var
    tries = 100000,
    cnt,
    name,
    names = ['a', 'b', 'c', 'd', 'e', 'f', 'g'],
    nLen  = names.length,
    switchTime,
    callTime;

function getName()
{
    for (var j, x, i = nLen; i; j = Math.floor(Math.random() * i), x = names[--i], names[i] = names[j], names[j] = x);
    return names[0];
}

function callByName(name)
{
    // ...
}

switchTime = (new Date()).getTime();
cnt = tries;
while (cnt--) {
    name = getName();
    switch (name) {

        case 'a':
        case 'b':
        case 'c':
        case 'd':
        case 'e':
        case 'f':
            // for_all_non_default()
            if (name == 'a') {
                // for_A()
            } else if (name == 'b') {
                // for_B()
            } else if (name == 'c') {
                // for_C()
            } else if (name == 'd') {
                // for_D()
            } else if (name == 'e') {
                // for_E()
            } else if (name == 'f') {
                // for_F()
            }
        break;

        default:
            // for_default()

    }

}
switchTime = (new Date()).getTime() - switchTime;


callTime = (new Date()).getTime();
cnt = tries;
while (cnt--) {
    name = getName();
    if (names.indexOf(name) != -1) {
        // for_all_non_default()
        callByName(name);
    } else {
        // for_default()
    }
}
callTime = (new Date()).getTime() - callTime;


console.log(switchTime, callTime);