separator Optional

A string to separate each pair of adjacent elements of the array. If omitted, the array elements are separated with a comma (",").

A string with all array elements joined. If array.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.

Array.prototype.join recursively converts each element, including other arrays, to strings. Because the string returned by Array.prototype.toString (which is the same as calling join()) does not have delimiters, nested arrays look like they are flattened. You can only control the separator of the first level, while deeper levels always use the default comma.

const matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9],
];

console.log(matrix.join()); // 1,2,3,4,5,6,7,8,9
console.log(matrix.join(";")); // 1,2,3;4,5,6;7,8,9

When an array is cyclic (it contains an element that is itself), browsers avoid infinite recursion by ignoring the cyclic reference.

const arr = [];
arr.push(1, [3, arr, 4], 2);
console.log(arr.join(";")); // 1;3,,4;2

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 property whose key is a nonnegative integer less than length.

const arrayLike = {
  length: 3,
  0: 2,
  1: 3,
  2: 4,
  3: 5, // ignored by join() since length is 3
};
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 guide
• Array
• Array.prototype.toString()
• TypedArray.prototype.join()
• String.prototype.split()