JS Software Design Patterns

TOC

• Usage
• Categories
  • Creational
    • Builder
    • curry
    • Factory
    • makeLazy
  • Behavioral
    • Chain of Responsibility
    • Command
    • Momento
    • PubSub/Observer
  • Structural
    • Bridge
• Changelog

Usage

import * as patterns from 'js-software-design-patterns';

Categories

Creational

Well known creational patterns

Builder

Used to build classes that have minor changes from instance to instance efficiently. Any class that has a set[A-Z][a-zA-Z0-9]* method, will have that method extracted and used to build the instance. Setters should only ever have one property passed.

import { Builder} from 'js-software-design-patterns'
 
const Dog {
    setAge(age){ this.age = age }
    setBreed(breed){ this.breed = breed }
}
 
const builder = new Builder(Dog);
builder.breed = 'husky';
builder.age = .5;
const puppy = builder.build();
// expected { breed: 'husky', age: .5 }
 
buider.age = 10;
const adult = builder.build();
// expected { breed: 'husky', age: 10 }

Curry

Provides a currying function for partial function calls. Calling curry requires at minimum a function, with any number of optional arguments to be pass to function pointer. The return value of the call is a function that will curry all arguments until no more arguments are passed, which at that point, it will execute the initial function passing all previous arguments the in the order they were received.

import { curry } from 'js-software-design-patterns';
 
const add = (...args) => args.reduce((sum, next) => sum + next, 0);
 
// start the curry
let curriedAdder = curry(add, 1, 2, 3);
 
// continue to curry
curriedAdder = curriedAdder(4);
curriedAdder = curriedAdder(5,6);
 
// end the curry and execute the initial function (add)
const sum = curriedAdder();
 
console.log(sum);
// expected output - 21

Factory

Provides a generic factory pattern. Because this is generic and meant to set up another factory of enumerated class instances, a method of setEnums is required to make the class strict on what is can instantiate. Also, during setup, a method setLine is provided that will associate an item name in the enum list to reference which class it should construct.

import { Factory } from 'js-software-design-patterns';
import { Cat, Dog, Parrot } from './my-animal-classes';
 
// construct a generic factory
const animalFactory = new Factory();
 
// factory needs to be fed well defined production lines
animalFactory.setEnums(['CAT', 'DOG', 'PARROT'])
 
// link the production line keys to the Class that they will construct
animalFactory.setLine('CAT', Cat);
animalFactory.setLine('DOG', Dog);
animalFactory.setLine('PARROT', Parrot);
 
export default animalFactory;

import AnimalFactory from './animal-factory'
 
// have factory construct basic classes
const dog = AnimalFactory.get('DOG');
const cat = AnimalFactory.get('CAT');
 
// have factory construct class with constructor arguments
const parrot = AnimalFactory.get('PARROT', 'Cockatoo', 6, 'Crackers')

makeLazy

Provides a generic lazy initialization pattern function. The methods of the class will act as a trigger to instantiate the class. This pattern is especially useful when you need objects in a wide scope but do not know when they will be used or by which instance methods to which the lazy object belongs. Note, methods must be instance methods when constructed; anonymous methods (arrow operators) will not be referenced. Static class methods are obviously spurious to an instance. Class instance properties will not trigger a construction of the instance; use set and get method instead.

import { makeLazy } from 'js-software-design-patterns';
 
class Dog {
    bark(){ console.log('bark!!!') }
    eat(){ }
    sleep: () => {}
    set age(age){ this._age = age }
    get age(){ return this._age }
}
 
// lazily set up a reference to the constructor
const lazyDog = makeLazy(Dog);
 
// all Dog methods are available to the lazyDog wrapper. Dog has not been instantiated yet
lazyDog.bark();
// A Dog instance has been created
// Expected output
//  - bark!!!
 
