A walkthrough loops and arrays for beginners

loops_variadic_arrays.js

// declare function
// function <name> (firstParameter secondParameter, ...otherParaemters)
// Notice the `...` before otherParameters
// It means it can either be zero or more entries
// These entries will be collapsed into an array called whatever you named the parameter
function walkthrough(param1, param2, ...otherVariadicParams) {
  // initialize sum to 0
  let sum = 0;
  // Here, this iteration varaibale is so you can track how many times the loop happened
  // It can be different from the index of the loop itself
  // In our case we dont start our index from zero always; 
  // It depends on user input
  // So to visualize how many times the loop runs, this is helpful
  let iteration = 0;
  // the index of the loop is defined here
  // the index = param1 means the index strats from the value of the first parameter
  // the `index < param2` means, keep looping as long as index is less than value of param2
  // if youre looping through an array
  // this will usually be written as `index < array.length`
  // since you want to loop till the end of the array
  // and start from beginning of array
  // if for instance you wanted to loop from end of array to begginning of array
  // You'd write the loop as
  // for (let index = array.length - 1; index <= 0; index--)
  // meaning start from end of array (array.length - 1); since an array is indexed from 0 till array.length - 1
  // keep looping till the index reaches 0
  // reduce index by 1 on every iteration
  // since we want to walk backwards
  // e.g array [0, 1, 2] (using 0... so its easier to keep up with indexes)
  // start loop from end of array (length = 3)
  // so start loop at index (3 - 1) sp array[2] (which is 2 here)
  // loop till beginning of array; index = 0; array[0]
  // reduce index by 1 on every iteration
  // so array[2], array[1], array[0]
  // Also here; we are increasing the index by 2
  // Not by 1 which is usually the default (index++)
  for (let index = param1; index < param2; index += 2) {
    console.log("Debug: iteration #", iteration, " Sum at beginning of iteration is: ", sum, " Index of loop is:  ", index, "  value of array at loop index is: ", otherVariadicParams[index]);
    // here because the index may be greater than the length / bounds of the array
    // and array[numberLargerTahnLength] will be undefined
    // we do the null check 
    // so if its undefined we turn it to 0
    // array[bigNUmber] || 0
    // basically means
    // if (!array[bigNUmber])
    // if (array[bigNUmber] === undefined)
    // if (array[bigNUmber] === null)
    // return 0
    // We are doing this since undefined + 3
    // undefined + number = NaN (not a number)
    sum = sum + (otherVariadicParams[index] || 0);
    // we have to manulaly increase the iteration variable
    // since the loop only increase the index automaticallt
    // although if we wrote the loop as
    //  for (let index = param1; index < param2; index += 2, iteration++)
    // Notice the comma above between `index +=2` and `iteration++`
    iteration = iteration + 1;
  }

  // Here we log the value of the array we are working on,
  // the sum
  // and the parameters for debug purposes
  console.log('Debug: ', 'Array is: ', otherVariadicParams, ' Sum from: ', param1, " to: ", param2, "is: ", sum);
  return sum;
}

// call function 
console.log('Result is: ', walkthrough(5, 20));
console.log("  ======================== ")
walkthrough(1, 5, 3, 4)
console.log("  ======================== ")
walkthrough(1, 5, 3, 4, 5, 6, 7, 8)
console.log("  ======================== ")
walkthrough(5, 5, 3, 4, 5)
console.log("  ======================== ")