As per the documentation:
Never mutate
this.statedirectly, as callingsetState()afterwards may replace the mutation you made. Treatthis.stateas if it wereimmutable.
- 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.
Answer from goto on Stack OverflowWhy use the spread operator when calling 'setState()' in React?
reactjs - Spread operator in react component props - Stack Overflow
The spread operator for props {...props} allows you to pass all props at once.
Curly braces and the spread operator questions
As per the documentation:
Never mutate
this.statedirectly, as callingsetState()afterwards may replace the mutation you made. Treatthis.stateas if it wereimmutable.
- 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"]
});
The ... spreads the next spreadable item. In your case if you evaluate the isActive to true you will find,
...{bg: "teal.200", rounded: "sm",}
So the spread operator spread this {bg: "teal.200", rounded: "sm",} object and returns bg: "teal.200", rounded: "sm" so that they can be passed as props.
To explain the spread operator in simple words, it spreads all the properties inside your component.
lets say, you pass a property title = "Side Link", this gets automatically assigned into the Side Nave link component so you don't need to specifically add this prop to retrieve the value.
If you had multiple props passed from parent, and you don't want to manually add them all in the Side Nav Link component, you can use the spread operator and it will spread all those props to the component.
<NavLink href={href}>
{(isActive) => (
<SideNavLink
{...(isActive && { // this spread operator
bg: "teal.200",
rounded: "sm",
})}
{...props} //====> title = "Side Link"
/>
)}
</NavLink>