mongo/jstests/core/eval2.js

t = db.eval2;
t.drop();
t.save({a: 1});
t.save({a: 1});

var f = db.group({
    ns: t.getName(),
    key: {a: true},
    cond: {a: 1},
    reduce: function(obj, prev) {
        prev.csum++;
    },
    initial: {csum: 0}
});

assert(f[0].a == 1 && f[0].csum == 2, "on db");

var f = t.group({
    key: {a: true},
    cond: {a: 1},
    reduce: function(obj, prev) {
        prev.csum++;
    },
    initial: {csum: 0}
});

assert(f[0].a == 1 && f[0].csum == 2, "on coll");