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);