What are these three dots in React doing?

Question

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

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

Timberlake

NOTE: the ... operator behaves differently in different contexts. In this context, it is the "spread" operator described below by @T.J. Crowder. In a different context this could also be the "rest" operator described below by @Tomas Nikodym. – Bedad

... is destructuring the this.props array into its individual values – Octogenarian

Sibling question: Spread Syntax vs Rest Parameter in ES2015 / ES6. – Saveall

It copies all your previous data from array – Falsecard

It acts different in these 2 situations: 1. when used behind the name of an object or an array, it acts takes all the elements and clones them into your new object or array. which in your case all the props are passed to the Modal component. This can be useful when you want to add new object into your array without manipulating the first one, like updating state. 2. when used in the end of a list of object/array-elements, like const {a,b, ...rest} = obj. now, you have a new obj/arr from the remaining elements except ones taken out like a, b. – Reck

Answer 1

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;
}

Juneberry

Does putting a child element between the opening and closing tags override the children property or are they combined? – Filiation

@anddero - That is a very interesting question. As far as I can see, it's not covered by [the documentation of children. Experimentation tells me the children you provide via an attribute called children are superceded by the one(s) you specify between the start and end tags, but if it's undefined behavior, I'd be sure not to rely on it. – Juneberry

Answer 2

... 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.

Humdrum

var person = { name:'Alex',age:35} is json, javascript object notation. ...person evaluates to name='Alex', age=35 and its purpose is for say you have 1,000,000 of these key value pairs in a json structure and you want to pass them all to the component all you have to do is do the ... notation and they are all passed. You do not have to enumerate them one by one. – Cu

This is not ECMA documentation, Don't take it too serious! I first answered this question in this manner, and after a while other people change their answer to look like mine, just achieving some vote-up. – Humdrum

would help to add an example of spreading an object into another object, since that's essentially what JSX spread does under the hood. – Duet

Seems on the whole very similar to pythonic "unpacking" notation e.g. **my_dict – Nigger

Answer 3

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.

4. Function arguments as array

     function fun1(...params) {
   
     }
   

Cholecyst

Mention rest parameters before the example for more clarity – Warrantee

The context of his example is json conversion only, not array conversion. – Cu

3.5. Destructuring an object – Duet

That's an awesome answer, and as a Python Developer I can translate the spread operator to the packing/unpacking with the * or ** notation, basically does the same thing. – Pteropod

Regarding number 4: why would one use rest parameters instead of the arguments object which is available in JavaScript functions? – Ares

@Ares developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… – Bookshelf

Number 4: it should be an unknown number of function arguments, which could be an array, strings or anything, depending on the function. – Aglimmer

The answer is great, however, didn't answer to the OP question case – Keyhole

Answer 4

... (three dots in JavaScript) is called the Spread Syntax or Spread Operator. This allows an iterable such as an array expression or string to be expanded or an object expression to be expanded wherever placed. This is not specific to React. It is a JavaScript operator.

All these answers here are helpful, but I want to list down the mostly used practical Use Cases of the Spread Syntax (Spread Operator).

1. Combine Arrays (Concatenate Arrays)

There are a variety of ways to combine arrays, but the spread operator allows you to place this at any place in an array. If you'd like to combine two arrays and place elements at any point within the array, you can do as follows:

var arr1 = ['two', 'three'];
var arr2 = ['one', ...arr1, 'four', 'five'];

// arr2 = ["one", "two", "three", "four", "five"]

2. Copying Arrays

When we wanted a copy of an array, we used to have the Array.prototype.slice() method. But, you can do the same with the spread operator.

var arr = [1,2,3];
var arr2 = [...arr];
// arr2 = [1,2,3]

3. Calling Functions without Apply

In ES5, to pass an array of two numbers to the doStuff() function, you often use the Function.prototype.apply() method as follows:

function doStuff (x, y, z) {}
var args = [0, 1, 2];

