Array.prototype.join()

The join() method of Array instances creates and returns a new string by concatenating all of the elements in this array, 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.

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Syntax

js
join()
join(separator)

Parameters

separator Optional

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

Return value

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

Description

The string conversions of all array elements are joined into one string. If an element is undefined or 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.

js
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.

js
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.

Examples

Joining an array four different ways

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.

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

Using join() on sparse arrays

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

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

Calling join() on non-array objects

The join() method reads the length property of this and then accesses each property whose key is a nonnegative integer less than length.

js
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

Specifications

Specification
ECMAScript Language Specification
# sec-array.prototype.join

Browser compatibility

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See also