useState Flashcards
userState
useState is a React Hook that lets you add a state variable to your component.
Adding state to a component
Call useState at the top level of your component to declare one or more state variables.
The convention is to name state variables like [something, setSomething] using array destructuring.
useState returns an array with exactly two items:
- The current state of this state variable, initially set to the initial state you provided.
- The set function that lets you change it to any other value in response to interaction.
To update what’s on the screen, call the set function with some next state:
function handleClick() {
setName('Robin');
}
React will store the next state, render your component again with the new values, and update the UI.
Calling the set function does not change the current state in the already executing code:
function handleClick() {
setName(‘Robin’);
console.log(name); // Still “Taylor”!
}
It only affects what useState will return starting from the next render.
Updating state based on the previous state
Suppose the age is 42. This handler calls setAge(age + 1) three times:
function handleClick() {
setAge(age + 1); // setAge(42 + 1)
setAge(age + 1); // setAge(42 + 1)
setAge(age + 1); // setAge(42 + 1)
}
However, after one click, age will only be 43 rather than 45! This is because calling the set function does not update the age state variable in the already running code. So each setAge(age + 1) call becomes setAge(43).
To solve this problem, you may pass an updater function to setAge instead of the next state:
function handleClick() {
setAge(a => a + 1); // setAge(42 => 43)
setAge(a => a + 1); // setAge(43 => 44)
setAge(a => a + 1); // setAge(44 => 45)
}
Here, a => a + 1 is your updater function. It takes the pending state and calculates the next state from it.
React puts your updater functions in a queue. Then, during the next render, it will call them in the same order:
a => a + 1 will receive 42 as the pending state and return 43 as the next state.
a => a + 1 will receive 43 as the pending state and return 44 as the next state.
a => a + 1 will receive 44 as the pending state and return 45 as the next state.
There are no other queued updates, so React will store 45 as the current state in the end.
Updating objects and arrays in state
You can put objects and arrays into state. In React, state is considered read-only, so you should replace it rather than mutate your existing objects. For example, if you have a form object in state, don’t update it like this:
// 🚩 Don't mutate an object in state like this: form.firstName = 'Taylor';
Instead, replace the whole object by creating a new one:
// ✅ Replace state with a new object
setForm({
...form,
firstName: 'Taylor'
});
Avoiding recreating the initial state
React saves the initial state once and ignores it on the next renders.
function TodoList() {
const [todos, setTodos] = useState(createInitialTodos());
// ...
Although the result of createInitialTodos() is only used for the initial render, you’re still calling this function on every render. This can be wasteful if it’s creating large arrays or performing expensive calculations.
To solve this, you may pass it as an initializer function to useState instead:
function TodoList() {
const [todos, setTodos] = useState(createInitialTodos);
// ...
Notice that you’re passing createInitialTodos, which is the function itself, and not createInitialTodos(), which is the result of calling it. If you pass a function to useState, React will only call it during initialization.
Resetting state with a key
Typically, you might encounter the key attribute when rendering lists. However, it also serves another purpose.
You can reset a component’s state by passing a different key to a component. In this example, the Reset button changes the version state variable, which we pass as a key to the Form. When the key changes, React re-creates the Form component (and all of its children) from scratch, so its state gets reset.
Storing information from previous renders
Usually, you will update state in event handlers. However, in rare cases you might want to adjust state in response to rendering — for example, you might want to change a state variable when a prop changes.
In most cases, you don’t need this:
- If the value you need can be computed entirely from the current props or other state, remove that redundant state altogether. If you’re worried about recomputing too often, the useMemo Hook can help.
- If you want to reset the entire component tree’s state, pass a different key to your component.
- If you can, update all the relevant state in the event handlers.
In the rare case that none of these apply, there is a pattern you can use to update state based on the values that have been rendered so far, by calling a set function while your component is rendering.
