Array.prototype.toString()

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

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

toString()

const arr = [];
arr.join = 1; // `join` 
console.log(arr.toString()); // [object Array] 

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

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

console.log(matrix.toString()); // 1,2,3,4,5,6,7,8,9

const arr = [];
arr.push(1, [3, arr, 4], 2);
console.log(arr.toString()); // 1,3,,4,2

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

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

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

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]"