SOURCE

Array.prototype.myMap = function(fn) {
    const res = [];
    if (!Array.isArray(this)) {
        return 'this is not array';
    }
    for (i = 0; i < this.length; i++ ) {
        res.push(fn(this[i], i, this));
    }
    return res;
}

const res1 = [1,2,3,4].myMap((item) => {
    return item + 1;
});

console.log(res1);