The Specification Pattern in JavaScript

Define a specification.

// no odd numbers allowed!
var even = spec(function (candidate) {
    return 0 === candidate % 2;
});

Negate that guy and save him for later.

// negate even to get an odd future
var odd = even.not();

Maybe get a little fancy with it.

// detest negativity
var positive = spec(function (candidate) {
    return candidate > 0;
});

// evenly positive
var evenAndPositive = even.and(positive);

// odd or negative
var oddOrPositive = odd.or(function (candidate) {
    return candidate < 0;
});

Now with more short circuiting.

// number five is alive
var shortCircuit = even.or(function (candidate) {
    // this will never run...need proof?
    console.log("No disassemble Number Five");
    return (null === candidate || "undefined" === typeof candidate);
});

Quench your thirst for truthiness.

console.log(even.isSatisfiedBy(3)); // false
console.log(odd.isSatisfiedBy(3)); // true
console.log(evenAndPositive.isSatisfiedBy(-2)); // false
console.log(oddOrNegative.isSatisfiedBy(3)); // true
console.log(shortCircuit.isSatisfiedBy(2)); // true