number become unique in mongoose in schema code example

Example: mongoose unique field

var SimSchema = new Schema({
    msisdn     : { type : String , unique : true, required : true, dropDups: true },
    imsi       : { type : String , unique : true, required : true, dropDups: true },
    status     : { type : Boolean, default: true},
    signal     : { type : Number },
    probe_name : { type:  String , required : true }
});