What do these three dots in React do?

Question

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

Answer 1

... (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.prototypr.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 the three dots ( ...) which is 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.