JavaScript Code Contract Libraries?

I am just starting up a new web application and I want to implement some form of contract'esque style validation in my JavaScript. I did some quick googling, and came across JsContact but the syntax isn't quite what I had in mind. Is anyone aware of other libraries?

I am thinking I want the syntax to be something like

String.prototype.padLeft = function(c, width) {
  Verify.value(c).isRequired().isNotNull().isChar();
  Verify.value(width).isRequired().isNotNull().isNumber().greaterThan(0);

  ...

  Verify.value(result).isNotNull();

  return result;
};

Although it won't take long to put together my own library that has the syntax/methods I want, if someone else has already done the work and it is close enough, it will save me some time. Thanks in advance.

UPDATE

I won't have time to work on this until this afternoon, so I will give it a few more hours to see if anyone has any recommendations. If not, I will post whatever I create up somewhere as an answer below for other people to reference if they desire.

I have also given a little more thought to the API that would make sense, and I am currently thinking something like (contrived examples):

 function searchUser(firstName, middleInit, lastName) {
   Verify.value(firstName).isString().matching(/\w+/);       // Must have value
   Verify.value(middleInit).whenNotNull().isChar();          // May be null, but not undefined
   Verify.value(lastName).isString().withMinimumLengthOf(2); // Must have value

   ...
 }

 function syncTime(serverTime, now) {
   Verify.value(serverTime).isDate();         // Must have value.
   Verify.value(now).whenDefined().isDate();  // May be undefined, but not null.

 }

My current thought is that tolerating NULL or UNDEFINED values is atypical (at least for me?), as such, rather than explicitly specifying that a value .isNotNull() you would actually disable the rule for .whenDefined() or .whenNotNull() as shown above. I may make .whenNotNull() not error on UNDEFINED, but I see NULL vs. UNDEFINED as an important distinction; we'll see... all other methods will be pretty typical... thoughts? comments?

Solution

Given that no one has recommended any existing libraries, or that I am crazy for thinking this is a good idea I went ahead and threw together a basic library. The code isn't fancy, but it does what I want, and it is reasonably fast to run (approx 40 chained checks per ms in IE).

I settled on a final syntax like:

function syncTime(serverTime, now) {
  Verify.value(serverTime).always().isDate();   // Cannot be undefined or null.
  Verify.value(now).whenDefined().isDate();     // Cannot be null, but must be date when defined.

  //Code
}

function searchForUser(firstName, middleInit, lastName) {
  Verify.value(firstName).always().isString().withMinimumLengthOf(2);  // Cannot be undefined or null.
  Verify.value(lastName).always().isString().withMinimumLengthOf(2);   // Cannot be undefined or null.
  Verify.value(middleInit).whenNotNull().isChar().between('A', 'Z');   // Cannot be undefined, but must be single char string when not null.

  //Code
}

I opted for an explicit 'Must Have Value' via the .always() check, personally I found it nicer to read; but I could see some going another way.

Given that the source is more than I want to post in this answer, please head to this CodePlex Wiki Page if you are interested in the source. I guess it turned in to more of a fluent assertion library; but it does what I need.

Update

I updated the source on the linked CodePlex page above. Specifically, I restructed the Verify class to make use of a 'value context' rather than always building new Verifier objects; improved IE's performance greatly (never was an issue with FireFox or Chrome)... now handles about 100 chained checks per ms in IE.