What do these three dots in React do? – Dev

The best answers to the question “What do these three dots in React do?” in the category Dev.

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:

... 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.

ANSWER:

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:

class Example extends React.Component {
  render() {
    const { className, children } = this.props;
    return (
      <div className={className}>
      {children}
      </div>
    );
  }
}
ReactDOM.render(
  [
    <Example className="first">
      <span>Child in first</span>
    </Example>,
    <Example className="second" children={<span>Child in second</span>} />
  ],
  document.getElementById("root")
);
.first {
  color: green;
}
.second {
  color: blue;
}
<div id="root"></div>

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>

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":

const obj = {
  foo: {
    a: 1,
    b: 2,
    c: 3
  }
};
console.log("original", obj.foo);
// Creates a NEW object and assigns it to `obj.foo`
obj.foo = {...obj.foo, a: "updated"};
console.log("updated", obj.foo);
.as-console-wrapper {
  max-height: 100% !important;
}

ANSWER:

The three dots in JavaScript are the spread / rest operator.

Spread operator

The spread syntax allows an expression to be expanded in places where multiple arguments are expected.

myFunction(...iterableObj);

[...iterableObj, 4, 5, 6]

[...Array(10)]

Rest parameters

The rest parameter syntax is used for functions with a variable number of arguments.

function(a, b, ...theArgs) {
  // ...
}

The spread / rest operator for arrays was introduced in ES6. There’s a State 2 proposal for object spread / rest properties.

TypeScript also supports the spread syntax and can transpile that into older versions of ECMAScript with minor issues.

ANSWER:

The three dots represent the spread operator in ES6. It allows us to do quite a few things in JavaScript:

  1. 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']
    
  2. 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']
    
  3. 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.

  1. Function arguments as array

      function fun1(...params) {
    
      }