Array.prototype.toString()

Try it

const array = [1, 2, "a", "1a"];

console.log(array.toString());
/ Expected output: "1,2,a,1a"

Syntax

toString()

Parameters

None.

Return value

A string representing the elements of the array.

Description

The Object.prototype.toString is used instead, returning [object Array].

const arr = [];
arr.join = 1; / re-assign `join` with a non-function
console.log(arr.toString()); / [object Array]

console.log(Array.prototype.toString.call({ join: () => 1 })); / 1

JavaScript calls the toString method automatically when an array is to be represented as a text value or when an array is referred to in a string concatenation.

Array.prototype.toString recursively converts each element, including other arrays, to strings. Because the string returned by Array.prototype.toString does not have delimiters, nested arrays look like they are flattened.

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

console.log(matrix.toString()); / 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.toString()); / 1,3,,4,2

Examples

Using toString()

const array = [1, 2, "a", "1a"];

console.log(array.toString()); / "1,2,a,1a"

Using toString() on sparse arrays

Following the behavior of join(), toString() treats empty slots the same as undefined and produces an extra separator:

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

Calling toString() on non-array objects

toString() is generic. It expects this to have a join() method; or, failing that, uses Object.prototype.toString() instead.

console.log(Array.prototype.toString.call({ join: () => 1 }));
/ 1; a number
console.log(Array.prototype.toString.call({ join: () => undefined }));
/ undefined
console.log(Array.prototype.toString.call({ join: "not function" }));
/ "[object Object]"

Specifications

Browser compatibility

See also

• Indexed collections guide
• Array
• Array.prototype.join()
• Array.prototype.toLocaleString()
• TypedArray.prototype.toString()
• String.prototype.toString()