Custom map prototype

array-prototype-map.js

// Create our custom map function and attach it to the prototype
Array.prototype.customMap = function(callback) {
  const arr = this;
  const newArr = [];
  
  for(let i = 0; i < arr.length; i++) {
    const alteredElement = callback(arr[i], i, arr);
    newArr.push(alteredElement);
  }
  
  return newArr;
}

// Execute our custom map function
const beachArr = ['waves', 'sharks', 'jellies', 'sand', 'pelicans'];
const newArr = beachArr.customMap(item => item.substring(0, 1));

// ['w', 's', 'j', 's', 'p']
console.log(`Our newly mapped array: ${newArr}`);