var evens = [2, 4, 6, 8, 10];
Array.prototype.customMap = function(fn) {
let results = [];
for (let i = 0; i < this.length; i++) {
results.push(fn(this[i], i))
}
return results;
}
const odds = evens.customMap(item => {
return item - 1;
})
console.log(odds);