How to concatenate JavaScript array using join

JavaScript join function is built in to every JS Array. It’s very useful to join the elements of the JavaScript array and convert it into a string.

Here’s the basic syntax:

const fruits = ['apple', 'orange', 'banana'];
const joinedFruits = fruits.join();

console.log(joinedFruits);

As you see, we’ve called .join() without any arguments which returned a string that consists of all the array elements separated by commas.

apple,orange,banana

How to use a custom separator with `join`

To use a custom separator with Array.join in JS, you should just pass it as an argument into join().

Let’s try use dashes this time.

const cities = ['London', 'Paris', 'Tokyo'];
const joinedCities = cities.join('-');

console.log(joinedCities);

As a result we’ll still get a string, but the array elements will be split (or joined) with dashes instead of commas.

London-Paris-Tokyo

You’re not limited in the length of the separator that you want to use.

For example, you can use the word cool instead of commas or dashes.

const numbers = [1, 2, 3];
const joinedNumbers = numbers.join('cool');

console.log(joinedNumbers);

Here’s the output:

1cool2cool3