Using the State Hook
Hooks are a new addition in React 16.8. They let you use state and other React features without writing a class.
The introduction page used this example to get familiar with Hooks:
We'll start learning about Hooks by comparing this code to an equivalent class example.
Equivalent Class Example
If you used classes in React before, this code should look familiar:
The state starts as { count: 0 }
, and we increment state.count
when the user clicks a button by calling this.setState()
. We'll use snippets from this class throughout the page.
Note
You might be wondering why we're using a counter here instead of a more realistic example. This is to help us focus on the API while we're still making our first steps with Hooks.
Hooks and Function Components
As a reminder, function components in React look like this:
or this:
You might have previously known these as "stateless components". We're now introducing the ability to use React state from these, so we prefer the name "function components".
Hooks don't work inside classes. But you can use them instead of writing classes.
What's a Hook?
Our new example starts by importing the useState
Hook from React:
What is a Hook? A Hook is a special function that lets you "hook into" React features. For example, useState
is a Hook that lets you add React state to function components. We'll learn other Hooks later.
When would I use a Hook? If you write a function component and realize you need to add some state to it, previously you had to convert it to a class. Now you can use a Hook inside the existing function component. We're going to do that right now!
Note:
There are some special rules about where you can and can't use Hooks within a component. We'll learn them in Rules of Hooks.
Declaring a State Variable
In a class, we initialize the count
state to 0
by setting this.state
to { count: 0 }
in the constructor:
In a function component, we have no this
, so we can't assign or read this.state
. Instead, we call the useState
Hook directly inside our component:
What does calling useState
do? It declares a "state variable". Our variable is called count
but we could call it anything else, like banana
. This is a way to "preserve" some values between the function calls — useState
is a new way to use the exact same capabilities that this.state
provides in a class. Normally, variables "disappear" when the function exits but state variables are preserved by React.
What do we pass to useState
as an argument? The only argument to the useState()
Hook is the initial state. Unlike with classes, the state doesn't have to be an object. We can keep a number or a string if that's all we need. In our example, we just want a number for how many times the user clicked, so pass 0
as initial state for our variable. (If we wanted to store two different values in state, we would call useState()
twice.)
What does useState
return? It returns a pair of values: the current state and a function that updates it. This is why we write const [count, setCount] = useState()
. This is similar to this.state.count
and this.setState
in a class, except you get them in a pair. If you're not familiar with the syntax we used, we'll come back to it at the bottom of this page.
Now that we know what the useState
Hook does, our example should make more sense:
We declare a state variable called count
, and set it to 0
. React will remember its current value between re-renders, and provide the most recent one to our function. If we want to update the current count
, we can call setCount
.
Note
You might be wondering: why is
useState
not namedcreateState
instead?"Create" wouldn't be quite accurate because the state is only created the first time our component renders. During the next renders,
useState
gives us the current state. Otherwise it wouldn't be "state" at all! There's also a reason why Hook names always start withuse
. We'll learn why later in the Rules of Hooks.
Reading State
When we want to display the current count in a class, we read this.state.count
:
In a function, we can use count
directly:
Updating State
In a class, we need to call this.setState()
to update the count
state:
In a function, we already have setCount
and count
as variables so we don't need this
:
Recap
Let's now recap what we learned line by line and check our understanding.
Line 1: We import the
useState
Hook from React. It lets us keep local state in a function component.Line 4: Inside the
Example
component, we declare a new state variable by calling theuseState
Hook. It returns a pair of values, to which we give names. We're calling our variablecount
because it holds the number of button clicks. We initialize it to zero by passing0
as the onlyuseState
argument. The second returned item is itself a function. It lets us update thecount
so we'll name itsetCount
.Line 9: When the user clicks, we call
setCount
with a new value. React will then re-render theExample
component, passing the newcount
value to it.
This might seem like a lot to take in at first. Don't rush it! If you're lost in the explanation, look at the code above again and try to read it from top to bottom. We promise that once you try to "forget" how state works in classes, and look at this code with fresh eyes, it will make sense.
Tip: What Do Square Brackets Mean?
You might have noticed the square brackets when we declare a state variable:
The names on the left aren't a part of the React API. You can name your own state variables:
This JavaScript syntax is called "array destructuring". It means that we're making two new variables fruit
and setFruit
, where fruit
is set to the first value returned by useState
, and setFruit
is the second. It is equivalent to this code:
When we declare a state variable with useState
, it returns a pair — an array with two items. The first item is the current value, and the second is a function that lets us update it. Using [0]
and [1]
to access them is a bit confusing because they have a specific meaning. This is why we use array destructuring instead.
Note
You might be curious how React knows which component
useState
corresponds to since we're not passing anything likethis
back to React. We'll answer this question and many others in the FAQ section.
Tip: Using Multiple State Variables
Declaring state variables as a pair of [something, setSomething]
is also handy because it lets us give different names to different state variables if we want to use more than one:
In the above component, we have age
, fruit
, and todos
as local variables, and we can update them individually:
You don't have to use many state variables. State variables can hold objects and arrays just fine, so you can still group related data together. However, unlike this.setState
in a class, updating a state variable always replaces it instead of merging it.
We provide more recommendations on splitting independent state variables in the FAQ.
Next Steps
On this page we've learned about one of the Hooks provided by React, called useState
. We're also sometimes going to refer to it as the "State Hook". It lets us add local state to React function components -- which we did for the first time ever!
We also learned a little bit more about what Hooks are. Hooks are functions that let you "hook into" React features from function components. Their names always start with use
, and there are more Hooks we haven't seen yet.
Now let's continue by learning the next Hook: useEffect
. It lets you perform side effects in components, and is similar to lifecycle methods in classes.
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