// Call the function, passing args
doStuff.apply(null, args);

However, by using the spread operator, you can pass an array into the function.

doStuff(...args);

4. Destructuring Arrays

You can use destructuring and the rest operator together to extract the information into variables as you'd like them:

let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
console.log(x); // 1
console.log(y); // 2
console.log(z); // { a: 3, b: 4 }

5. Function Arguments as Rest Parameters

ES6 also has three dots (...) which indicates a rest parameter that collects all remaining arguments of a function into an array.

function f(a, b, ...args) {
  console.log(args);
}

f(1, 2, 3, 4, 5); // [3, 4, 5]

6. Using Math Functions

Any function where spread is used as the argument can be used by functions that can accept any number of arguments.

let numbers = [9, 4, 7, 1];
Math.min(...numbers); // 1

7. Combining Two Objects

You can use the spread operator to combine two objects. This is an easy and cleaner way to do it.

var carType = {
  model: 'Toyota',
  yom: '1995'
};

var carFuel = 'Petrol';

var carData = {
  ...carType,
  carFuel
}

console.log(carData);
// {
//  model: 'Toyota',
//  yom: '1995',
//  carFuel = 'Petrol'
// }

8. Separate a String into Separate Characters

You can use the spread operator to spread a string into separate characters.

let chars = ['A', ...'BC', 'D'];
console.log(chars); // ["A", "B", "C", "D"]

You can think of more ways to use the Spread Operator. What I have listed here are the popular use cases of it.

Pansypant

Answer 5

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.

Limpid

Spread/rest is now Stage4, finished. I think included in ES9/2018 github.com/tc39/proposal-object-rest-spread/blob/master/… – Haddock

Answer 6

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 a maximum of three 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

Spina

Answer 7

Kudos to Brandon Morelli. He explained perfectly here, but links may die so I am just pasting the content below:

The spread syntax is simply three dots: ... It allows an iterable to expand in places where 0+ arguments are expected. Definitions are tough without context. Let's explore some different use cases to help understand what this means.

Example 1 — Inserting Arrays

Take a look at the code below. In this code, we don’t use the spread syntax:

var mid = [3, 4];
var arr = [1, 2, mid, 5, 6];

console.log(arr);

Above, we’ve created an array named mid. We then create a second array which contains our mid array. Finally, we log out the result. What do you expect arr to print? Click run above to see what happens. Here is the output:

[1, 2, [3, 4], 5, 6]

Is that the result you expected?

By inserting the mid array into the arr array, we’ve ended up with an array within an array. That’s fine if that was the goal. But what if you want only a single array with the values of 1 through 6? To accomplish this, we can use the spread syntax! Remember, the spread syntax allows the elements of our array to expand.

Let’s look at the code below. Everything is the same — except we’re now using the spread syntax to insert the mid array into the arr array:

var mid = [3, 4];
var arr = [1, 2, ...mid, 5, 6];

console.log(arr);

And when you hit the run button, here’s the result:

[1, 2, 3, 4, 5, 6]

Awesome!

Remember the spread syntax definition you just read above? Here’s where it comes into play. As you can see, when we create the arr array and use the spread operator on the mid array, instead of just being inserted, the mid array expands. This expansion means that each and every element in the mid array is inserted into the arr array. Instead of nested arrays, the result is a single array of numbers ranging from 1 to 6.

Example 2 — Math

JavaScript has a built-in math object that allows us to do some fun math calculations. In this example we’ll be looking at Math.max(). If you’re unfamiliar, Math.max() returns the largest of zero or more numbers. Here are a few examples:

Math.max();
// -Infinity
Math.max(1, 2, 3);
// 3
Math.max(100, 3, 4);
// 100

As you can see, if you want to find the maximum value of multiple numbers, Math.max() requires multiple parameters. You unfortunately can’t simply use a single array as input. Before the spread syntax, the easiest way to use Math.max() on an array is to use .apply().

var arr = [2, 4, 8, 6, 0];

function max(arr) {
  return Math.max.apply(null, arr);
}

console.log(max(arr));

It works, it’s just really annoying.

