Adding an auto-incrementing UserID field

I am creating an API to get and post values from a Database using the MEAN stack.

I want to automatically generate the userID starting from 0 when each entry is created in the usesrschema.

var userSchema = new mongoose.Schema({
    //userId: Number,
    username: String,
    password: String, //hash created from password
    created_at: {type: Date, default: Date.now}
})

How is it that I add a userID starting from 0 and autoincrementing with each insertion?

via Dilinieee