Mongoose validation: required : false, validate : regex, issues with empty values

I think your regex is failing validation on empty string which should in this case be valid since this field is not required. Why don't you try this regex:

/^$|^\d{10}$/ 

This will match an empty string or 10 digits.

You may try with a custom validator as they are only triggered when there is a value on a given key because the key selection for custom validation is done via path() :

var user = new Schema({  // ...  phone    : { type: String }, // using default - required:false  // ...});// Custom validationuser.path('phone').validate(function (value) {  // Your validation code here, should return bool}, 'Some error message');

Have a look at this question: Why Mongoose doesn't validate empty document?

This will also effectively prevent the document to be persisted to the DB if validation fails, unless you handle the error accordingly.

BonusTip: Try to approach custom validations in a simplistic way, for example try to avoid loops when possible and avoid using libraries like lodash or underscore for in my experience I've seen that these may have a significant performance cost when working with lots of transactions.

You can simply check if the value entered exists (not null or undefined). If it exists, then test the regex:

var user = new Schema({     _id      : { type: String, required: true },    name     : { type: String, required: true},    phone    : { type: String,/*not required by default**/                  validate: {                     validator: function(v) {                         var re = /^\d{10}$/;                         return (v == null || v.trim().length < 1) || re.test(v)                     },                     message: 'Provided phone number is invalid.'                 }    },    password : { type: String },    added    : { type: Date,    default: Date.now },},{collection : 'users'});