Now take a look at how we do the same exact thing with the spread syntax:

var arr = [2, 4, 8, 6, 0];
var max = Math.max(...arr);

console.log(max);

Instead of having to create a function and utilize the apply method to return the result of Math.max() , we only need two lines of code! The spread syntax expands our array elements and inputs each element in our array individually into the Math.max() method!

Example 3 — Copy an Array

In JavaScript, you can’t just copy an array by setting a new variable equal to already existing array. Consider the following code example:

var arr = ['a', 'b', 'c'];
var arr2 = arr;

console.log(arr2);

When you press run, you’ll get the following output:

['a', 'b', 'c']

Now, at first glance, it looks like it worked — it looks like we’ve copied the values of arr into arr2. But that’s not what has happened. You see, when working with objects in JavaScript (arrays are a type of object) we assign by reference, not by value. This means that arr2 has been assigned to the same reference as arr. In other words, anything we do to arr2 will also affect the original arr array (and vice versa). Take a look below:

var arr = ['a', 'b', 'c'];
var arr2 = arr;

arr2.push('d');

console.log(arr);

Above, we’ve pushed a new element d into arr2. Yet, when we log out the value of arr, you’ll see that the d value was also added to that array:

['a', 'b', 'c', 'd']

No need to fear though! We can use the spread operator! Consider the code below. It’s almost the same as above. Instead though, we’ve used the spread operator within a pair of square brackets:

var arr = ['a', 'b', 'c'];
var arr2 = [...arr];

console.log(arr2);

Hit run, and you’ll see the expected output:

['a', 'b', 'c']

Above, the array values in arr expanded to become individual elements which were then assigned to arr2. We can now change the arr2 array as much as we’d like with no consequences on the original arr array:

var arr = ['a', 'b', 'c'];
var arr2 = [...arr];

arr2.push('d');

console.log(arr);

Again, the reason this works is because the value of arr is expanded to fill the brackets of our arr2 array definition. Thus, we are setting arr2 to equal the individual values of arr instead of the reference to arr like we did in the first example.

Bonus Example — String to Array

As a fun final example, you can use the spread syntax to convert a string into an array. Simply use the spread syntax within a pair of square brackets:

var str = "hello";
var chars = [...str];

console.log(chars);

Fault

Thank you for explaining, me and my colleague just discussed the spread operator and your explanation was tremendously helpful. – Secant

Answer 8

For someone who wants to understand this simple and fast:

First of all, this is not a syntax only to React. This is syntax from ES6 called spread syntax which iterate (merge, add, etc.) the array and object. Read more about it here.

So to answer the question:

Let's imagine you have this tag:

<UserTag name="Supun" age="66" gender="male" />

And you do this:

const user = {
  "name": "Joe",
  "age": "50"
  "test": "test-val"
};

<UserTag name="Supun" gender="male"  {...user} age="66" />

Then the tag will be equal to this:

<UserTag name="Joe" gender="male" test="test-val" age="66" />

So when you used the spread syntax in a React tag, it took the tag's attribute as object attributes which merge (replace if it exists) with the given object user. Also, you might have noticed one thing that it only replaces before attribute, not after attributes. So in this example, age remains as it is.

Darcydarda

shouldn't age=50 at the end? – Jingoism

@DonCheadle no, because I added {...user} before age so age tag won't replace – Darcydarda

Answer 9

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.

Crumb

What is "props"? "Properties"? Or literal? – Schnorkle

Answer 10

For those who come from the Python world, JSX Spread Attributes are equivalent to Unpacking Argument Lists (the Python **-operator).

I'm aware this is a JSX question, but working with analogies sometimes helps to get it faster.

Gavrila

Answer 11

Three dots ... represent spread operators or rest parameters.

