I did research but the answer was related with auto generated id using mongodb, not express and not for a nested object.

I'll wonder if someone helps me to generate the auto-id for a nested object dep4.id present in

my server.js file

function create() {
    let shop = {};
    shop.id = "shop1";
    shop.departmentList = [];

    let dep1 = {};
    dep1.id = "d1"
    dep1.name = "my department 1";
    shop.departmentList.push(dep1);

    let dep2 = {};
    dep2.id = "d2"

    dep2.name = "my department 2";
    dep2.departmentVariationList = [];
    let dep2v1 = {};
    dep2v1.id = "dep2v1"
    dep2v1.name = "my department variation dep2v1";
    dep2.departmentVariationList.push(dep2v1);
    //shop.departmentList.push(dep2);
    let dep2v2 = {};
    dep2v2.id = "dep2v2"
    dep2v2.name = "my department variation dep2v2";
    dep2.departmentVariationList.push(dep2v2);
    shop.departmentList.push(dep2);

    let dep3 = {};
    dep3.id = "d3"

    dep3.name = "my department 3";
    shop.departmentList.push(dep3);

    return shop;
}

Info: Here I am adding department 4 using postman and want to generate an auto id for department 4 and so on.

app.post('/shops/:shopId', (req, res) => {
    const shop = req.body;

    let shopId = req.params.shopId;
    if (shopId === "shop1") {

        shop1 = req.body;
    } else {
 
        shop2 = req.body;
    }
 
});

Your time and help will be really appreciated. Thanks :)