Bypassing props (useContext)

Whenever you're utilizing a component architecture, as your application grows, the ability to share state amongst different components will inevitably become an issue.

Let's pretend we had an app with the following architecture, each circle representing a different component.

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Now let's pretend that we had a piece of state that was needed throughout various levels of our application.

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The recommended solution for this problem is to move that state up to the nearest parent component and then pass it down via props.

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This works, and most of the time it's the right solution. However, there are times when passing props through intermediate components can become overly redundant or downright unmanageable. Take a tool like React Router for example. React Router needs to have the ability to pass routing props to any component in the component tree, regardless of how deeply nested the components are. Because this is such a significant problem, React comes with a built-in API to solve it called Context.

Context provides a way to pass data through the component tree without having to pass props down manually at every level. - The React Docs

Now that we know the problem that Context solves, how do we use it?

The Context API

For our example, let's say we're building an app that is used by both English and Spanish speaking countries. We want to expose a button that when it's clicked, can toggle the text of our entire application between English and Spanish.

From a high level, if you think about what's needed to solve this problem, there are two aspects to it.

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React gives us the ability to do both of those things whenever we create a new Context using the **React.createContext** method. Typically, you create a new Context for each unique piece of data that needs to be available throughout your component tree. Based on our example, we'll create a **LocaleContext**.

const LocaleContext = React.createContext();

Now if we examine our **LocaleContext**, you'll notice that it has two properties, both of which are React components, **Provider**, and **Consumer**.

**Provider** allows us to "declare the data that we want available throughout our component tree".

**Consumer** allows "any component in the component tree that needs that data to be able to subscribe to it".

Provider

You use **Provider** just like you would any other React component. It accepts a **value**prop which is the data that you want available to any of its **children** who need to consume it.

In our example, we want **locale** to be available anywhere in the component tree. We also want to update the UI (re-render) whenever it changes, so we'll stick it on our component's state.

Now, any component in our component tree that needs the value of **locale** will have the option to subscribe to it using **LocaleContext.Consumer**.

Consumer

Again, the whole point of the **Consumer** component is it allows you to get access to the data that was passed as a **value** prop to the Context's **Provider** component. To do this, **Consumer** uses a render prop.

Now in our example, because we passed **locale** as the **value** prop to **LocaleContext.Provider**, we can get access to it by passing **LocaleContext.Consumer** a render prop.



Updating Context State

At this point, we've seen that because we wrapped our whole app in **<LocaleContext.Provider value={locale}>**, any component in our application tree can get access to **locale** by using **LocaleContext.Consumer**. However, what if we also want to be able to toggle it (**en** -> **es**) from anywhere inside of our component tree?

Your first intuition might be to do something like this.

What we've done is added a new property to the object we pass to **value**. Now, anywhere in our component tree, using **LocaleContext.Consumer**, we can grab **locale** OR **toggleLocale**.

Sadly, the idea is right, but the execution is a little off. Can you think of any downsides to this approach? Hint, it has to do with performance.

Just like React re-renders with prop changes, whenever the data passed to **value**changes, React will re-render every component which used **Consumer** to subscribe to that data. The way in which React knows if the data changes is by using "reference identity" (which is kind of a fancy way of saving **oldObject** === **newObject**).

Currently with how we have it set up (**value={{}}**), we're passing a new object to **value** every time that **App** re-renders. What this means is that when React checks if the data passed to **value** has changed, it'll always think it has since we're always passing in a new object. As a result of that, every component which used **Consumer** to subscribe to that data will re-render as well, even if **locale** or **toggleLocale** didn't change.

To fix this, instead of passing a new object to **value** every time, we want to give it a reference to an object it already knows about. To do this, we can use the **useMemo**Hook.

React will make sure the **value** that **useMemo** returns stays the same unless **locale**changes. This way, any component which used **Consumer** to subscribe to our **locale**context will only re-render if **locale** changes.

Now, anywhere inside of our component tree, we can get access to the **locale** value or the ability to change it via **toggleLocale**.

Here's a link to the full **locale** app if you want to play around with it. Admittedly, it's not the best use of Context as it's a pretty shallow app, but it gives you the general idea how to use Context in an app with multiple routes/components.

defaultValue

Whenever you render a **Consumer** component, it gets its value from the **value** prop of the nearest **Provider** component of the same Context object. However, what if there isn't a parent **Provider** of the same Context object? In that case, it'll get its value from the first argument that was passed to **createContext** when the Context object was created.

const MyContext = React.creatContext("defaultValue");

And adapted to our example.

const LocaleContext = React.createContext("en");

Now, if we use **<LocaleContext.Consumer>** without previously rendering a **<LocaleContext.Provider>**, the value passed to **Consumer** will be **en**.




Here's a very clever example my good friend chantastic came up with. I've modified it a bit, but the core idea is his.

Can you follow what's going on? First, we create a new **ExpletiveContext** and set its default value to **shit**. Then we render two components, **VisitFriendsHouse** and **VisitGrandmasHouse**.

Because we're allowed to swear at our friend's house, **VisitFriendsHouse** renders **ExpletiveContext.Consumer** whose value will default to **shit** since there's not an **ExpletiveContext.Provider** in the tree above it.

Unlike at our friends, with Grandma, we're not allowed to swear. So instead of just rendering **ExpletiveContext.Consumer**, we wrap it in **ExpletiveContext.Provider** passing it a value of **poop**. This way when the **Consumer** looks for its nearest **Provider**, it'll find it and get a value of **poop** rather than the default value of **shit**.

useContext

At this point, you've seen that in order to get access to the data that was passed as a **value** prop to the Context's **Provider** component, you use **Consumer** as a render prop.

This works, but as always the render-props syntax is a little funky. The problem gets worse if you have multiple context values you need to grab.

Oof. Luckily for us, there's a Hook that solves this problem - **useContext****useContext ** takes in a Context object as its first argument and returns whatever was passed to the **value** prop of the nearest **Provider** component. Said differently, it has the same use case as **.Consumer** but with a more composable API.

As always, this API really shines when you need to grab multiple values from different Contexts.

Warnings

Here's the thing, when you're a hammer, everything looks like a nail. Typically when you first learn about Context, it appears like it's the solution to all your problems. Just remember, there's nothing wrong with passing props down multiple levels, that's literally how React was designed. I don't have a universal rule for when you should and shouldn't use Context, just be mindful that it's common to overuse it.

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