Why You Should be Using React Context More Often

under React

When people think of context in a React JS app, they often think of global state. "I need to manage the theme my app is set to use, I can use context for this." While this isn't wrong, context can be used for so much more than that use case. I like to think of it as "encapsulated state".

Think of context as a component in your tree that can provide props directly to any child below it. Combine that with hooks, and you can write some clean APIs for your components. Let's take a look at a simple example using a Modal component.

Here's a look at a modal that accepts an onClose prop which should be called to close the modal. The modal manages its own transition system, so when closing the modal, the modal's custom closeModal needs to be called. After the transition is complete, the passed-in onClose prop will be called.

1// inside your Modal component...
2 
3const Modal = ({
4 title,
5 onClose
6}) => {
7 const closeModal = useCallback(() => {
8 // Fake code, pretend to start the closing transition
9 // and call `onClose` when the transition is done
10 startCloseTransition(onClose);
11 }, [onClose]);
12 
13 return (
14 <ModalOverlay closeModal={closeModal}>
15 <ModalContent>
16 <ModalHeader title={title} closeModal={closeModal} />
17 
18 <ModalBody>
19 {React.cloneElement(children, { closeModal })}
20 </ModalBody>
21 </ModalContent>
22 </ModalOverlay>
23 );
24};

Here's the corresponding component that uses the Modal component above:

1const SomeComponent = () => {
2 const [modalOpen, setModalOpen] = useState(false);
3 
4 return (
5 <div>
6 <button type="button" onClick={() => setModalOpen(true)}>Open Modal</button>
7 
8 {modalOpen ? (
9 <Modal title="Some Modal" onClose={() => setModalOpen(false)}>
10 <SomeComponentModalContent />
11 </Modal>
12 ) : null}
13 </div>
14 );
15};
16 
17const SomeComponentModalContent = ({
18 closeModal
19}) => {
20 // `closeModal` is available here because the `Modal`
21 // component passes it via the `cloneElement` call
22 return (
23 <div>
24 <p>The content of the modal goes here</p>
25 <button type="button" onClick={() => closeModal()}>Close the modal</button>
26 </div>
27 );
28};

The example above is about as simple as I could make it. However, in a real application the button in SomeComponentModalContent may be multiple levels deep, and you'd have to pass closeModal down through the tree.

My proposal for an improved API is to introduce a ModalContext, which will expose the closeModal function. Let's start with the context implementation.

1// modal context file...
2 
3export const ModalContext = React.createContext();
4 
5export const useModal = () => {
6 return useContext(ModalContext);
7}
8 
9export const ModalContextProvider = ({ closeModal, children }) => {
10 const context = useMemo(() => {
11 return {
12 closeModal
13 };
14 }, [closeModal]);
15 
16 return (
17 <ModalContext.Provider value={context}>{children}</ModalContext.Provider>
18 );
19};

Note that there's a useModal hook in there! That will be important in a moment.

Here's the updated Modal component, using the new context:

1// inside your Modal component...
2 
3const Modal = ({
4 onClose
5}) => {
6 const closeModal = useCallback(() => {
7 // Fake code, pretend to start the closing transition
8 // and call `onClose` when the transition is done
9 startCloseTransition(onClose);
10 }, [onClose]);
11 
12 return (
13 <ModalContextProvider closeModal={closeModal}>
14 <ModalOverlay>
15 <ModalContent>
16 <ModalHeader />
17 
18 <ModalBody>
19 {children}
20 </ModalBody>
21 </ModalContent>
22 </ModalOverlay>
23 </ModalContextProvider>
24 );
25};

Notice how ModalOverlay and ModalHeader no longer accept the closeModal prop? Since we've exposed the closeModal function via context, you no longer have to pass it as a prop anymore. Instead, ModalOverlay would look like this:

1// inside ModalOverlay component...
2 
3const ModalOverlay = ({
4 children
5}) => {
6 const { closeModal } = useModal();
7 
8 return (
9 <div onClick={() => closeModal()}>
10 {children}
11 </div>
12 );
13};

You might argue that the example above is worse off because closeModal was a prop before, so we've now added a line by using the hook. You would be right, but let's take a look at an example with nested components, the ModalHeader component.

1// inside ModalHeader component...
2 
3const ModalCloseButton = () => {
4 const {closeModal} = useModal();
5 
6 return (
7 <button type="button" onClick={() => closeModal()}>X</button>
8 );
9}
10 
11const ModalHeader = ({
12 title
13}) => {
14 return (
15 <header>
16 {title}
17 
18 <ModalCloseButton />
19 </header>
20 );
21};

By using context here, we can create a generic ModalCloseButton component that can be used anywhere within a modal to close the modal. No need to worry about ensuring you have access to the correct props because the ModalCloseButton component is responsible for pulling what it needs out of context.

Let's continue by looking at how the parent of the Modal component changes.

1// No changes to `SomeComponent`...
2const SomeComponent = () => {
3 const [modalOpen, setModalOpen] = useState(false);
4 return (
5 <div>
6 <button type="button" onClick={() => setModalOpen(true)}>Open Modal</button>
7 
8 {modalOpen ? (
9 <Modal title="Some Modal" onClose={() => setModalOpen(false)}>
10 <SomeComponentModalContent />
11 </Modal>
12 ) : null}
13 </div>
14 );
15};
16 
17const SomeComponentModalContent = () => {
18 // `SomeComponentModalContent` has access to the modal context
19 const {closeModal} = useModal();
20 
21 return (
22 <div>
23 <p>The content of the modal goes here</p>
24 <button type="button" onClick={() => closeModal()}>Close the modal</button>
25 </div>
26 );
27};

Instead of passing closeModal down however many levels of the tree, we leave it to the component needing closeModal to grab it out of context.

I've tried to make the examples as simple as possible. In the real world application I pulled this from, the ModalContext exposes more than just a closeModal function.

We recently built a custom Stepper component which allows the user to flow through a series of steps in a form. We used the context/hook pattern to allow custom components in each step to control the flow of the Stepper component. The API is so much simpler than prop passing because the custom components only pull what they need out of a hook. Don't need anything? Don't use the hook!


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