Neat Little React Tricks

JSX can ‘dynamic’, in the sense that the component type can be determined at runtime as long the variable name is capitalized ¹. It’s just how JSX .

This can be useful when having to switch between two different components that take the same props, like variants of styled components:

import { LeftSidebar, RightSidebar } from "./sidebars";
function Sidebar({ position, size, children }) {
  const SidebarComponent = position === "left" ? LeftSidebar : RightSidebar;
  return <SidebarComponent size={size}>{children}</SidebarComponent>;
}

Another real world use case is conditionally replacing an old component with a newly refactored one:

// class-based or perhaps just wonky
import { UserPermission as OldUserPermission } from "./old-user-permissions";
// functional and clean
import { UserPermission as NewUserPermission } from "./new-user-permissions";
function UsersManagement({ users, isNewAPI }) {
  // ...
  // capitalized variable name!
  const UserPermissions = isNewAPI ? UserPermissions : NewUserPermissions;
  return (<UserPermissions
    permission={permission}
    users={users}
    currentUser={currentUser}
  />)

Yeah you could conditionally render the element with a as well but this is less work when the component has a lot of props and the props are the same for both.

Have some state that only ever updates in one particular way? Consider using a single-action useReducer over useState!

Take for example, a boolean in useState that can only be toggled. Maybe you’re already using a state updater function to avoid reading the state variable from component scope: setIsOpen((prevIsOpen) => !prevIsOpen). Knock it off and put that logic in a reducer instead, then just call the dispatch without any arguments.

const [isOpen, toggleOpen] = useReducer((prevIsOpen) => !prevIsOpen, false);
// Flip the switch
toggleOpen();

A nice benefit is that the is , whereas a callback for setting state is not. If a state setter-wrapping callback is called in a useEffect, it will have to be wrapped in useCallback to make it referentially stable. And that works but it’s quite some boilerplate for something as simple as flipping a switch.

Also, a reducer restricts what can be done to the state ². Suppose you’re working in a team on a gigantic app and pass a state setter for a toggle down to parts of the app that fall under your teammates responsibilty. They can now set the state to whatever, which is not what a toggle should be able to do! OK, this might be slightly farfetched but you get what I mean, limiting possibilites is good design✨ .

Real simple, barely a trick but here you go. Just like setting default values on props, you can set a default for the special children prop ³. Set it to a sane default string or element and render children where you want.

I often use it on small reusable (styled) UI components, like buttons and links, that contain the same text most of the time, but need to overriden every now and then. I’m talking about text like “Read more…”, or “Open”,

export const DonateLink = ({ children = "Donate" }) => (
  <Link href="/donate" className="styledLink">
    {children}
    <HeartIcon />
  </Link>
);
// In practice:
<DonateLink /> // default text
<DonateLink>Donate now!</DonateLink> // overridden text

JSX is just another way to React.createElement, and are just plain objects ⁴. We can declare element variables before the return for reuse.

Here’s an example, it’s a component I use on this blog, it’s a fleshed out <blockquote> as seen on its MDN page. You can pass the component a url props which will both be set as the url attribute on the blockquote element and as the href on the inside the cite element. If no url is given, no anchor element should be inside the <cite>.

export function Quote({ children, url, source }) {
  
  const citeEl = <cite>{source}</cite>;
  return (
    <figure>
      <blockquote cite={url}>{children}</blockquote>
      <figcaption>
        {url ? <a href={url} rel="noopener">{citeEl}</a> : citeEl}
      </figcaption>
    </figure>
  );
}

Another use case could be to conditionally wrap an element with a tooltip.

function TableCell({ value, isOutdated }) {
  const tableCell = <td>{value}</td>;
  return isOutdated ? (
    <Tooltip text="This value is not up to date">{tableCell}</Tooltip>
  ) : (
    tableCell
  );
}

Take it a step further with React.cloneElement to conditionally add a prop. I think this is only worth if the wrapped element takes a lot of props, so those don’t have to be repeated. Repeating the element is fine if it’s a DOM node or a component without props.

function TableCell({ value, setCell, isOutdated }) {
  const tableCell = (
    <EditableCell onChange={setCell} long={long} list={list} ofProps={ofProps}>
      {value}
    </EditableCell>
  );
  return isOutdated ? (
    <Tooltip text="The value is not up to date">
      {cloneElement(tableCell, { className: "outdated" })}
    </Tooltip>
  ) : (
    tableCell
  );
}

First off, putting things in state that don’t belong there is a bad idea, you’ll have a hard time when you do so. Remember that state is reserved for data that changes over time ⁵.

There’s one exception to this rule and it’s using useState to initialize a value once, and it sure is a neat trick.

Why would we want to initialize a value once? Well, values defined in React components are reevaluated every render, that can be problematic. Sometimes it’s important that a value is initialized strictly once, like an external library. Other times, an object can be ‘expensive’ to create, for instance a huge array or heavy class. If you need to initialize such a value per component, useState can help.

First, the state value is guaranteed to be stable. On top of that, useState can take an initializer function to ensure the initial state is . The value should never change, so we can just ignore the state setter by not destructuring it.

function Image(props) {
  const [intersectionObserver] = useState(() => new IntersectionObserver());
  return (
    // JSX
  )
}

Voila, a lazily-created, stable value.

“Isn’t this what useMemo is for?” No, not really. useMemo should be seen as a performance optimization for expensive computations based on , we shouldn’t rely on it for one-time initializations ⁶. Or as the docs put it:

useMemo lets you memoize an expensive calculation if the dependencies are the same. However, it only serves as a hint, and doesn’t guarantee the computation won’t re-run.

React Hooks FAQ - How to create expensive objects lazily?

Write your code so that it still works without useMemo — and then add it to optimize performance.

React Hooks Reference - useMemo

I have spotted this pattern a few times in @react-three/fiber examples and @react-three/drei source code where it’s used to initialize ‘heavy’ three.js objects once on a component basis.

You can achieve the same with useRef, below is the example from How to create expensive objects lazily? section Hooks FAQ.

function Image(props) {
  const ref = useRef(null);
  // ✅ IntersectionObserver is created lazily once
  function getObserver() {
    if (ref.current === null) {
      ref.current = new IntersectionObserver(props.onIntersect);
    }
    return ref.current;
  }
  // When you need it, call getObserver()
  // ...
}

But I think it’s not as nice as useState with an initializer. I’d love for useRef to have an initializer function too but it doesn’t.

And those were all the tricks for this time!

1. Conditionally Swap Component Type

2. Single-Action Reducer for Toggle State

3. Default Children

4. Conditionally Wrap an Element

5. useState to Initialize a Value Once

Footnotes