var original = [1,2,3,4,5,6];
var i, newList = [];

for (i = 0; i < original.length; i++) {
    newList.push(original[i] * 2);
}

var original = [1,2,3,4,5,6];

var newList = original.map(function(o){
    return o * 2;
});

var obj = {a: 1, b: 2, c: 3},
    k, v;

for (k in obj) {
    if (obj.hasOwnProperty(k)) {
        v = obj[k];
        // ...
    }
}

var obj = {a: 1, b: 2, c: 3};

Object.keys(obj).map(function(k){
    var v = obj[k];
    // ...
});