Array.prototype.toString()

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.

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'

Same as join(), toString() is generic and only reads the length property of this and then accesses each integer index.

const arrayLike = {
  length: 3,
  0: 1,
  1: 2,
  2: 3,
};
console.log(Array.prototype.toString.call(arrayLike));
// 1,2,3

• Array.prototype.join()