The join() method is used to join the elements of an array into a string and return the string. The method consists of an optional separator parameter which is used to specify how you want to join the elements.

let numbers = [5, 6, 7, 8, 9];

let str = numbers.join(' ');

console.log(str); // 5 6 7 8 9

Another example, with strings as elements.

let numbers = ["I", "love", "Javascript."];

let str = numbers.join(' ');

console.log(str); // I love Javascript.

The separator can be any string. For example,

let numbers = ["I", "love", "Javascript."];

let str = numbers.join('-');
console.log(str); // I-love-Javascript.

str = numbers.join('00');
console.log(str); // I00love00Javascript.

This method can be useful when you are using a split() method to split a string into an array. The join() method can help convert the array to a string.

let string = "lorem ipsum dolor sit amet.";

let arr = string.split(' ').reverse();

console.log(arr); // ['amet.', 'sit', 'dolor', 'ipsum', 'lorem']

console.log(arr.join(' ')); // amet. sit dolor ipsum lorem

If you do not pass any argument to the join() method, it will use , by default.

let arr = ['lorem', 'ipsum', 'dolor', 'sit', 'amet.'];

let newStr = arr.join();

console.log(newStr); // lorem,ipsum,dolor,sit,amet.