Double every other

doubleEveryOther.js

var zipWith = function (fn, a, b) {
    var l = Math.min(a.length, b.length),
        r = [];
    for(var i = 0; i < l; i += 1) {
        r.push(fn(a[i], b[i]));
    }
    return r;
};

var mul = function (a, b) { return a * b; }

var cycle = function (a, n) {
    var output = [];

    for (var i = 0; i < n; i++) {
        output.push(a[i % a.length]);
    }
    return output;
};

// doubleEveryOther
var list = [1,2,3,4,5];
zipWith(mul, cycle([1, 2], list.length), list);
/*
1,4,3,8,5
*/

doubleEveryOtherGeneric.js

var zipWith = function (fn, a, b) {
    var l = Math.min(a.length, b.length),
        r = [];
    for(var i = 0; i < l; i += 1) {
        r.push(fn(a[i], b[i]));
    }
    return r;
};

var apply = function (fn, v) {
    return fn(v);  
};

var id = function (v) { return v; }

var dbl = function (n) { return 2 * n; }

var cycle = function (a, n) {
    var output = [];

    for (var i = 0; i < n; i++) {
        output.push(a[i % a.length]);
    }
    return output;
};

// doubleEveryOther
zipWith(apply, cycle([id, dbl], 10), [1,2,3,4,5]);
/*
1,4,3,8,5
*/