Here’s an example. This CountLabel component displays the count prop passed to it:
export default function CountLabel({ count }) {
return <h1>{count}</h1>
}
Say you want to show whether the counter has increased or decreased since the last change. The count prop doesn’t tell you this — you need to keep track of its previous value. Add the prevCount state variable to track it. Add another state variable called trend to hold whether the count has increased or decreased. Compare prevCount with count, and if they’re not equal, update both prevCount and trend. Now you can show both the current count prop and how it has changed since the last render.
export default function CountLabel({ count }) {
const [prevCount, setPrevCount] = useState(count);
const [trend, setTrend] = useState(null);
if (prevCount !== count) {
setPrevCount(count);
setTrend(count > prevCount ? ‘increasing’ : ‘decreasing’);
}
return (
<>
<h1>{count}</h1>
{trend && <p>The count is {trend}</p>}
>
);
}
Note that if you call a set function while rendering, it must be inside a condition like prevCount !== count, and there must be a call like setPrevCount(count) inside of the condition. Otherwise, your component would re-render in a loop until it crashes. Also, you can only update the state of the currently rendering component like this. Calling the set function of another component during rendering is an error. Finally, your set call should still update state without mutation — this special case doesn’t mean you can break other rules of pure functions.
Reference
Call useState at the top level of your component to declare a state variable.
Parameters
initialState: The value you want the state to be initially. It can be a value of any type, but there is a special behavior for functions. This argument is ignored after the initial render.
If you pass a function as initialState, it will be treated as an initializer function. It should be pure, should take no arguments, and should return a value of any type. React will call your initializer function when initializing the component, and store its return value as the initial state.
Returns
- The current state. During the first render, it will match the initialState you have passed.
- The set function that lets you update the state to a different value and trigger a re-render.
Caveats
useState is a Hook, so you can only call it at the top level of your component or your own Hooks. You can’t call it inside loops or conditions. If you need that, extract a new component and move the state into it.
In Strict Mode, React will call your initializer function twice in order to help you find accidental impurities. This is development-only behavior and does not affect production. If your initializer function is pure (as it should be), this should not affect the logic of your component. The result from one of the calls will be ignored.
- set functions, like setSomething(nextState)
The set function returned by useState lets you update the state to a different value and trigger a re-render. You can pass the next state directly, or a function that calculates it from the previous state
Parameters
nextState: The value that you want the state to be. It can be a value of any type, but there is a special behavior for functions.
If you pass a function as nextState, it will be treated as an updater function. It must be pure, should take the pending state as its only argument, and should return the next state. React will put your updater function in a queue and re-render your component. During the next render, React will calculate the next state by applying all of the queued updaters to the previous state
Returns
set functions do not have a return value.
Caveats
- The set function only updates the state variable for the next render. If you read the state variable after calling the set function, you will still get the old value that was on the screen before your call.
- If the new value you provide is identical to the current state, as determined by an Object.is comparison, React will skip re-rendering the component and its children. This is an optimization. Although in some cases React may still need to call your component before skipping the children, it shouldn’t affect your code.
- React batches state updates. It updates the screen after all the event handlers have run and have called their set functions. This prevents multiple re-renders during a single event. In the rare case that you need to force React to update the screen earlier, for example to access the DOM, you can use flushSync.
- Calling the set function during rendering is only allowed from within the currently rendering component. React will discard its output and immediately attempt to render it again with the new state. This pattern is rarely needed, but you can use it to store information from the previous renders. See an example above.
- In Strict Mode, React will call your updater function twice in order to help you find accidental impurities. This is development-only behavior and does not affect production. If your updater function is pure (as it should be), this should not affect the logic of your component. The result from one of the calls will be ignored.
Troubleshooting
- I’ve updated the state, but logging gives me the old value
This is because states behaves like a snapshot. Updating state requests another render with the new state value, but does not affect the count JavaScript variable in your already-running event handler.
- I’ve updated the state, but the screen doesn’t update
React will ignore your update if the next state is equal to the previous state, as determined by an Object.is comparison. This usually happens when you change an object or an array in state directly
- I’m getting an error: “Too many re-renders”
You might get an error that says: Too many re-renders. React limits the number of renders to prevent an infinite loop. Typically, this means that you’re unconditionally setting state during render, so your component enters a loop: render, set state (which causes a render), render, set state (which causes a render), and so on. Very often, this is caused by a mistake in specifying an event handler
If you can’t find the cause of this error, click on the arrow next to the error in the console and look through the JavaScript stack to find the specific set function call responsible for the error.
- I’m trying to set state to a function, but it gets called instead
You can’t put a function into state like this:
const [fn, setFn] = useState(someFunction);
function handleClick() {
setFn(someOtherFunction);
}
Because you’re passing a function, React assumes that someFunction is an initializer function, and that someOtherFunction is an updater function, so it tries to call them and store the result. To actually store a function, you have to put () => before them in both cases. Then React will store the functions you pass.
const [fn, setFn] = useState(() => someFunction);
function handleClick() {
setFn(() => someOtherFunction);
}
