execCommand and contenteditable

Formatting

Users can add formatting to contenteditable documents or elements using their browser's features, such as common keyboard shortcuts for formatting (Ctrl-B for bold, Ctrl-I for italic, etc.) or by dragging and dropping images, links, or markup from the clipboard.

Additionally, developers can use JavaScript to apply formatting to the current selection (highlighted text).

document.execCommand('bold', false, null); // toggles bold formatting
document.execCommand('italic', false, null); // toggles italic formatting
document.execCommand('underline', false, null); // toggles underline

Listening to Changes of contenteditable

Events that work with most form elements (e.g., change, keydown, keyup, keypress) do not work with contenteditable.

Instead, you can listen to changes of contenteditable contents with the input event. Assuming contenteditableHtmlElement is a JS DOM object that is contenteditable:

contenteditableHtmlElement.addEventListener("input", function() {
    console.log("contenteditable element changed");
});

Getting started

The HTML attribute contenteditable provides a simple way to turn a HTML element into a user-editable area

<div contenteditable>You can <b>edit</b> me!</div>

Native Rich-Text editing

Using JavaScript and execCommandW3C you can additionally pass more editing features to the currently focused contenteditable element (specifically at the caret position or selection).

The execCommand function method accepts 3 arguments

document.execCommand(commandId, showUI, value)
  • commandId String. from the list of available **commandId**s
    (see: Parameters→commandId)
  • showUI Boolean (not implemented. Use false)
  • value String If a command expects a command-related String value, otherwise "".
    (see: Parameters→value)

Example using the "bold" command and "formatBlock" (where a value is expected):

document.execCommand("bold", false, "");          // Make selected text bold
document.execCommand("formatBlock", false, "H2"); // Make selected text Block-level <h2>

Quick Start Example:

<button data-edit="bold"><b>B</b></button>
<button data-edit="italic"><i>I</i></button>
<button data-edit="formatBlock:p">P</button>
<button data-edit="formatBlock:H1">H1</button>
<button data-edit="insertUnorderedList">UL</button>
<button data-edit="justifyLeft">&#8676;</button>
<button data-edit="justifyRight">&#8677;</button>
<button data-edit="removeFormat">&times;</button>

<div contenteditable><p>Edit me!</p></div>

<script>
[].forEach.call(document.querySelectorAll("[data-edit]"), function(btn) {
  btn.addEventListener("click", edit, false);
});

function edit(event) {
  event.preventDefault();
  var cmd_val = this.dataset.edit.split(":");
  document.execCommand(cmd_val[0], false, cmd_val[1]);
}
<script>

jsFiddle demo
Basic Rich-Text editor example (Modern browsers)

Final thoughts
Even being present for a long time (IE6), implementations and behaviors of execCommand vary from browser to browser making "building a Fully-featured and cross-browser compatible WYSIWYG editor" a hard task to any experienced JavaScript developer.
Even if not yet fully standardized you can expect pretty decent results on the newer browsers like Chrome, Firefox, Edge. If you need better support for other browsers and more features like HTMLTable editing etc. a rule of thumbs is to look for an already existent and robust Rich-Text editor.

Copy to clipboard from textarea using execCommand("copy")

Example:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
</head>
<body>
    <textarea id="content"></textarea>
    <input type="button" id="copyID" value="Copy" />
    <script type="text/javascript">
        var button = document.getElementById("copyID"),
            input = document.getElementById("content");

        button.addEventListener("click", function(event) {
            event.preventDefault();
            input.select();
            document.execCommand("copy");
        });
    </script>
</body>
</html>

document.execCommand("copy") copies the current selection to the clipboard