None.

A string representing the elements of the array.

The Array object overrides the toString method of Object. The toString method of arrays calls join() internally, which joins the array and returns one string containing each array element separated by commas. If the join method is unavailable or is not a function, 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

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'

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]"

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