Array.prototype.join()

The join() method creates and returns a new string by concatenating all of the elements in an array (or an array-like object), separated by commas or a specified separator string. If the array has only one item, then that item will be returned without using the separator.

separator Optional

Specifies a string to separate each pair of adjacent elements of the array. The separator is converted to a string if necessary. If omitted, the array elements are separated with a comma (","). If separator is an empty string, all elements are joined without any characters in between them.

A string with all array elements joined. If arr.length is 0, the empty string is returned.

The string conversions of all array elements are joined into one string.

The following example creates an array, a, with three elements, then joins the array four times: using the default separator, then a comma and a space, then a plus and an empty string.

const a = ['Wind', 'Water', 'Fire'];
a.join();      // 'Wind,Water,Fire'
a.join(', ');  // 'Wind, Water, Fire'
a.join(' + '); // 'Wind + Water + Fire'
a.join('');    // 'WindWaterFire'

The following example joins array-like object (arguments), by calling Function.prototype.call on Array.prototype.join.

function f(a, b, c) {
  const s = Array.prototype.join.call(arguments);
  console.log(s); // '1,a,true'
}
f(1, 'a', true);
//expected output: "1,a,true"

• String.prototype.split()
• Array.prototype.toString()
• TypedArray.prototype.join()