const t = db[jsTestName()];
t.drop();

const o = {
    _id: 1,
};
assert.commandWorked(t.insert(o));

assert.commandWorked(t.update({}, {$addToSet: {"kids": {"name": "Bob", "age": "4"}}}));
let res = t.findOne();
assert.eq(res["kids"].length, 1, res);
assert.commandWorked(t.update({}, {$addToSet: {"kids": {"name": "Dan", "age": "2"}}}));
res = t.findOne();
assert.eq(res["kids"].length, 2, res);