useState

useState lets you use local state within a function component. You can view the docs for it here and view a live, editable cheat sheet here.

Declare state variable

Declaring a state variable is as simple as calling useState with some initial state value.

useState(initialStateValue)

Update state variable

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

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

Multiple state variables

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

Use object state variable

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.

Initialize state from function

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

Functional setState

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.

useEffect

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

Basic side effect

Effect with cleanup

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 effects

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.

Skipping effects (array dependency)

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:

Skipping effects (empty array dependency )

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:

Skipping effects (no array dependency)

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

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:

useLayoutEffect

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])

Similar usage as useEffect

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

useLayoutEffect vs. useEffect

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

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.

Basic usage

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.

Initialize state lazily

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.

Imitate this.setState’s behavior

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:

useCallback

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

Starter example

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 with referenced function

useCallback with inline function

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

useMemo

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

Starter example

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

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?

Basic usage

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

useRef

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.

Accessing the DOM

Consider the sample application below:

Instance-like variables (generic container)

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:

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

Other examples

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: