react.dev/docs/component-lifecycle.md

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docs-component-lifecycle	Component Lifecycle	What happens when I render a React component?	docs	component-data.html	event-handling.html

Mounting

We have previously seen how to render components into existing DOM elements on the page:

React.renderComponent(<div>Hello, world!</div>, document.body);

In this one simple line, we have accomplished the following:

• A <div> (defined by React.DOM.div) component is instantiated.
• The component is mounted into document.body.

Mounting is the process of initializing a React component by creating its DOM nodes and inserting the them into a supplied container node.

At this point, the entire page consists of a single <div> with "Hello, world!".

Updating

Let's add a second call to React.renderComponent() after three seconds:

React.renderComponent(<div>Hello, world!</div>, document.body);
setTimeout(function() {
  React.renderComponent(<div>Goodbye, world.</div>, document.body);
}, 3000);

The second call to React.renderComponent() will trigger the following:

• The <div> component will check the new props to see if anything changed.
• The set of changes are used to update the DOM node as necessary.

Updating is the process of mutating the rendered DOM nodes and occurs whenever either props or state has changed. This ensures that the rendered DOM is consistent with the data.

Unmounting

Let's add one final call to React.renderComponent() after another three seconds:

React.renderComponent(<div>Hello, world!</div>, document.body);
setTimeout(function() {
  React.renderComponent(<div>Goodbye, world.</div>, document.body);
}, 3000);
setTimeout(function() {
  React.renderComponent(<img src="/images/fin.png" />, document.body);
}, 6000);

The third call to React.renderComponent() will trigger the following:

• An <img> (defined by React.DOM.img) component is instantiated.
• React will compare the <div> component with the <img> component.
• Since the component class is different, the <div> component will be unmounted.
• The <img> component will then be mounted into document.body.

Unmounting is the process of releasing resources that have been allocated by a component. This allows user interfaces built with React to live long without memory leaks.

Components can also be unmounted using React.unmountAndReleaseReactRootNode():

React.unmountAndReleaseReactRootNode(document.body);

This will unmount any components mounted immediately within document.body.