Rendering of Elements

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In the previous chapters I have used them a few times without explaining them: what actually are React elements?
React elements are the smallest building blocks in a React application. They describe what is going to be rendered on the screen. Although they sound similar to DOM elements, they differ in one important point: React elements are only simple objects — thus they are easy to create and performant. Calling React.createElement() to create a React element does not trigger a DOM operation.
React elements are often confused with React components and used interchangeably. This is not correct though. Elements make up the building blocks of components. We are going to talk about components in great depths in the following chapters, but you should read and understand this chapter about elements first.
We have already learned how we can create a React element. JSX allows us to save many lines of code by avoiding lengthy React.createElement() calls. But how do we render an element to the screen, i.e. show it in the browser?
In order to achieve this, we are making use of the render() method of ReactDOM. Additionally, we need a root node or a mount node to render a React element. This node works as a placeholder and informs React where the element should be rendered to. Theoretically, there can be many different root nodes in the HTML document. React controls these independently and keeps track of the different mount nodes. So instead of having one large React application, you could easily choose to have many smaller (or larger) apps in a single HTML document. In most situations, you are likely to only have a single root node for your React application.
But let's get to the important parts: how to render a React element. Pass the element you want to render as the first argument to ReactDOM.render() and then pass the root node as the second argument, which is the DOM node that the element will be rendered into.
Imagine that we want to render a div with the id root in the HTML document, which will work as our root node:
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<!DOCTYPE html>
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<html>
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  <head>
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    <title></title>
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  </head>
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  <body>
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    <div id="root"></div>
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  </body>
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</html>
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The call matching this is the following:
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const myFirstElement = <div>My first React Element</div>;
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ReactDOM.render(myFirstElement, document.getElementById('root'));
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If you execute this code in the browser, you will see <div>My first React element</div inside of the root div.
React elements are immutable meaning that they do not change. Once the element has been created, it represents a particular state in the user interface. The official React documentation metaphorically speaks of a single frame in a film. If we wanted to update the user interface of our application, we would need to create a new React element with the updated and changed data — don't be afraid, it sounds more complex than it actually is!
React is clever enough to figure out which parts of an application have changed and will only update those parts that have actually been updated, thanks to a clever comparison algorithm. React elements and their children will be compared to the previous version and only invoke a DOM operation if a change is present. If used correctly, React's reconciliation process can drastically improve the performance of our application as regular DOM updates are very costly and negatively impact performance. Depending on the changes in the specific React element, you might not even need to update a full DOM element every time and often only a few attributes need to change.
But let's look at that in practice:
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function showTime() {
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  const time = new Date().toLocaleTimeString();
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  const timeElement = (
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    <div>
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      <p>It is now {time} o' clock.</p>
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    </div>
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  );
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  ReactDOM.render(timeElement, document.getElementById('root'));
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}
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setInterval(showTime, 1000);
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Once again a React element was created. Once it is invoked in ReactDOM.render(), it will tell us the time. Because we only care about punctuality, we pass the element and the ReactDOM.render() call into a function which is called every 1000ms.
Inspecting the elements in the Chrome Dev Tools reveals: with each ReactDOM.render() call only the time itself is updated. The remaining elements as well as the DOM nodes or even parts not impacted of the shown text remain the same:
React only updates the time, nothing else.
We've just met one of the most fundamental principles of React in practice: React's declarative way of creating user interfaces. Instead of telling our app in an imperative fashion to update itself every second, we declaratively define in the React element that we want to see the current time in a particular place at each re-render.
Implementing this said functionality without React might look something like the following:
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function changeTime() {
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  const time = new Date().toLocaleTimeString();
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  const target = document.getElementById('root');
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  target.textContent = 'It is now ' + time + " o' clock";
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}
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setInterval(changeTime, 1000);
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The main advantage of declarative programming is that we only describe states and mention how something should render instead of deciding on each and every step and how to reach our final state. Especially complex applications benefit greatly from declarative approaches, as both readability and simplicity are drastically improved and makes applications less error prone.
In practice, ReactDOM.render() is only called once. Mostly when opening a page. We have only used render() repeatedly in the examples to illustrate how ReactDOM and React elements work together
Components (instead of elements) trigger a re-render if their state changes or if they have been passed new props, but let's look at components in the next chapter!
JSX – an Introduction
Components in React
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