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Pen Settings

CSS Base

Vendor Prefixing

Add External Stylesheets/Pens

Any URL's added here will be added as <link>s in order, and before the CSS in the editor. If you link to another Pen, it will include the CSS from that Pen. If the preprocessor matches, it will attempt to combine them before processing.

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You're using npm packages, so we've auto-selected Babel for you here, which we require to process imports and make it all work. If you need to use a different JavaScript preprocessor, remove the packages in the npm tab.

Add External Scripts/Pens

Any URL's added here will be added as <script>s in order, and run before the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

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Use npm Packages

We can make npm packages available for you to use in your JavaScript. We use webpack to prepare them and make them available to import. We'll also process your JavaScript with Babel.

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Code Indentation


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Auto-Updating Preview

If enabled, the preview panel updates automatically as you code. If disabled, use the "Run" button to update.

HTML Settings

Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

              function createElement(tag, attrs, ...children) {
	return {

function setAttribute(dom, name, value) {
	// 如果属性名是className,则改回class
	if (name === "className") name = "class";

	// 如果属性名是onXXX,则是一个事件监听方法
	if (/on\w+/.test(name)) {
		name = name.toLowerCase();
		dom[name] = value || "";
		// 如果属性名是style,则更新style对象
	} else if (name === "style") {
		if (!value || typeof value === "string") {
			dom.style.cssText = value || "";
		} else if (value && typeof value === "object") {
			for (let name in value) {
				// 可以通过style={ width: 20 }这种形式来设置样式,可以省略掉单位px
				dom.style[name] =
					typeof value[name] === "number" ? value[name] + "px" : value[name];
		// 普通属性则直接更新属性
	} else {
		if (name !== "class" && name in dom) {
			dom[name] = value || "";
		if (value) {
			dom.setAttribute(name, value);
		} else {

function render(vnode, container) {
	// 当vnode为字符串时,渲染结果是一段文本
	if (typeof vnode === "string") {
		const textNode = document.createTextNode(vnode);
		return container.appendChild(textNode);

	const dom = document.createElement(vnode.tag);

	if (vnode.attrs) {
		Object.keys(vnode.attrs).forEach(key => {
			const value = vnode.attrs[key];
			setAttribute(dom, key, value); // 设置属性

	vnode.children.forEach(child => render(child, dom)); // 递归渲染子节点

	return container.appendChild(dom); // 将渲染结果挂载到真正的DOM上

const React = {

const ReactDOM = {
	render: (vnode, container) => {
		container.innerHTML = "";
		return render(vnode, container);

const tick = () => {
	const element = (
			<h1>Hello, world!</h1>
			<h2>It is {new Date().toLocaleTimeString()}.</h2>
	ReactDOM.render(element, document.getElementById("app"));

setInterval(tick, 1000);

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