Add a TypeKit font to the TinyMCE editor in WordPress.

typekit.editor.php

add_filter( 'mce_external_plugins', 'my_theme_mce_external_plugins' );
function my_theme_mce_external_plugins( $plugin_array ) {
	$plugin_array['typekit'] = get_template_directory_uri() . '/typekit.tinymce.js';
	return $plugin_array;
}

typekit.tinymce.js

/* global tinymce */
tinymce.PluginManager.add( 'typekit', function( editor ) {

	function addScriptToHead() {
		// Get the DOM document object for the IFRAME
		var doc = editor.getDoc();

		// Create the script we will add to the header asynchronously
		var jscript = "(function() {\n\
			var config = {\n\
				kitId: 'xxxxxxx'\n\
			};\n\
			var d = false;\n\
			var tk = document.createElement('script');\n\
			tk.src = '//use.typekit.net/' + config.kitId + '.js';\n\
			tk.type = 'text/javascript';\n\
			tk.async = 'true';\n\
			tk.onload = tk.onreadystatechange = function() {\n\
				var rs = this.readyState;\n\
				if (d || rs && rs != 'complete' && rs != 'loaded') return;\n\
				d = true;\n\
				try { Typekit.load(config); } catch (e) {}\n\
			};\n\
			var s = document.getElementsByTagName('script')[0];\n\
			s.parentNode.insertBefore(tk, s);\n\
		})();";

		// Create a script element and insert the TypeKit code into it
		var script = doc.createElement( 'script' );
		script.type = 'text/javascript';
		script.appendChild( doc.createTextNode( jscript ) );

		// Add the script to the header
		doc.getElementsByTagName( 'head' )[0].appendChild( script );
	}

	// Support both TinyMCE 3 and 4.
	if ( 3 < parseInt( tinymce.majorVersion ) ) {
		editor.on( 'preInit', function() {
			addScriptToHead();
		});
	} else {
		editor.onPreInit.add( function( editor ) {
			addScriptToHead();
		});
	}
});