Mongoose middleware are functions that can intercept the process of the init, validate, save, and remove instance methods. Middleware are executed at the instance level and have two types: pre middleware and post middleware.
Pre middleware gets executed before the operation happens. For instance, a pre-save middleware will get executed before the saving of the document. This functionality makes pre middleware perfect for more complex validations and default values assignments.
A pre middleware is defined using the pre() method of the schema object, so validating your model using a pre middleware will look like what is shown in the following code snippet:
UserSchema.pre('save', function(next){
if (...) {
next()
} else {
next(new Error('An Error Occurred'));
}
});