Find an existing id object in post web service bill - node.js

Question

I want to find an existing id object when I post a bill, but I don't know how find it to save the bill. I want it works from back end and front end.
This is my bill model:
const mongoose = require("mongoose");
const { Schema } = mongoose;
const billSchema = new Schema({
number: Number,
date: { type: Date, default: Date.now() },
type: String,
local: String,
client: {
type: mongoose.Schema.Types.ObjectId,
ref: "clients"
},
provider: {
type: mongoose.Schema.Types.ObjectId,
ref: "providers"
},
detail: [
{
quantity: Number,
product: {
code: Number,
name: String,
price: Number
},
undertotal: Number
}
],
total: Number
});
module.exports = mongoose.model("bills", billSchema);
And this is my post service:
app.post("/api/bills", async (req, res) => {
const { number, type, local, detail, total } = req.body;
let existingClient = await Client.findById(Client._id);
if (!existingClient) {
return res.status(404).json({
message: "not found client"
});
}
let existingProvider = await Provider.findById(Provider._id);
if (!existingProvider) {
return res.status(404).json({
message: "not found provider"
});
}
if (
!existingClient._id ||
(existingClient._id &&
mongoose.Types.ObjectId() ===
(await Client.findById(existingClient._id).select("_id")))
) {
const clientId = mongoose.Types.ObjectId();
this.existingClient._id = clientId;
}
if (
!existingProvider._id ||
(existingProvider._id &&
mongoose.Types.ObjectId() ===
(await Provider.findById(existingProvider._id).select("_id")))
) {
const providerId = mongoose.Types.ObjectId();
this.existingProvider._id = providerId;
}
const bill = new Bill({
number,
date: new Date(),
type,
local,
client: clientId,
provider: providerId,
detail,
total
});
try {
let newBill = await bill.save();
res.status(201).send(newBill);
} catch (err) {
if (err.name === "MongoError") {
res.status(409).send(err.message);
}
console.log(err);
res.status(500).send(err);
}
});
The expected output is bill saved with client and provider ids but real output is an error message saying "cannot read property _id of undefined"
What is wrong?