sandy.js

//Let's say you have a function that accepts a callback, and applies that call back to each element:

forEach(array, callback) {
  // loop start at 0 index, until it reaches the end of the array, one at a time
  for(var i=0; i < array.length; i++) {
    // at the index of that array, have it equal to the result of the callback with "the result of that index of that array" as a parameter
    array[i] = callback(array[i])
  }
}

example:
var arr = [1,2,3]
var addOne = function (number) {
  return number + 1
}

var addedOne = forEach(arr, addOne) // returns [2,3,4]

/**
 * Under the hood this is what it looks like
 * forEach([1,2,3], addOne) {
    for(var i=0; i < array.length; i++) {
      array[i] = addOne(array[i])
    }
  }
  For the first loop:
  forEach([1,2,3], addOne) {
    for(var i=0; i < array.length; i++) {
      array[0] = addOne(array[0])
    }
  }
  forEach([1,2,3], addOne) {
    for(var i=0; i < array.length; i++) {
      array[0] = addOne(1)
    }
  }
  forEach([1,2,3], addOne) {
    for(var i=0; i < array.length; i++) {
      array[0] = addOne= function (1) {  return 1 + 1 }
    }
  }
  forEach([1,2,3], addOne) {
    for(var i=0; i < array.length; i++) {
      array[0] = 2
    }
  }
  end 1st loop
  repeat for 2nd loop
  I hope that helps! text me back if you need clarification.
*/