In this pen, I am having a hard time understanding why when using the push function on a newly created const (line 24) it matters whether one uses the spread operator or simply copies "state" over. How is it that replacing [...state] with state causes state to be overwritten with the content of newState?
Am I correct in assuming that it is the reference to state which is overwritten if state is directly passed to newState? I.e. in the case of const newState = state
I assume this is the section or the pen you are referencing:
const newState = [...state]; // copy contents of state, NOT the reference to it (state)
newState.push(action.payload);
return newState;
Let's look at the difference between what this code is doing now, and what it would do if const newState = state was used instead.
const newState = [...state];
This line creates a new array, called newState with the contents of state. It is essentially a one liner for:
const newState = [];
for (let i = 0; i < state.length; i++) {
newState.push(state[i]);
}
When newState.push(action.payload) is called, the state array is unchanged, while newState has an extra element pushed into it.
const newState = state;
This line sets the variable newState to the same reference as state, so there is only one array in play.
When newState.push(action.payload) is called, the new element is pushed into the array, so both newState and state have changed.
Using the spread operator like this will take all the items from state and create a new array with these items. So state and newState will reference different arrays (which have the same elements, so if one of these elements is an object and you change one of it's properties, that change will be visible in the other array). Writing const newState = state will result in one array and two references to that array, since you're not creating a second array anywhere.
Related
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
})
}
I want to be able to copy/clone the store state and modify it locally. How can I copy the object and get rid of the read-only property?
let a = store.getState();
console.log(a.property) // 'property'
let b = copy(a)
b.property = 'newProperty';
console.log(b.property) // 'newProperty'
One way is that you can get state from store and then spread it into a new object.
Like:
const state = store.getState()
const newState = {...state}
Now, you can modify state object.
But, if you have multi level objects in state then use JSON.stringify and then use JSON.parse to parse it.
Like this.
const state = store.getState();
const newState = JSON.parse(JSON.stringify(state));
It will make a whole new clone and now you can modify state.
I recommend here using JSON.stringify because you can have multi level objects in your state.
And spread only do shadow copy.
Copy using
let b = JSON.parse(JSON.stringify(a))
I just start picking up react.js so I went through a lot of tutorials and I've stumbled upon this bit which basically meant to delete an item from the state.
this is how the guy introduced to me the delete function
delTodo = id => {
this.setState({
todos: [...this.state.todos.filter(todo => todo.id !== id)]
});
};
Since I am not so familiar with javascript I had a hard time figuring out what the ... operator is doing and why exactly is he using it in the given scenario. So in order to have a better understanding of how it works, I played a bit in the console and I've realised that array = [...array]. But is that true?
Is this bit doing the same exact thing as the one from above?
delTodo = id => {
this.setState({
todos: this.state.todos.filter(todo => todo.id !== id)
});
};
Could someone more experienced clarify to me why he has chosen to be using that approach instead of the one I've come up with?
As per the documentation:
Never mutate this.state directly, as calling setState() afterwards may replace the mutation you made. Treat this.state as if it were immutable.
reactjs.org/docs/react-component.html#state
So, in the example from the tutorial you've mentioned, you wouldn't need to make a copy of the array to update your state.
// GOOD
delTodo = id => {
this.setState({
todos: this.state.todos.filter(...)
})
}
Array.filter method creates a new array and does not mutate the original array, therefore it won't directly mutate your state. Same thing applies to methods such as Array.map or Array.concat.
If your state is an array and you're applying methods that are mutable, you should copy your array.
See more to figure out which Array methods are mutable:
doesitmutate.xyz
However, if you were to do something like the following:
// BAD
delTodo = id => {
const todos = this.state.todos
todos.splice(id, 1)
this.setState({ todos: todos })
}
Then you'd be mutating your state directly, because Array.splice changes the content of an existing array, rather than returning a new array after deleting the specific item. Therefore, you should copy your array with the spread operator.
// GOOD
delTodo = id => {
const todos = [...this.state.todos]
todos.splice(id, 1)
this.setState({ todos: todos })
}
Similarly with objects, you should apply the same technique.
// BAD
updateFoo = () => {
const foo = this.state.foo // `foo` is an object {}
foo.bar = "HelloWorld"
this.setState({ foo: foo })
}
The above directly mutates your state, so you should make a copy and then update your state.
// GOOD
updateFoo = () => {
const foo = {...this.state.foo} // `foo` is an object {}
foo.bar = "HelloWorld"
this.setState({ foo: foo })
}
Hope this helps.
Why use the spread operator at all?
The spread operator ... is often used for creating shallow copies of arrays or objects. This is especially useful when you aim to avoid mutating values, which is encouraged for different reasons. TLDR; Code with immutable values is much easier to reason about. Long answer here.
Why is the spread operator used so commonly in react?
In react, it is strongly recommended to avoid mutation of this.state and instead call this.setState(newState). Mutating state directly will not trigger a re-render, and may lead to poor UX, unexpected behavior, or even bugs. This is because it may cause the internal state to differ from the state that is being rendered.
