What's the difference between
1
const [state, setState] = useState(0)
setState(state+1)
2
const [state, setState] = useState(0)
setState(...prevState => prevState+1)
In the first option, based on the documentation:
The setState function is used to update the state. It accepts a new state value and enqueues a re-render of the component.
In the second option, called functional update:
If the new state is computed using the previous state, you can pass a function to setState. The function will receive the previous value, and return an updated value.
So basically if you'd like to be sure your state will be updated based on the previous state, you need to use the second option.
Read further in the official documentation of useState.
I hope this clarifies!
Related
I am new to react Hooks. Am trying to make use of useState in my code. While I was using it I found a term "Lazy initial state"
https://reactjs.org/docs/hooks-reference.html#lazy-initial-state
const [state, setState] = useState(() => {
const initialState = someExpensiveComputation(props);
return initialState;
});
But I am not able to think of any use case where this lazy initialising of state will be useful.
Like say my DOM is rendering and it needs the state value, but my useState has not initialised it yet! And say if you have rendered the DOM and the someExpensiveComputation has finished, the DOM will re-render!
The value passed to the useState hook in the initial render is the initial state value, and gets disregarded in subsequent renders. This initial value can be the result of calling a function as in the following situation:
const Component = () => {
const [state, setState] = useState(getInitialHundredItems())
}
But note that getInitialHundredItems is unconditionally and needlessly called on each render cycle.
For use cases like this instead of just calling a function that returns a value you can pass a function which returns the initial state. This function will only be executed once (initial render) and not on each render like the above code will. See Lazy Initial State for details.
const Component = () =>{
const [state, setState] = useState(getInitialHundredItems)
}
As per React official documentation, if a component new state is computed using the previous state, one can pass a function to setState. The function will receive the previous value, and return an updated value.
Consider the following code snippet example:
const [counter, setCounter] = useState(0);
const btnClick = useCallback(() => setCounter(counter+1), [counter]); //(1)
const btnClick = useCallback(() => setCounter(previous => previous+1), []);//(2)
btnClick is passed as a callback function for the 'onClick' event of a rendered button.
In (1), the btnClick function reference is updated each time the counter state variable is modified.
In (2), the btnClick function reference is created during the first render cycle and is cached for the rest of the component lifetime; at this level, the state update function will receive the previous state value and return an updated value.
At this level, I have 2 questions:
1- As a good practice, should I opt for (1) or (2)? And are there any advantages for using (1) instead of (2)?
2- Does (2) apply to both primitive and reference types? (check below, I switched the state from being an Integer to an Object)
const [counter, setCounter] = useState({cntr:1});
const btnClick = useCallback(() => setCounter({...counter,cntr:counter.cntr+1}), [counter]); //(1)
const btnClick = useCallback(() => setCounter(previous => return {...previous ,cntr:previous.cntr+1}), []);//(2)
Does "previous" in (2) still reference the previous state value or does it refer to a stale state that was cached during the component first render cycle?
Your help is appreciated.
So, it is a deep question, let's try to explain how it works.
useCallback just promise you that btnClick will not reinitialize. This does not related to useState and body of function at all.
In react community we do not have any good practices on this point. The difference is when you call setState react promise you that sometimes value will be updated.
For example:
setValue(1);
setValue(2);
setValue(3);
It does not mean react will update the state three times. React can combine it to one update and set just last value.
Pass function as a argument to setState
const myFunc = (oldValue) => ({})
useValue(myFunc)
it means that react sees you pass function then all useValue will be called immediately and after useValue(myFunc) will be called.
Developers need it to work with last data which is placed in the state.
Does "previous" in (2) still reference the previous state value or
does it refer to a stale state that was cached during the component
first render cycle?
So, as a result of the question previous in setState will be recent value from state.
What's the difference between
1
const [state, setState] = useState(0)
setState(state+1)
2
const [state, setState] = useState(0)
setState(...prevState => prevState+1)
In the first option, based on the documentation:
The setState function is used to update the state. It accepts a new state value and enqueues a re-render of the component.
In the second option, called functional update:
If the new state is computed using the previous state, you can pass a function to setState. The function will receive the previous value, and return an updated value.
So basically if you'd like to be sure your state will be updated based on the previous state, you need to use the second option.
Read further in the official documentation of useState.
I hope this clarifies!
What is the case where you use useState's initial value as a function?
Is there any difference from just passing a value?
e.g.
const [state, setState] = useState(() => someValue)
You use it when you want the computation of that initial state to happen only once. Because if you use an expression instead say:
const [state, setState] = useState(compute())
The compute runs on other renders too, just its value is ignored after first* render.
