I have the following code :-
const [dice, setDice] = React.useState(allNewDice()) // Array of objects with the following properties: `value` (random digit between 1 and 6), `id` (unique id), `isHeld` (boolean)
function holdDice(id) {
const heldDie = dice.findIndex(die => die.id === id)
setDice(prevDice => {
prevDice[heldDie].isHeld = !prevDice[heldDie].isHeld
return prevDice
})
}
The function holdDice is working as expected. It updates prevDice's first element's isHeld property to the opposite of what was previously, as evident from logging it to the console. But the only problem is, the state is not getting updated.
So I tried making a copy of prevDice and then returning the copy (using the spread (...) operator) :-
const [dice, setDice] = React.useState(allNewDice()) // Array of objects with the following properties: `value` (random digit between 1 and 6), `id` (unique id), `isHeld` (boolean)
function holdDice(id) {
const heldDie = dice.findIndex(die => die.id === id)
setDice(prevDice => {
prevDice[heldDie].isHeld = !prevDice[heldDie].isHeld
return [...prevDice]
})
}
And this works perfectly.
Can anyone please explain why this behaviour? Why can't I return the previous state passed in to be set as the new state, even when I am not returning the previous state, exactly as it was?
I understand that it's expensive to re-render the component and that maybe React does not update the state when the previous and the new states are equal. But here, the previous and the new states are not equal.
My first guess was that it might have been related to React not deep checking whether the objets of the array prevDice have been changed or not, or something like that. So I created an array of Numbers and tried changing it's first element to some other number, but unless i returned a copy of the array, the state still did not change.
Any help would be greatly appreciated. Thank you!
The first time you call setDice() after the initial mount of your component or rerender, prevDice refers to the same array in memory that your dice state refers to (so prevDice === dice). It is not a unqiue copy of the array that you're able to modify. You can't/shouldn't modify it because the object that prevDice refers to is the same object that dice refers to, so you're actually modifying the dice state directly when you change prevState. Because of this, when the modified prevDice is used as the value for setDice(), React checks to see if the new state (which is prevState) is different from the current state (dice) to see if it needs to rerender. React does this equality check by using === (or more specifically Object.is()) to check if the two arrays/objects are the same, and if they are, it doesn't rerender. When React uses === between prevState and dice, JS checks to see if both variables are referring to the same arrays/objects. In our case they are, so React doesn't rerender. That's why it's very important to treat your state as immutable, which you can think of as meaning that you should treat your state as readonly, and instead, if you want to change it, you can make a "copy" of it and change the copy. Doing so will correctly tell React to use that new modified copy as the new state when passed into setDice().
Do note that while your second example does work, you are still modifying your state directly, and this can still cause issues. Your second example works because the array you are returning is a new array [], but the contents of that array (ie: the object references) still refers to the same objects in memory that your original dice state array referred to. To correctly perform your update in an immutable way, you can use something like .map(), which by nature returns a new array (so we'll rerender). When you map, you can return a new inner object when you find the one you want to update (rather than updating it directly). This way, your array is a new array, and any updated objects are also new:
function holdDice(id) {
setDice(prevDice => prevDice.map( // `.map()` returns a new array, so different to the `dice`/`prevDice` state
die => die.id === id
? {...die, isHeld: !die.isHeld} // new object, with overwritten property `isHeld`
: die
));
}
State doesn't update because you update it yourself in the setDice function. dice and prevDice refer to the same object so it won't trigger a re-render.
const [dice, setDice] = React.useState(allNewDice())
function holdDice(id) {
const heldDie = dice.findIndex(die => die.id === id)
setDice(prevDice => {
// Create a copy of the dice
// Use whatever method you wish.Spread. JSON parse and stringify.
const currentDice = JSON.parse(JSON.stringify(prevDice))
currentDice[heldDie].isHeld = !prevDice[heldDie].isHeld
return currentDice
})
}