What do these three dots in React do?
That's property spread notation. It was added in ES2018 (spread for arrays/iterables was earlier, ES2015), but it's been supported in React projects for a long time via transpilation (as "JSX spread attributes" even though you could do it elsewhere, too, not just attributes).
{...this.props}
spreads out the "own" enumerable properties in props
as discrete properties on the Modal
element you're creating. For instance, if this.props
contained a: 1
and b: 2
, then
<Modal {...this.props} title='Modal heading' animation={false}>
would be the same as
<Modal a={this.props.a} b={this.props.b} title='Modal heading' animation={false}>
But it's dynamic, so whatever "own" properties are in props
are included.
Since children
is an "own" property in props
, spread will include it. So if the component where this appears had child elements, they'll be passed on to Modal
. Putting child elements between the opening tag and closing tags is just syntactic sugar — the good kind — for putting a children
property in the opening tag. Example:
Spread notation is handy not only for that use case, but for creating a new object with most (or all) of the properties of an existing object — which comes up a lot when you're updating state, since you can't modify state directly:
this.setState(prevState => { return {foo: {...prevState.foo, a: "updated"}};});
That replaces this.state.foo
with a new object with all the same properties as foo
except the a
property, which becomes "updated"
:
...
are called spread attributes which, as the name represents, it allows an expression to be expanded.
var parts = ['two', 'three'];var numbers = ['one', ...parts, 'four', 'five']; // ["one", "two", "three", "four", "five"]
And in this case (I'm going to simplify it).
// Just assume we have an object like this:var person= { name: 'Alex', age: 35 }
This:
<Modal {...person} title='Modal heading' animation={false} />
is equal to
<Modal name={person.name} age={person.age} title='Modal heading' animation={false} />
So in short, it's a neat short-cut, we can say.
The three dots represent the spread operator in ES6. It allows us to do quite a few things in JavaScript:
Concatenate arrays
var shooterGames = ['Call of Duty', 'Far Cry', 'Resident Evil']; var racingGames = ['Need For Speed', 'Gran Turismo', 'Burnout']; var games = [...shooterGames, ...racingGames]; console.log(games) // ['Call of Duty', 'Far Cry', 'Resident Evil', 'Need For Speed', 'Gran Turismo', 'Burnout']
Destructuring an array
var shooterGames = ['Call of Duty', 'Far Cry', 'Resident Evil']; var [first, ...remaining] = shooterGames; console.log(first); //Call of Duty console.log(remaining); //['Far Cry', 'Resident Evil']
Combining two objects
var myCrush = { firstname: 'Selena', middlename: 'Marie' }; var lastname = 'my last name'; var myWife = { ...myCrush, lastname } console.log(myWife); // {firstname: 'Selena', // middlename: 'Marie', // lastname: 'my last name'}
There's another use for the three dots which is known as Rest Parameters and it makes it possible to take all of the arguments to a function in as one array.
Function arguments as array
function fun1(...params) { }