What do these three dots in React do?

Question

What does the ... do in this React (using JSX) code and what is it called?

<Modal {...this.props} title='Modal heading' animation={false}>

Answer 1

It's just defining props in a different way in JSX for you!

It's using ... array and object operator in ES6 (object one not fully supported yet), so basically if you already define your props, you can pass it to your element this way.

So in your case, the code should be something like this:

function yourA() {
  const props = {name='Alireza', age='35'};
  <Modal {...props} title='Modal heading' animation={false} />
}

so the props you defined, now separated and can be reused if necessary.

It's equal to:

function yourA() {
  <Modal name='Alireza' age='35' title='Modal heading' animation={false} />
}

These are the quotes from React team about spread operator in JSX:

JSX Spread Attributes If you know all the properties that you want to place on a component ahead of time, it is easy to use JSX:

var component = <Component foo={x} bar={y} />;

Mutating Props is Bad
If you don't know which properties you want to set, you might be tempted to add them onto the object later:

var component = <Component />;
component.props.foo = x; // bad
component.props.bar = y; // also bad

This is an anti-pattern because it means that we can't help you check the right propTypes until way later. This means that your propTypes errors end up with a cryptic stack trace.

The props should be considered immutable. Mutating the props object somewhere else could cause unexpected consequences so ideally it would be a frozen object at this point.

Spread Attributes
Now you can use a new feature of JSX called spread attributes:

var props = {};
    props.foo = x;
    props.bar = y;
    var component = <Component {...props} />;

The properties of the object that you pass in are copied onto the component's props.

You can use this multiple times or combine it with other attributes. The specification order is important. Later attributes override previous ones.

var props = { foo: 'default' };
var component = <Component {...props} foo={'override'} />;
console.log(component.props.foo); // 'override'

What's with the weird ... notation?
The ... operator (or spread operator) is already supported for arrays in ES6. There is also an ECMAScript proposal for Object Rest and Spread Properties. We're taking advantage of these supported and developing standards in order to provide a cleaner syntax in JSX.

Answer 2

Spread Attributes used to Pass the the multiple Properties in a Simple Way

{ ... this.props } is Holding the property of this.props

Use of the { ... } Spread Operator with below props

this.props = 
 { 
    firstName: 'Dan', 
    lastName: 'Abramov', 
    city: 'New York',
    country: 'USA' 
}

Without { ... } Spread

<Child 
  firstName={this.props.firstName}
  lastName={this.props.lastName}
  city={this.props.city}
  country={this.props.country}

> 

With { ... } Spread

<Child { ...this.props } />

Dan Abramov's Tweet about Spread operator (Creator of Redux) https://twitter.com/dan_abramov/status/694519379191545856?lang=en

Answer 3

This is a new feature in ES6/Harmony. It is called the Spread Operator. It lets you either separate the constituent parts of an array/object, or take multiple items/parameters and glue them together. Here is an example:

let array = [1,2,3]
let array2 = [...array]
// array2 is now filled with the items from array

And with an object/keys:

// lets pass an object as props to a react component
let myParameters = {myKey: 5, myOtherKey: 7}
let component = <MyComponent {...myParameters}/>
// this is equal to <MyComponent myKey=5 myOtherKey=7 />

What's really cool is you can use it to mean "the rest of the values".

const myFunc = (value1, value2, ...values) {
    // Some code
}

myFunc(1, 2, 3, 4, 5)
// when myFunc is called, the rest of the variables are placed into the "values" array

Answer 4

This is a feature of es6 which is used in React as well. Look at the below example:

function Sum(x,y,z) {
   return x + y + z;
}
console.log(Sum(1,2,3)); //6

This way is fine if we have maximum 3 parameters but what if we need to add for example 110 parameters. Should we define them all and add them one by one?! Of course there is an easier way to do which is called SPREAD. Instead of passing all those parameters you write :

function (...numbers){} 

We have no idea how many parameters we have but we know there are heaps of those. Based on es6 we can rewrite the above function as below and use the spread and mapping between them to make it as easy as a piece of cake:

let Sum = (...numbers) => {
return numbers.reduce((prev, current) => prev + current );
}
console.log(Sum(1, 2, 3, 4, 5, 6, 7, 8, 9));//45