How to do Prototype Validation in Javascript

I hacked together a quick model for a prototype REST API written in Node.js, and wanted to validate an object prior to saving it to a database. I didn’t want or need some wacky full blown ORM but I did want to ensure that the records I was storing were consistent.

Validating in Javascript can be done in many ways but this is a method I came up with after a couple iterations. Let’s take a look at the code.

PictureValidations = function() {
 
  this.name = function(name) {
    return /^[A-Za-z0-9_-]{8,64}$/.test(name);
  }
 
  this.type = function(type) {
    return /^(jpeg|png|gif|bmp)$/.test(type);
  }
 
  this.tags = function(tags) {
    for (i in tags) {
      if(!(/^[A-Za-z0-9]+$/.test(tags[i]))) { return false };
    }
  }
 
}
 
exports.Picture = function(options) {
  this.name =  options.name ||"unassigned" ;
  this.tags = options.tags;
  this.description = options.description;
  this.created_on = new Date().getTime();
}
 
exports.Picture.prototype.valid = function() {
  var validations = new PictureValidations();
  var valid = true;
  for (v in validations) {
    valid = validations[v](this[v]);
    if (valid == false) { break ; } // stop checking on first failure
  };
  return valid
}

How it works

We’ve defined a new Object called Picture which has all the properties associated with it, as well as some defaults.

We also specify another object called PictureValidations which contains a bunch of functions. Each function is named after the properties in the Picture that we want to validate. Each function will return either True or False when it is called with the valid() method which has been added to the Picture prototype. The key is the line that says:


valid = validations[v](this[v]);

This is storing the status returned from the function in the validations Object we’ve instantiated with the value of the current objects properties (this).
It will then check to see whether or not it passed or failed.

The nice thing about this method is that you can write a function that does just about anything to determine whether or not something is valid.

Anyway, it seems to work, but I’d appreciate any feedback!

This entry was posted on Wednesday, November 24th, 2010 at 9:42 am and is filed under General. You can follow any comments to this entry through the RSS 2.0 feed. You can leave a comment, or trackback from your own site.

  • http://twitter.com/kevinykchan Kevin Chan

    Hey Trevor, great series of posts!

    Your validator halts on the first invalid field. I think if I was to do it, I would delegate the valid method on the Picture to the PictureValidator and have PictureValidator.valid return an object containing an array of the errors mapped to each field. The consumer would get to know exactly which field failed validation and why. It also allows more more complicated validations that validates across multiple fields.

    Keep posting!

  • http://trevoro.ca/ Trevor Orsztynowicz

    Hey Kevin,

    Yeah that’s a good idea. I was actually thinking about that earlier. It would definitely be nicer, especially in an API response as your error message.