JS uses for in and for of to simplify ordinary for loops

let arr = [10, 20, 30, 40];
 
// Common writing 
for (let i = 0; i < arr.length; i++) {
    
  console.log(arr[i]);
}
 
//  Abbreviation 

for (const val of arr) {
    
  console.log(val);
}

for (const index in arr) {
    
  console.log(arr[index]);
}