It allows an array expression or string or anything which can be iterating to be expanded in places where zero or more arguments for function calls or elements for array are expected.

• Merge two arrays

var arr1 = [1,2,3];
var arr2 = [4,5,6];

arr1 = [...arr1, ...arr2];
console.log(arr1);  //[1, 2, 3, 4, 5, 6]

• Copying array:

var arr = [1, 2, 3];
var arr2 = [...arr];

console.log(arr); //[1, 2, 3]

Note: Spread syntax effectively goes one level deep while copying an array. Therefore, it may be unsuitable for copying multidimensional arrays as the following example shows (it's the same with Object.assign() and spread syntax).

• Add values of one array to other at specific index e.g 3:

var arr1 = [4, 5]
var arr2 = [1, 2, 3, ...arr1, 6]
console.log(arr2);    // [1, 2, 3, 4, 5, 6]

• When calling a constructor with new:

var dateFields = [1970, 0, 1];  // 1 Jan 1970
var d = new Date(...dateFields);

console.log(d);

• Spread in object literals:

var obj1 = { foo: 'bar', x: 42 };
var obj2 = { foo: 'baz', y: 13 };

var clonedObj = { ...obj1 };
console.log(clonedObj);    // {foo: "bar", x: 42}

var mergedObj = { ...obj1, ...obj2 };
console.log(mergedObj);    // {foo: "baz", x: 42, y: 13}

Note that the foo property of obj1 has been overwritten by the obj2 foo property.

• As a rest parameter syntax which allows us to represent an indefinite number of arguments as an array:

function sum(...theArgs) {
  return theArgs.reduce((previous, current) => {
    return previous + current;
  });
}

console.log(sum(1, 2, 3));    //6
console.log(sum(1, 2, 3, 4));    //10

Note: The spread syntax (other than in the case of spread properties) can be applied only to iterable objects:

So the following will throw an error:

var obj = {'key1': 'value1'};
var array = [...obj]; // TypeError: obj is not iterable

Reference1

Reference2

Elbaelbart

Answer 12

The ...(spread operator) is used in React to:

provide a neat way to pass props from parent to child components. E.g., given these props in a parent component,

this.props = {
  username: "danM",
  email: "[email protected]"
}

they could be passed in the following manner to the child,

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

which is similar to this

<ChildComponent username={this.props.username} email={this.props.email} />

but way cleaner.

Capitalistic

suppose , if we want to pass only 'username' property to the child component , then can we use <ChildComponent {...this.props.username} /> ? – Odaniel

why is it asking me to use spread operator before this keyword in h1 tag? class Test { obj = { user1: "vijay", age: 27 }; m1() { <h1 {this.obj.user}> </h1>; } } – Odaniel

//ex-2 : for this example , its not throwing error to use spread operator when i use img tag . class Test { obj = { user1: "vijay", imageUrl: "picsum.photos/id/1/200/300" }; m1() { // <h1 {this.obj.user1}> </h1>; <img src={this.obj.imageUrl} />; } } – Odaniel

Answer 13

The three dots (...) are called the spread operator, and this is conceptually similar to the ES6 array spread operator, JSX taking advantage of these supported and developing standards in order to provide a cleaner syntax in JSX

Spread properties in object initializers copies own enumerable properties from a provided object onto the newly created object.

let n = { x, y, ...z };
n; // { x: 1, y: 2, a: 3, b: 4 }

References:

1. Spread Properties

2. JSX In Depth

Chromatics

That's a proposal for spread operator on objects in ECMAScript. The question was about the JSX spread operator. They're not the same even though they work the same way. – Ferromagnesian

@Ferromagnesian Update the answer based on context, hope this fits with context – Chromatics

"The three dots (...) are called the spread operator" Only incorrectly. :-) Spread and rest aren't operators, and they can't be, because an operator has to produce a single result value. Spread and rest are primary syntax, not operators. – Juneberry

Answer 14

Spread operator! As most ppl have already answered the question elegantly, I wanted to suggest a quick list of ways to use the spread operator:

The ... spread operator is useful for many different routine tasks in JavaScript, including the following:

• Copying an array
• Concatenating or combining arrays
• Using Math functions
• Using an array as arguments
• Adding an item to a list
• Adding to state in React
• Combining objects
• Converting NodeList to an array

Check out the article for more details. How to use the Spread Operator. I recommend getting used to it. There are so many cool ways you can use spread operators.

