0px;', 'width: 0px;', 'height: 0px;', 'z-index: 100;', 'opacity: 0;'].join(''))
            document.body.appendChild(container)
        }
        container.style.display = 'block'
        textarea = document.createElement('textarea')
        textarea.setAttribute('style', [, 'width: 1px;', 'height: 1px;', 'padding: 0px;'].join(''))
        textarea.id = textareaId
        container.innerHTML = ''
        container.appendChild(textarea)

        textarea.appendChild(document.createTextNode(me.value))
        textarea.focus()
        textarea.select()
    }

    var keyDownMonitor = function (e) {
        var code = e.keyCode || e.which;
        if (!(e.ctrlKey || e.metaKey)) {
            return
        }
        var target = e.target
        if (utils.nodeName(target, 'textarea') || utils.nodeName(target, 'input')) {
            return
        }
        if (window.getSelection && window.getSelection() && window.getSelection().toString()) {
            return
        }
        if (document.selection && document.selection.createRange().text) {
            return
        }
        setTimeout(createTextarea, 0)
    }

    var keyUpMonitor = function (e) {
        var code = e.keyCode || e.which;
        if (e.target.id !== textareaId || code !== 67) {
            return
        }
        container.style.display = 'none'
    }

    document.addEventListener('keydown', keyDownMonitor)
    document.addEventListener('keyup', keyUpMonitor)
}

TrelloClipboard.prototype.setValue = function (value) {
    this.value = value;
}

var clip = new TrelloClipboard();
clip.setValue("test");

See a working example: http://jsfiddle.net/AGEf7/

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).

Daniel LeCheminant's code didn't work for me after converting it from CoffeeScript to JavaScript (js2coffee). It kept bombing out on the _.defer() line.

I assumed this was something to do with jQuery deferreds, so I changed it to $.Deferred() and it's working now. I tested it in Internet Explorer 11, Firefox 35, and Chrome 39 with jQuery 2.1.1. The usage is the same as described in Daniel's post.

var TrelloClipboard;

TrelloClipboard = new ((function () {
    function _Class() {
        this.value = "";
        $(document).keydown((function (_this) {
            return function (e) {
                var _ref, _ref1;
                if (!_this.value || !(e.ctrlKey || e.metaKey)) {
                    return;
                }
                if ($(e.target).is("input:visible,textarea:visible")) {
                    return;
                }
                if (typeof window.getSelection === "function" ? (_ref = window.getSelection()) != null ? _ref.toString() : void 0 : void 0) {
                    return;
                }
                if ((_ref1 = document.selection) != null ? _ref1.createRange().text : void 0) {
                    return;
                }
                return $.Deferred(function () {
                    var $clipboardContainer;
                    $clipboardContainer = $("#clipboard-container");
                    $clipboardContainer.empty().show();
                    return $("<textarea id='clipboard'></textarea>").val(_this.value).appendTo($clipboardContainer).focus().select();
                });
            };
        })(this));

        $(document).keyup(function (e) {
            if ($(e.target).is("#clipboard")) {
                return $("#clipboard-container").empty().hide();
            }
        });
    }

    _Class.prototype.set = function (value) {
        this.value = value;
    };

    return _Class;

})());