CODESAMPLE

Specification - JavaScript

Specification JavaScript

The Specification pattern is a functional technique for encapsulating business logic into reusable, composable objects. Instead of embedding conditions directly within code, it defines a Specification object that represents a declarative criteria. This allows for dynamic querying and validation, and simplifies complex conditional logic.

The code defines a base Specification class with a isSatisfiedBy method. Concrete specifications (e.g., IsEven, GreaterThan) inherit from this base and implement the criteria. Specifications can be combined using and, or, and not to create more complex rules. This approach is idiomatic JavaScript due to its reliance on object composition and first-class functions, promoting a more declarative and testable style.

// Specification.js
class Specification {
  constructor(callback) {
    this.callback = callback;
  }

  isSatisfiedBy(candidate) {
    return this.callback(candidate);
  }

  and(other) {
    return new Specification(candidate => this.isSatisfiedBy(candidate) && other.isSatisfiedBy(candidate));
  }

  or(other) {
    return new Specification(candidate => this.isSatisfiedBy(candidate) || other.isSatisfiedBy(candidate));
  }

  not() {
    return new Specification(candidate => !this.isSatisfiedBy(candidate));
  }
}

class IsEven extends Specification {
  constructor() {
    super(x => x % 2 === 0);
  }
}

class GreaterThan extends Specification {
  constructor(threshold) {
    super(x => x > threshold);
  }
}

// Example Usage:
const isEven = new IsEven();
const greaterThanTen = new GreaterThan(10);

const combinedSpec = isEven.and(greaterThanTen);

console.log(combinedSpec.isSatisfiedBy(12)); // true
console.log(combinedSpec.isSatisfiedBy(11)); // false
console.log(combinedSpec.isSatisfiedBy(8));  // false

const notEven = isEven.not();
console.log(notEven.isSatisfiedBy(7)); // true
console.log(notEven.isSatisfiedBy(4)); // false