setInterval(tick, 1000);

componentWillReceiveProps(object nextProps)

React doesn’t need error boundaries to recover from errors in event handlers. Unlike the render method and lifecycle methods, the event handlers don’t happen during rendering. So if they throw, React still knows what to display on the screen.

As of React 16, errors that were not caught by any error boundary will result in unmounting of the whole React component tree

Error boundaries work like a JavaScript catch {} block, but for components

Since an uncontrolled component keeps the source of truth in the DOM, it is sometimes easier to integrate React and non-React code when using uncontrolled components.

an uncontrolled component keeps the source of truth in the DOM

In a controlled component, form data is handled by a React component

Two elements of different types will produce different trees. The developer can hint at which child elements may be stable across different renders with a key prop.

React builds and maintains an internal representation of the rendered UI. It includes the React elements you return from your components. This representation lets React avoid creating DOM nodes and accessing existing ones beyond necessity, as that can be slower than operations on JavaScript objects. Sometimes it is referred to as a “virtual DOM”, but it works the same way on React Native.

Note that a HOC doesn’t modify the input component, nor does it use inheritance to copy its behavior. Rather, a HOC composes the original component by wrapping it in a container component. A HOC is a pure function with zero side-effects.

accessing their DOM nodes may be unavoidable for managing focus, selection, or animations.

Context is primarily used when some data needs to be accessible by many components at different nesting levels. Apply it sparingly because it makes component reuse more difficult. If you only want to avoid passing some props through many levels, component composition is often a simpler solution than context.

Context is designed to share data that can be considered “global” for a tree of React components, such as the current authenticated user, theme, or preferred language

The dynamic import() syntax is a ECMAScript (JavaScript) proposal not currently part of the language standard. It is expected to be accepted in the near future.

If you can’t find a component where it makes sense to own the state, create a new component simply for holding the state and add it somewhere in the hierarchy above the common owner component.

Simply ask three questions about each piece of data: Is it passed in from a parent via props? If so, it probably isn’t state. Does it remain unchanged over time? If so, it probably isn’t state. Can you compute it based on any other state or props in your component? If so, it isn’t state.

A good rule of thumb is that elements inside the map() call need keys.

When you don’t have stable IDs for rendered items, you may use the item index as a key as a last resort: const todoItems = todos.map((todo, index) =>

Keys help React identify which items have changed, are added, or are removed. Keys should be given to the elements inside the array to give the elements a stable identity:

In JavaScript, class methods are not bound by default

Because this.props and this.state may be updated asynchronously, you should not rely on their values for calculating the next state. For example, this code may fail to update the counter: // Wrong this.setState({ counter: this.state.counter + this.props.increment, }); To fix it, use a second form of setState() that accepts a function rather than an object. That function will receive the previous state as the first argument, and the props at the time the update is applied as the second argument: // Correct this.setState((state, props) => ({ counter: state.counter + props.increment }));

By default, React DOM escapes any values embedded in JSX before rendering them. Thus it ensures that you can never inject anything that’s not explicitly written in your application. Everything is converted to a string before being rendered. This helps prevent XSS (cross-site-scripting) attacks.

We split JSX over multiple lines for readability. While it isn’t required, when doing this, we also recommend wrapping it in parentheses to avoid the pitfalls of automatic semicolon insertion.

Most React developers use a special syntax called “JSX” which makes these structures easier to write. The <div /> syntax is transformed at build time to React.createElement('div')

test()

You can also add other assets to the public folder. Note that we normally encourage you to import assets in JavaScript files instead. For example, see the sections on adding a stylesheet and adding images and fonts. This mechanism provides a number of benefits: Scripts and stylesheets get minified and bundled together to avoid extra network requests. Missing files cause compilation errors instead of 404 errors for your users. Result filenames include content hashes so you don’t need to worry about browsers caching their old versions.

To reduce the number of requests to the server, importing images that are less than 10,000 bytes returns a data URI instead of a path

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React is fast, thanks to the VirtualDOM. Using a diffing algorithm, the browser DOM nodes are manipulated only when there is a state change. This algorithm is computationally expensive. Using webworkers to perform the calculations can make React even faster.