• Better understand JS iterators
• Gain a deeper understanding of callbacks

1. Open the index.html file in the browser
2. Test your JS code inside the index.js file

Remember to refresh the page whenever you make changes and to use the console in the Chrome Dev Tools to try things on the fly.

Here's what a forEach might look like if we wrote it ourselves:

function myEach(arr, callback){
  for(let i = 0; i < arr.length; i++){
    const element = arr[i]
    callback(element)
  }
}

Discuss every line of this code. If the objective of a forEach is to run some bit of code on every element of an array, how does this code allow us to do this? Discuss how you would invoke this function and where its arguments come from.

Invoke myEach several times with your own array and callback.

For the next three exercises, pay attention to return values. In particular, discuss how the return value of the callback is used. Then discuss the return value of the higher-order function (i.e. myMap, myFilter, myFind).

Use the above myEach as a template to write your own map called myMap. Remember: the purpose of map is to execute some bit of code on each element and collect the results (the return values) of those executions into a new array and return that array.

For example: If our callback multiplies each number by 2, if [1,2,3,4] goes in to map, [2,4,6,8] should be returned by myEach.

Use the above myEach as a template to write your own filter called myFilter. Remember: the purpose of filter is to collect only the elements that match a given criteria into a new array and return that array. The callback is used to determine whether or not an element meets your given criteria.

For example: If our callback checks to see if a number is even, if [1,2,3,4] goes in to filter, [2,4] should be returned.

Use the above myEach as a template to write your own find called myFind, which is similar to filter but instead returns only the first element it finds.

For example: If our callback checks to see if a number is greater than 5, if [4,2,5,7,2,6,8], 7 should be returned.