copy.js

'use-strict';

(function() {
    function copyTextArea($selTextArea) {
        $selTextArea.select();
        var successful = false;
        try {
            successful = document.execCommand('copy');
        } catch (err) {
            successful = false;
        }
        if (! successful) {
            if (BootstrapDialog) {
                BootstrapDialog.show({
                    title: "Copy",
                    message: "Copy to clipboard: Ctrl+C (Cmd+C Mac), Enter.<br/><input type='text'/>",
                    onshown: function(dialog) {
                        dialog.getModalBody().find('input').val($selTextArea.val()).select();
                    },
                    buttons: [
                        {
                            label: 'Close',
                            hotkey: 13,
                            action: function(dialog) {
                                dialog.close();
                            }
                        }
                    ]
                });
            } else {
                window.prompt("Copy to clipboard: Ctrl+C (Cmd+C Mac), Enter", $selTextArea.val());
            }
        }
    }
    function clickCopyBtn() {
        copyTextArea($($(this).attr("x-target")));
    }
    $(function() {
        $("button.copy").click(clickCopyBtn);
    });
})();

Example usage in HTML:

<input type="text" id="txtTest"/>
<button class="btn btn-lg btn-primary copy" x-target="#txtTest">Copy</button>