None.

A string representing the elements of the array.

TheArray object overrides thetoString method ofObject. ThetoString method of arrays callsjoin() internally, which joins the array and returns one string containing each array element separated by commas. If thejoin 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-functionconsole.log(arr.toString()); // [object Array]console.log(Array.prototype.toString.call({ join: () => 1 })); // 1

JavaScript calls thetoString 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 byArray.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 ofjoin(),toString() treats empty slots the same asundefined and produces an extra separator:

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

toString() isgeneric. It expectsthis to have ajoin() method; or, failing that, usesObject.prototype.toString() instead.

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