Cnidoblast

Answer 15

It is called spreads syntax in JavaScript.

It use for destructuring an array or object in JavaScript.

Example:

const objA = { a: 1, b: 2, c: 3 }
const objB = { ...objA, d: 1 }
/* Result of objB will be { a: 1, b: 2, c: 3, d: 1 } */
console.log(objB)

const objC = { ....objA, a: 3 }
/* result of objC will be { a: 3, b: 2, c: 3, d: 1 } */
console.log(objC)

You can do it same result with Object.assign() function in JavaScript.

Reference: Spread syntax

Chirrup

Answer 16

It is common practice to pass props around in a React application. In doing this we able to apply state changes to the child component regardless of whether it is Pure or Impure (stateless or stateful). There are times when the best approach, when passing in props, is to pass in singular properties or an entire object of properties. With the support for arrays in ES6 we were given the "..." notation and with this we are now able to achieve passing an entire object to a child.

The typical process of passing props to a child is noted with this syntax:

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

This is fine to use when the number of props is minimal but becomes unmanageable when the prop numbers get too much higher. A problem with this method occurs when you do not know the properties needed within a child component and the typical JavaScript method is to simple set those properties and bind to the object later. This causes issues with propType checking and cryptic stack trace errors that are not helpful and cause delays in debugging. The following is an example of this practice, and what not to do:

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

This same result can be achieved but with more appropriate success by doing this:

var props = {};
props.foo = x;
props.bar = y;
var component = Component(props); // Where did my JSX go?

But does not use JSX spread or JSX so to loop this back into the equation we can now do something like this:

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

The properties included in "...props" are foo: x, bar: y. This can be combined with other attributes to override the properties of "...props" using this syntax:

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

In addition we can copy other property objects onto each other or combine them in this manner:

var oldObj = { foo: 'hello', bar: 'world' };
var newObj = { ...oldObj, foo: 'hi' };
console.log(newObj.foo); // 'hi';
console.log(newObj.bar); // 'world';

Or merge two different objects like this (this is not yet available in all react versions):

var ab = { ...a, ...b }; // merge(a, b)

Another way of explaining this, according to Facebook's react/docs site is:

If you already have "props" as an object, and you want to pass it in JSX, you can use "..." as a SPREAD operator to pass the whole props object. The following two examples are equivalent:

function App1() {
  return <Greeting firstName="Ben" lastName="Hector" />;
}



function App2() {
  const props = {firstName: 'Ben', lastName: 'Hector'};
  return <Greeting {...props} />;
}

Spread attributes can be useful when you are building generic containers. However, they can also make your code messy by making it easy to pass a lot of irrelevant props to components that don't care about them. This syntax should be used sparingly.

Waiwaif

Are "Pure" and "Impure" literal? For emphasis, we have italics and bold on this platform. – Schnorkle

Answer 17

The spread operator (triple operator) introduced in ECMAScript 6 (ES6). ECMAScript (ES6) is a wrapper of JavaScript.

The spread operator enumerable properties in props.

this.props = { firstName: 'Dan', lastName: 'Abramov', city: 'New York', country: 'USA' } <Modal {...this.props} title='Modal heading' animation={false}>

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

But the main feature spread operator is used for a reference type.

