foo.js

let arr = [10, 11, 12, 13];
console.log(arr.length); // 4
arr.foo = "bar";
console.log(arr.length); // 4

console.log(1 in arr); // true

console.log(arr[10] === undefined); // true
console.log(10 in arr); // false

arr[12] = "thirteenth value";

console.log(arr.length); // 13
console.log(arr[10] === undefined); // still undefined
console.log(10 in arr); // false <-- does not create empty properties.