Rest and Spread Operator

Rest Operator

Often times we want all our arguments to be an array of arguments. We don't know how many arguments will be passed into our function, but we want them to be an array in our function. We do that using the rest operator. ...whatWeWillNameOurArray

function addNumbers(...numbers) {
  // numbers is now an array that we can use .reduce() on
  return numbers.reduce((sum, number) => {
    return sum + number;
  }, 0);

addNumbers(1, 2, 3, 4, 5, 6, 7, 8);

Spread Operator

In this example, we will have two arrays that we want to be joined into one. Before, we would have had to use .concat().

const defaultColors = ['red', 'green'];
const userFavoriteColors = ['orange', 'yellow'];


Now we can do this.

[ ...defaultColors, ...userFavoriteColors ];

The ES6 way is much easier to read and identify quickly was is being done.

Not only that, but now we can add more that two arrays.

[ ...defaultColors, ...userFavoriteColors, ...fallColors ];

We aren't done yet! We can also just add values!

[ "blue", "green", ...defaultColors, ...userFavoriteColors, ...fallColors ];

Let's write a shopping list validator that always includes milk. We will check for milk, and add it to our array if it's not there.

function validateShoppingList(...items) {
  if (items.indexOf('milk') < 0) {
    return [ 'milk', ...items ];
  return items;

validateShoppingList('oranges', 'bread', 'eggs');

Practical example of Rest and Spread.

Let's assume we have a library with a depreciated method. We would like users of our library to use a better method, but don't what to break the code of people still using the old method.

const MathLibrary = {
  calculateProduct( {
    console.log('Please use the multiply method instead');
    return this.multiply(;
  multiply(a, b) {
    return a * b;

We just pass on all of the arguments straight into the call of the new method. The arguments do get turned into an array, but then "spread" out and dumped into the new method and individual parameters.

Try to make the following function more ES6xy:

function product(a, b, c, d, e) {
  const numbers = [a,b,c,d,e];
  return numbers.reduce(function(acc, number) {
    return acc * number;
  }, 1)

Do this one too. Use at least both the rest and spread operators:

function unshift(array, a, b, c, d, e) {
  return [a, b, c, d, e].concat(array);