arr = [1,2,3,4,5]

function map(arr,fn) {
    res = [];
    for(i in arr) {
        res.push(fn(arr[i]));
    }
    return res;
}

// maps the function and the flattens it to a depth of one.
function flatMap(arr,fn) {
    return Array.prototype.concat.apply([],map(arr,fn));
}

console.log(map(arr,x=>[2*x])); // [ [ 2 ], [ 4 ], [ 6 ], [ 8 ], [ 10 ] ]
console.log(flatMap(arr,x=>[2*x])); // [ 2, 4, 6, 8, 10 ]