Toolbar: Minor improvements (#1818)

The `registerButton` function will now log a warning and do nothing if a button is registered twice.

plugins/toolbar/index.html

@@ -44,7 +44,7 @@ <h1>How to use</h1>
 		<code>Prism.plugins.toolbar.registerButton</code>.</p>
 
 	<p>The function accepts a key for the button and an object with a <code>text</code> property string and an optional
-		<code>onClick</code> function or <code>url</code> string. The <code>onClick</code> function will be called when the button is clicked, while the
+		<code>onClick</code> function or a <code>url</code> string. The <code>onClick</code> function will be called when the button is clicked, while the
 		<code>url</code> property will be set to the anchor tag's <code>href</code>.</p>
 
 	<pre><code class="language-javascript">Prism.plugins.toolbar.registerButton('hello-world', {

plugins/toolbar/prism-toolbar.js

@@ -9,11 +9,18 @@
 
 	Prism.plugins.toolbar = {};
 
+	/**
+	 * @typedef ButtonOptions
+	 * @property {string} text The text displayed.
+	 * @property {string} [url] The URL of the link which will be created.
+	 * @property {Function} [onClick] The event listener for the `click` event of the created button.
+	 */
+
 	/**
 	 * Register a button callback with the toolbar.
 	 *
 	 * @param {string} key
-	 * @param {Object|Function} opts
+	 * @param {ButtonOptions|Function} opts
 	 */
 	var registerButton = Prism.plugins.toolbar.registerButton = function (key, opts) {
 		var callback;
@@ -43,6 +50,11 @@
 			};
 		}
 
+		if (key in map) {
+			console.warn('There is a button with the key "' + key + '" registered already.');
+			return;
+		}
+
 		callbacks.push(map[key] = callback);
 	};