Array.prototype.join()

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. If an element is undefined, null, it is converted to an empty string instead of the string "null" or "undefined".

The join method is accessed internally by Array.prototype.toString() with no arguments. Overriding join of an array instance will override its toString behavior as well.

When used on sparse arrays, the join() method iterates empty slots as if they have the value undefined.

The join() method is generic. It only expects the this value to have a length property and integer-keyed properties.

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'

join() treats empty slots the same as undefined and produces an extra separator:

console.log([1, , 3].join()); // '1,,3'
console.log([1, undefined, 3].join()); // '1,,3'

The join() method reads the length property of this and then accesses each integer index.

const arrayLike = {
  length: 3,
  0: 2,
  1: 3,
  2: 4,
};
console.log(Array.prototype.join.call(arrayLike));
// 2,3,4
console.log(Array.prototype.join.call(arrayLike, "."));
// 2.3.4

• Polyfill of Array.prototype.join in core-js
• Indexed collections
• Array
• Array.prototype.toString()
• TypedArray.prototype.join()
• String.prototype.split()