React Hooks cheat sheet: Unlock solutions to common problems

April 17, 2019 0 Comments

React Hooks cheat sheet: Unlock solutions to common problems

 

 

While Hooks may have been the talk of the community for a while now, beyond the fuss, they do have a very simple API.

This article will highlight examples and use cases, from simple to advanced. I’ve also built an accompanying web app for live interaction with the examples herein.

Please note that this article includes a lot of code snippets and assumes some Hooks fluency. You may want to start here if you’re completely new to Hooks.

Let’s get started.

Updating a state variable is as simple as invoking the updater function returned by the useState invocation.

const [stateValue, updaterFn] = useState(initialStateValue);

Note how the age state variable is being updated.
Here’s the code responsible for the screencast above.

Multiple state variables may be used and updated from within a functional component, as shown below:

Here’s the code responsible for the screencast above.

As opposed to strings and numbers, you could also use an object as the initial value passed to useState.

Note that you have to pass the entire object to the useState updater function because the object is replaced, not merged.

setState vs. useState updater function.
Multiple state objects updated via a state object variable.
Here’s the code for the screencast above.

As opposed to just passing an initial state value, state could also be initialized from a function, as shown below:

The updater function returned from invoking useState can also take a function similar to the good ol’ setState:

const [value, updateValue] = useState(0)
// both forms of invoking "updateValue" below are valid 👇
updateValue(1);
updateValue(previousValue => previousValue + 1);

This is ideal when the state update depends on some previous value of state.

A counter with functional setState updates.
Here’s the code for the screencast above.

useEffect accepts a function, which can perform any side effects. View the docs here, and check out the live, editable cheat sheet.

Watch the title of the document update.
Here’s the code responsible for the screencast above.

It’s pretty common to clean up an effect after some time. This is possible by returning a function from within the effect function passed to useEffect. Below is an example with addEventListener.

Multiple useEffect calls can happen within a functional component, as shown below:

Note thatuseEffect calls can be skipped — i.e., not invoked on every render. This is done by passing a second array argument to the effect function.

In the example above, useEffect is passed an array of one value: [randomNumber].

Thus, the effect function will be called on mount and whenever a new random number is generated.

Here’s the “Generate random number” button being clicked and the effect function being rerun upon generating a new random number:

In this example, useEffect is passed an empty array, []. Therefore, the effect function will be called only on mount.

Here’s the button being clicked and the effect function not invoked:

Without an array dependency, the effect function will be run after every single render.

useEffect(() => {
  console.log(“This will be logged after every render!”)
})

useContext saves you the stress of having to rely on a Context consumer. It has a simpler API when compared to MyContext.Consumer and the render props API it exposes. View the docs here, and view a live, editable cheat sheet.

The following example highlights the difference between consuming a context object value via useContext or Context.Consumer:

Here’s a live example with useContext:

Here’s the code responsible for the example above.

useLayoutEffect has the very same signature as useEffect. We’ll discuss the difference between useLayoutEffect and useEffect below. Again, view the docs and the live, editable cheat sheet.

useLayoutEffect(() => {
    //do something
}, [arrayDependency])

Here’s the same example for useEffect built with useLayoutEffect:

Here’s the code responsible for the screencast above.

The function passed to useEffect fires after layout and paint, i.e., after the render has been committed to the screen. This is OK for most side effects that shouldn’t block the browser from updating the screen.

There are cases where you may not want the behavior useEffect provides, though; for example, if you need to make a visual change to the DOM as a side effect, useEffect won’t be the best choice.

To prevent the user from seeing flickers of changes, you can use useLayoutEffect. The function passed to useLayoutEffect will be run before the browser updates the screen.

useReducer may be used as an alternative to useState. It’s ideal for complex state logic where there’s a dependency on previous state values or a lot of state sub-values.

Depending on your use case, you may find useReducer quite testable. View the docs and the live, editable cheat sheet.

As opposed to calling useState, call useReducer with a reducer and initialState, as shown below. The useReducer call returns the state property and a dispatch function.

Increase/decrease bar size by managing state with useReducer
Here’s the code responsible for the screencast above.

useReducer takes a third function parameter. You may initialize state from this function, and whatever’s returned from this function is returned as the state object. This function will be called with initialState — the second parameter.

Same increase/decrease bar size — with state initialized lazily.
Here’s the code responsible for the screencast above.

useReducer uses a reducer that isn’t as strict as Redux’s. For example, the second parameter passed to the reducer, action, doesn’t need to have a type property.

This allows for interesting manipulations, such as renaming the second parameter and doing the following:

The results remain the same with a setState like api imitated.

useCallback returns a memoized callback. View the docs and the view live, editable cheat sheet here.

The following example will form the basis of the explanations and code snippets that follow.

In the example above, the parent component, <Age />, is updated (and re-rendered) whenever the “Get older” button is clicked.

Consequently, the <Instructions /> child component is also re-rendered because the doSomething prop is passed a new callback with a new reference.

Note that even though the Instructions child component uses React.memo to optimize performace, it is still re-rendered.

How can this be fixed to prevent <Instructions /> from re-rendering needlessly?

useCallback also works with an inline function as well. Here’s the same solution with an inline useCallback call:

useMemo returns a memoized value. View the docs and the live, editable cheat sheet.

The following example will form the basis of the explanations and code snippets that follow.

Here’s the code responsible for the screenshot above.

The example above is similar to the one foruseCallback. The only difference here is that someValue is an object, not a string. Owing to this, the Instructions component still re-renders despite the use of React.memo

Why? Objects are compared by reference, and the reference to someValue changes whenever <App /> re-renders.

Any solutions?

The object someValue may be memoized using useMemo. This prevents the needless re-render.

useRef returns a “ref” object. Values are accessed from the .current property of the returned object. The.current property could be initialized to an initial value — useRef(initialValue), for example. The object is persisted for the entire lifetime of the component. Reference the docs and the live, editable cheat sheet.

Consider the sample application below:

Accessing the DOM via useRef.
Here’s the code responsible for the screencast above.

Other than just holding DOM refs, the “ref” object can hold any value. Consider a similar application below, where the ref object holds a string value:

Here’s the code responsible for the screencast above.

You could do the same as storing the return value from a setInterval for cleanup.

Working on a near-real-world example can help bring your knowledge of Hooks to life. Until data fetching with React Suspense is released, fetching data via Hooks proves to be a good exercise for more Hooks practice.

Below’s an example of fetching data with a loading indicator:

Here’s the code responsible for the screencast above.

Hooks give a lot of power to functional components. I hope this cheat sheet proves useful in your day-to-day use of Hooks. Cheers!

Thanks to Hooks and a couple other new React features. Illustration by me :)

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