Specification pattern for NodeJS

Specification pattern implementation for NodeJS. Suitable for any kind of validations, simplifying, improving reusability and and making code clearer.

Although the specification pattern is mainly use in DDD to check business rules, I the idea of combine rules offers great flexibility.

This is a JavaScript implementation of the pattern, both synchronous and asynchronous versions, so you can create your custom validation rules and combine them easily into a new rule.

How to use ?

First step is to include the required version (synchronous or asynchronous):

``javascript
// Asynchronous version
var specification = require("specification").Specification;

// Synchronous version
var specificationSync = require("specification").SpecificationSync;
`

For each business rule (or validation) you need to check, a specification must be created. Next code creates a specification that checks if a number is greater than the one indicated at the specification:

`javascript
function GreaterThan(num) {
this.num = num;
}
// Make it a subclass of SpecificationSync base class.
GreaterThan.prototype = Object.create(SpecificationSync);

// Implement the 'isSatisfiedBy' method.
GreaterThan.prototype.isSatisfiedBy = function(n) {
return (n > this.num);
};
`

Later, to use the previous specification:

`javascript
var greaterThan6 = new GreaterThan(6);

if (greaterThan6.isSatisfiedBy(9)) {
console.log("9 is greater than 6");
}
`

The base class SpecificationSync offers the and, or and not methods we can use to chain specifications and build complex ones. For example:

`javascript
var greaterThan6 = new GreaterThan(6);
var greaterThan8 = new GreaterThan(8);

var greaterThan6And8 = greaterThan6.and(greaterThan8);

if (greaterThan6And8.isSatisfiedBy(9)) {
console.log("9 is greater than 6 and 8");
}
`


The asynchronous version is suitable if you need to check agains an asynchronous source, like a database, files, etc. The only difference is the way to implement the isSatisfiedBy() method, which must be use a callback, for example:

`javascript
function GreaterThan(num) {
this.num = num;
}
GreaterThan.prototype = Object.create(Specification);
GreaterThan.prototype.isSatisfiedBy = function(n, cb) {
return cb(null, n > this.num);
};
`

and to use it you can make via the callback:

`javascript
var greaterThan6 = new GreaterThan(6);

greaterThan6.isSatisfiedBy(9, function(err, satisfies) {
if (satisfies) {
console.log("9 is greater than 6");
}
});
`

Chaining specifications works in the same way as the synchronous version, simply remember the only difference is the way to use the isSatisfiedBy() method:

`javascript
new GreaterThan(1)
.and(new GreaterThan(2))
.and(new GreaterThan(3))
.isSatisfiedBy(6, function(err, satisfies) {
if (satisfies) {
console.log("6 is greater than 1, 2 and 3");
}
});
``

---

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Copyright (c) 2015 Antonio Santiago (@acanimal)

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