So if you do:
const [state, setState] = useState(() => compute())
Now, compute will run only once.
From the docs:
const [state, setState] = useState(initialState);
The initialState argument is the state used during the initial render.
In subsequent renders, it is disregarded. If the initial state is the
result of an expensive computation, you may provide a function
instead, which will be executed only on the initial render
const [state, setState] = useState(() => {
const initialState = someExpensiveComputation(props);
return initialState;
});
* Well if it is strict mode then it could be the value of first render gets ignored too due to double invoking the render method. But this is not important for this answer. Because the value would now be ignored after second render.
If you want to use useState's initial value as a function, you need to use currying :
const [state, setState] = useState(() => () => someValue);
This is because in the documentation, useState executes the provided function and considers its result as the initial value.
Using currying, () => someValue is returned and considered to be the intial value.
When we have some heavy computation to initialize the state, we should use the function. And this is lazy initialization of the state. Here is a well-written blog on react state lazy initialization by kentcdodds.
The reason why wrapping in a function is less computationally heavy is because react will rerun the component function when props change. If useState() has a computation inside like useState(heavyComputation()), JavaScript will run heavyComputation(), but in useState(()=>heavyComputation()), JavaScript will not run heavyComputation(), but will pass in the curried function, and useState knows not to rerun this function.
Normal
Render 1: useState(heavyComputation()) //JavaScript calls heavyComputation
Render 2: useState(heavyComputation()) //JavaScript calls heavyComputation
Render 3: useState(heavyComputation()) //JavaScript calls heavyComputation
...
Wrapped in Function (curried)
Render 1: useState(()=>heavyComputation()) //useState() calls heavyComputation
Render 2: useState(()=>heavyComputation()) //useState() uses the value from the previous render so heavyComputation() is not called
Render 3: useState(()=>heavyComputation()) //useState() uses the value from the previous render so heavyComputation() is not called
...
Sorry if someone has already answered this question, but I didn't find what I am looking for.
I recently started learning react and notice that there are quite a few ways to set the state. For example, I have a counter in an object, and I want to increase it.
const [state, setState] = React.useState({ counter: 0 });
And all the functions below give the same result, but as I understood, they do it asynchronously.
setState({ ...state, counter: counter + 1 }):
setState(() => ({ ...state, counter: counter + 1 }));
setState(prevState => ({...prevState, counter: counter + 1 }));
setState(counter = counter + 1);
How can I update the state instantly and properly after calling the setState function? Thank you in advance!
In order to do some logic with the newly updated value of the state, you should use
The UseEffect Hook
useEffect(() => {
/* use the new state value here */
}, [state])
or, [if you're using component classes], the callback function
this.setState({ ...state, counter: counter++ },
() => { /* use the new state value here */ }
);
Also for a simple counter and in order not to be confused betweeen react components and react hooks... I would recommend using the useState like this :
const [counter, setCounter] = React.useState(0);
setCounter(counter++);
What do you mean by
How can I update the state instantly and properly after calling the setState function?
?
You actually are updating state in the setState function, so the question itself sound strangely.
If you want to perform some action synchronously right after new value will be set, you can use callback function, which was mentioned by Seba99. Here are link to docs (basically it's setState docs). Optional callback is executed after state update, and this.state inside it will always be up-to-date with latest changes, you've made in setState.
So, if you need to synchronously get latest state and perform some actions with it (even update state one more time) - use callback of this.setState.
How can I update the state instantly
You can't, because you've already declared your state with const, right?
const /*const*/ [state, setState] = React.useState({ counter: 0 });
function handleClick() {
setState(anything)
// you declared state with const, you "obviously" shouldn't expect this
// to "somehow" immediately change `state` to anything
}
You can only make it work as expected even when state's updated asynchronously, or not instantly, depends on circumstances. If you want to get the newly updated value of your state to use later in a consequence, cache that new value to use is the right way. For example:
const newStateA = changeState(stateA)
setStateA(newStateA)
setStateB(calculateBFromA(newStateA))
// instead of
setStateB(calculateBFromA(stateA)) // will get the old value of stateA
Of course, you could just set state = something if you haven't declared it with const, like let or var instead, it would change state instantly, but it wouldn't tell React to rerender the component later with the newly updated state value, unless you just set it with with the "second array-destructured" param setState (talking about Hooks), and btw this (declare state with let/var`) is obviously the wrong way