what is of in javascript code example

Example 1: use these instead of a for loop javascript

const array = [1, 2, 3];array.forEach(function(elem, index, array) {    array[index] = elem * 2;});console.log(array); // [2,4,6]

Example 2: for in and for of in js

const iterable = [10, 20, 30];

for (const value of iterable) {
  console.log("fef"+value);
}
// 10
// 20
// 30