What is the Virtual DOM?
The Virtual DOM was created to deal with issues that arise from directly modifying the actual-DOM. It's hard to keep track of changes - it can become difficult to keep track of current (and prior) state of the DOM to manipulate it into the form we need. Additionally, modifying the actual-DOM is a costly operation, and modifying the DOM on every change can cause poor performance.
To deal with these issues, React created a Virtual DOM - a tree of JavaScript objects that represents the actual DOM elements. This tree is updated by React when your application's state changes. When you update a component's state or props, React will re-render the Virtual DOM tree and then make the necessary changes to the actual-DOM in one operation.
This approach makes it much easier to manage the complexity of managing the DOM and ensures that your application remains fast and efficient.
What do I need to get started?
In React, we don't directly manipulate the actual DOM. Instead, we must manipulate the virtual representation and let React take care of changing the browser's DOM. This is a very powerful feature but it requires us to think differently about how we build web apps.
Why Not Modify the Actual DOM? It's worth asking: why do we need a Virtual DOM? Can't we just use the "actual-DOM"? When we do "classic-" (e.g. jQuery-) style web development, we would typically:
locate an element (using document.querySelector or document.getElementById) and then
modify that element directly (say, by calling .innerHTML() on the element).
This style of development is problematic in that:
It's hard to keep track of changes - it can become difficult to keep track of current (and prior) state of the DOM to manipulate it into the form we need
It can be slow - modifying the actual-DOM is a costly operation, and modifying the DOM on every change can cause poor performance
The magic behind the scenes
When we build web apps with React, we don't directly manipulate the actual DOM. Instead, we manipulate a virtual representation and let React take care of changing the browser's DOM. This is where the Virtual DOM comes in.
The Virtual DOM is a tree of JavaScript objects that represents the actual DOM simultaneously. It allows us to batch updates to the DOM, making it easy to use and optimized for performance. But what does this Virtual DOM consist of? React's Virtual DOM is a tree of ReactElements.
Understanding how these ReactElements interact with the actual DOM is key to building efficient web apps. We'll explore this further in upcoming sections, but let's focus on creating a basic ReactElement first.
For instance, we can create a ReactElement that represents a <b> (bold) element using the React.createElement method:
var boldElement = React.createElement('b');
This code creates a new ReactElement for a <b> tag. The resulting object is an instance of a ReactElement.
Why is it so useful?
The Virtual DOM was created to deal with issues that arise when modifying the actual DOM directly. When building a web app in React, we don't work directly with the browser's "actual" DOM, but instead provide React with enough information to build a JavaScript object that represents what the browser will render.
This approach has two significant benefits. Firstly, it makes it easier to keep track of changes - React can compare the virtual representation with the previous state and figure out what needs to be updated in the actual DOM. This reduces the complexity of managing the DOM's state, making it easier to reason about our application's behavior.
Secondly, it optimizes performance by minimizing the number of costly DOM operations. Instead of modifying the actual DOM on every change, React can compute the differences between the virtual and actual representations and update only what needs to be updated. This results in a much faster and more efficient way to build web apps.
Glossary of essential terms
Understanding Virtual DOM in React requires grasping certain fundamental concepts. Let's start with a few essential terms.
Virtual DOM: A tree of JavaScript objects that represents the actual DOM simultaneously. This allows batch updates to the DOM, making it easy to optimize and improve performance.
ReactElements: The building blocks of the Virtual DOM. Think of them as instructions on how to render specific parts of your user interface.
Shadow DOM: Not to be confused with the Virtual DOM! A Shadow DOM is a form of encapsulation, allowing you to isolate certain elements from the rest of the document. It's like creating a private space for your video controls or other UI components.
Hands-on, step-by-step instructions
Here is the content for this section:
Let's walk through how we render a <b> tag in our (actual) DOM using React. Instead of creating a <b> tag directly in the DOM (like we might if we were using jQuery), React expects us to provide a Virtual DOM tree.
That is, we're going to give React a set of JavaScript objects which React will turn into a real DOM tree. The objects that make up the tree will be ReactElements. To create a ReactElement, we use the createElement method provided by React.
For instance, to create a ReactElement that represents a <b> (bold) element in React, type the following in the browser console:
var boldElement = React.createElement('b');
boldElement is an instance of a ReactElement. Now we have this boldElement, but it's not visible without giving it to React to render in the actual DOM tree.
Let's give it a try by rendering our boldElement into the real DOM:
ReactDOM.render(<div>{boldElement}</div>, document.getElementById('root'));
This will create a <b> element and add it to the <div> with an id of 'root'.
Tips and tricks for success
Tips and tricks for success with Virtual DOM in React:
When working with the Virtual DOM in React, it's essential to remember that you're not directly manipulating the actual DOM. Instead, you're providing React with enough information to build a JavaScript object that represents what the browser will render. This can take some getting used to, but once you grasp this concept, you'll be able to write more efficient and scalable code.
One key tip is to think of the Virtual DOM as a tree of ReactElements. Each ReactElement represents a single DOM element or a group of elements, and you can use the React.createElement method to create new elements. For example:
var boldElement = React.createElement('b');
This code creates a new ReactElement that represents a <b> tag. You can then pass this element to the ReactDOM.render method to render it in the actual DOM.
Another important consideration is that you should only modify the Virtual DOM, not the actual DOM directly. This is because React will take care of updating the actual DOM for you when necessary, which makes your code more efficient and easier to maintain.
By following these tips and understanding how the Virtual DOM works, you'll be well on your way to writing effective and scalable React applications.
What not to do
When building web apps in React, it's tempting to modify the actual DOM directly. You might be familiar with this approach from classic (e.g., jQuery-) style web development. Here, you would:
Locate an element using
document.querySelectorordocument.getElementById.Modify that element directly, for instance, by calling
.innerHTML()on the element.
This style of development is problematic because:
It's hard to keep track of changes - it can become difficult to keep track of the current (and prior) state of the DOM to manipulate it into the desired form.
It can be slow - modifying the actual-DOM is a costly operation, and modifying the DOM on every change can cause poor performance.
In React, we don't directly modify the actual DOM. Instead, we work with the Virtual DOM, a tree of JavaScript objects that represents the real DOM elements.
Answers to common questions
Maybe you've heard of the "Shadow DOM" and you're wondering, is the Shadow DOM the same thing as the Virtual DOM? The answer is no.
The Virtual DOM is a tree of JavaScript objects that represent the real DOM elements. It's a representation of what our browser will render.
On the other hand, the Shadow DOM is a form of encapsulation on our elements. Think about using the <video> tag in your browser. In a video tag, your browser will create a set of video controls such as play button, time display, and volume control. The Shadow DOM is like an invisible layer that contains these controls, keeping them separate from other parts of the page.
In React, we're not working with the actual DOM directly, but instead a virtual representation of it. Our job is to provide React with enough information to build a JavaScript object that represents what the browser will render.
Final thoughts and next steps
As we've explored the Virtual DOM in React, it's clear that this feature is a game-changer for building web applications. By working with a virtual representation of the DOM rather than modifying the actual DOM directly, we can improve performance and reduce complexity. This requires us to think differently about how we build our applications, but the benefits are well worth the extra effort.
In conclusion, understanding the Virtual DOM is crucial for building efficient and scalable React applications. By using JSX to create elements and the ReactDOM.render() method to render those elements in the actual DOM, we can take advantage of React's virtual DOM features without sacrificing performance or ease of use. With this knowledge under our belts, we're ready to start building powerful and engaging web applications with React.