To avoid manipulating values, it has become common practice to use the spread operator to create derivatives of objects (or arrays), without mutating the original:
// current state
let initialState = {
user: "Bastian",
activeTodo: "do nothing",
todos: ["do nothing"]
}
function addNewTodo(newTodo) {
// - first spread state, to copy over the current state and avoid mutation
// - then set the fields you wish to modify
this.setState({
...this.state,
activeTodo: newTodo,
todos: [...this.state.todos, newTodo]
})
}
// updating state like this...
addNewTodo("go for a run")
// results in the initial state to be replaced by this:
let updatedState = {
user: "Bastian",
activeTodo: "go for a run",
todos: ["do nothing", "go for a run"]
}
Why is the spread operator used in the example?
Probably to avoid accidental state mutation. While Array.filter() does not mutate the original array and is safe to use on react state, there are several other methods which do mutate the original array, and should not be used on state. For example: .push(), .pop(),.splice(). By spreading the array before calling an operation on it, you ensure that you are not mutating state. That being said, I believe the author made a typo and instead was going for this:
delTodo = id => {
this.setState({
todos: [...this.state.todos].filter(todo => todo.id !== id)
});
};
If you have a need to use one of the mutating functions, you can choose to use them with spread in the following manner, to avoid mutating state and potentially causing bugs in your application:
// here we mutate the copied array, before we set it as the new state
// note that we spread BEFORE using an array method
this.setState({
todos: [...this.state.todos].push("new todo")
});
// in this case, you can also avoid mutation alltogether:
this.setState({
todos: [...this.state.todos, "new todo"]
});
As .filter gives you a new array (than mutating the source array), it is acceptable and results in the same behaviour, making spreading redundant here.
What's not acceptable is:
const delIndex = this.state.todos.findIndex(todo => todo.id !== id);
this.state.todos.splice(delIndex, 1); // state mutation
this.setState({
todos: this.state.todos
});
slice is fine though:
const delIndex = this.state.todos.findIndex(todo => todo.id !== id);
this.setState({
todos: [
...this.state.todos.slice(0, delIndex),
...this.state.todos.slice(delIndex + 1)
]
});
If you mutate state (which keeps ref same) React may not be able to ascertain which part of your state actually changed and probably construct a tree on next render which is different from expected.
The spread operator that the guy's code is applying causes the array returned from the filter function to be copied again.
Since [.filter is returning a new array][https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter], you will already avoid mutating the array in state directly. It seems like the spread operator may be redundant.
One thing I wanted to point out too is while the values in a copied array may be the same as the values in an old array (array = [...array]), the instances change so you wouldn't be able to use '===' or '==' to check for strict equivalency.
const a = ['a', 'b']
const b = [...a]
console.log(a === b) // false
console.log(a == b) // false
console.log(a[0] === b[0]) // true
console.log(a[1] === b[1]) // true
Hope this helps!
I am trying to update the state value but first I have loaded it in another varand after changes made on the new var it is going to setState and update the original state value. but unexpectedly changes made to the temp var already changes the component state value!
var postData = this.state.postData;
postData.likes = postData.likes + 1;
console.log(postData.likes, this.state.postData.likes);
the console.log values:
(1, 1)
(2, 2)
...
Since postData is an object, don't save it directly in another variable since that will create a reference to the one in the state, not a copy. And if it's a reference, changing one will change the other cos they're both pointing to the same object.
Make a copy of it first"
var postData = Object.assign({}, this.state.postData)
and then change it. Then when you're done, use setState({postData})
you should never ever mutate state, always use setState, and for copying object user spread notation "..."
this.setState(((prevState) => ({
postData:{
...prevState.postData,
likes: prevState.postData.likes + 1,
}
});
You can also use ES2018 spread notation like var postData = {...this.state.postData}; to clone the object and manipulate it before assigning back to state.
I've read that it is not advisable to update state directly, e.g:
this.state.array = ['element'];
But, for example, consider this state:
this.state = {
array: [],
}
updateArray = element => {
temp = this.state.array;
temp.push(element);
this.setState({array: temp});
}
Isn't this essentialy the same as the first example? We are setting temp to point to this.state.array, thus mutating it and then overwriting it with this.setState().
I've seen this example in various tutorials, Wouldn't it be 'better' to make a copy of the array?
Check out the docs here, https://reactjs.org/docs/state-and-lifecycle.html.
According to the React team. Setting the state directly will not re-render a component. You must use setState.
As far as using a temp variable when setting state, that is used when something needs to be done to 'element' before setting the state variable. As setState can be called asynchronously. It is acceptable in your case to just do
updateArray = (element) => {
this.setState({ array: [ ...this.state.array, element ] });
}
Yes, you are right it's the same thing. You should read more about pass by value and pass by reference in JS.
When we say
let x = some.array
We are copying a reference(remember copy and paste as shortcut)? Both the things actually point to the same real thing and takes no extra space.
You should instead use some JS that can do this for you.
let x = some.array.slice()
This creates real copy of the array. On modern ES2015 you can do
let x = [...some.array]
a more elegent way.