For example,

let person= {
    name: 'Alex',
    age: 35
}
person1 = person;

person1.name = "Raheel";

console.log( person.name); // Output: Raheel

This is called a reference type. One object affects other objects, because they are shareable in memory. If you are getting a value independently means spread memory and both use the spread operator.

 let person= {
        name: 'Alex',
        age: 35
    }
person2 = {...person};

person2.name = "Shahzad";

console.log(person.name); // Output: Alex

Sibilate

Answer 18

... 3 dots represent the spread operator in JS.

Without a spread operator.

let a = ['one','one','two','two'];
let unq = [new Set(a)];

console.log(a);
console.log(unq);

Output:

(4) ['one', 'one', 'two', 'two']
[Set(2)]

With spread operator.

let a = ['one','one','two','two'];
let unq = [...new Set(a)];

console.log(a);
console.log(unq);

Output:

(4) ['one', 'one', 'two', 'two']
(2) ['one', 'two']

Lordly

Answer 19

Those 3 dots ... is not React terms. Those are JavaScript ES6 spread operators. Which helps to create a new array without disturbing the original one to perform a deep copy. This can be used for objects also.

const arr1 = [1, 2, 3, 4, 5]
const arr2 = arr1 // [1, 2, 3, 4, 5]
/*
 This is an example of a shallow copy.
 Where the value of arr1 is copied to arr2
 But now if you apply any method to
 arr2 will affect the arr1 also.  
*/

/*
 This is an example of a deep copy. 
 Here the values of arr1 get copied but they 
 both are disconnected. Meaning if you
 apply any method to arr3 it will not
 affect the arr1.
 */
 const arr3 = [...arr1] // [1, 2, 3, 4, 5]

Daughterly

Answer 20

in Reactjs , the ... is called spread operator .

It is used to expand objects or arrays into a list of arguments or elements.

For Example:

<MyComponent {...props} />

Lavin

Answer 21

In React, the ... syntax is called the "spread operator" or "object spread syntax". It is used to pass all the properties of an object as individual props to a React component.

In the code snippet you provided:

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

he spread operator {...this.props} is used to pass all the props (this.props) of the current component to the Modal component. For example, if this.props contains { prop1: 'value1', prop2: 'value2' }, then using the spread operator {...this.props} would pass prop1 and prop2 as individual props to the Modal component, resulting in:

<Modal prop1='value1' prop2='value2' title='Modal heading' animation={false}>

Spider

Answer 22

Those are called spreads. Just as the name implies, it means it's putting whatever the value of it in those array or objects.

Such as:

let a = [1, 2, 3];
let b = [...a, 4, 5, 6];
console.log(b);
> [1, 2, 3, 4, 5, 6]

Doubleton

Answer 23

The spread syntax allows an data structures like array and object to destructure them for either to extract a value from them or to add a value to them. e.g const obj={name:"ram",age:10} const {name}=obj from above example we can say that we destructured the obj and extracted name from that object. similarly, const newObj={...obj,address:"Nepal"} from this example we added a new property to that object. This is similar in case of array too.

Hamrah

Answer 24

In React (using JSX), the three dots (...) in the code you provided are called the "spread operator" or "object spread syntax." This operator is used to spread the properties of an object or iterable (e.g., an array) into another object or component props.

In the code you provided:

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

The spread operator ...this.props is used to take all the properties and values of the this.props object and spread them as individual props into the Modal component. This is a convenient way to pass down all the props of the current component (this) to the Modal component without explicitly specifying each prop.

So, if this.props contains properties like prop1, prop2, etc., they will be passed as props to the Modal component, along with the explicitly defined title and animation props.

Here's an example of how the props would be passed down:

<Modal prop1={this.props.prop1} prop2={this.props.prop2} title='Modal heading' animation={false}>

Using the spread operator can help reduce repetition and make your code more concise when you want to pass most or all of the props of one component to another.

Talishatalisman

I don't see how this answer improves anything said by the existing answers, including the accepted answer from 8 years ago... – Septime

Answer 25

The Spread operator lets you expand an iterable like an object, string, or array into its elements
However, the Rest operator does the inverse by reducing a set of elements into array.

Parsimony