// from this point on, all methods called belonging to Dog are called from the managed instance
lazyDog.eat();
 
 
// example of failure where arrow operators will not work
const brokenLazyDog = makeLazy(Dog);
brokenLazyDog.sleep();
// expected output - sleep is not defined
 
 
// example of using set and get syntax
const blueDog = makeLazy(Dog);
blueDog.color = 'blue';
const color = blueDog.color;
console.log(color === 'blue');
// expected output - true
 
 
// example of getting the actual lazy instance if needed
const instance = lazyDog.getInstance();
console.log(instance instanceof Dog)
// expected output - true

Behavioral

Well known behavioral patterns

Chain of Responsibility

Implementation of a generic Chain of Responsibility class.

import { ChainOfResponsibility } from 'js-software-design-patterns';
 
class Example extends ChainOfResponsibility {
    constructor(number = 0){
        super();
        this.number = number;
    }
 
    executeOn = (number = 0) => this.number + number;
}
 
 
const one = new Example(1);
const two = new Example(2);
 
one.appendNext(two);
 
console.log(one.execute());
// outputs 3
 
console.log(two.execute());
// outputs 2

Command Pattern

Implementation of the command pattern using function pointers rather than command objects.

import Commander from 'js-software-design-patterns';
 
const commander = new Commander();
 
class LightSwitch {
    static turnOn = () => console.log('Turned on the light')
}
 
class Fan {
    static turnOn = () => console.warn('Turned on the fan')
}
 
class AC {
    static coolHouse = (temp) => console.log(`Turned down the AC to ${temp}`)
}
 
commander.register('turn on', LightSwitch.turnOn);
commander.register('turn on', Fan.turnOn);
commander.register('make cooler', AC.coolHouse);
commander.register('make cooler', Fan.turnOn);
 
 
// event driven execution
commander.execute('turn on');
// expected output
//  Turned on the light
//  Turned on the fan
 
 
// event drive execution
commander.execute('make cooler', 65);
// expected output
//  - Turned down the AC to 65
//  - Turned on the fan

Momento

Implementation of a momento pattern. The momento pattern allows you to save a state of an object or instance and then recover that previously saved state. This mutates the state of the object and will not work on frozen/immutable objects. A single momento instance, once used to save a momento of an originating object, is permanently bound to that originator of the momento and cannot be used by other originating instances.

import { Momento } from 'js-software-design-patterns';
 
const fooBar = {
    foo: 'foo',
    bar: 'bar',
}
 
const foobarMomento = new Momento();
 
// Save the current state of fooBar
foobarMomento.save(fooBar);
 
fooBar.foo = 'bar';
 
// Now recover the previous state of fooBar
foobarMomento.recover(fooBar)
console.log(fooBar.foo);
// Expected output - foo
 
delete fooBar.bar;
foobarMomento.recover(fooBar);
console.log(fooBar.bar);
// Expected output - bar
 
fooBar.fooBar = 'fooBar';
foobarMomento.recover(fooBar);
console.log(fooBar.fooBar);
// Expected output - undefined
 
fooBar.fooBar = 'fooBar';
foobarMomento.save(fooBar);
foobarMomento.recover(fooBar);
console.log(fooBar);
// Expected output
// {
//      foo: 'foo'
//      bar: 'bar'
//      fooBar: 'fooBar'
// }

PubSub (Observer Pattern)

A declarative namespaced pubsub pattern.

import { PubSub } from 'js-software-design-patterns';
 
const pubSub = new PubSub();
 
const fox = pubSub.getPublisher('Fox Network', 'Fox Sports', 'Fox News');
const espn = pubSub.getPublisher('ESPN Network', 'ESPN College', 'ESPN 2', 'ESPN Ocho');
 
const subscriber = pubSub.getSubscriber('Sam Subscriber', {'Fox Network': ['Fox Sports'],'ESPN Network': ['ESPN Ocho']});
subscriber.setOnPublish( (publisher, channel, data) => { console.log(publisher, channel, data) });
 
// Positive case
fox.publish('Fox Sports', 'Bears win');
// Expect "Fox", "Fox Sports", "Bears win" to be printed
 
// Negative case
espn.publish('ESPN College', 'Texas loses');
// Expect nothing to happen
 
// Add and subscribe to channel after it is created.
espn.createChannel('ESPN Golf');
subscriber.subscribe(espn, 'ESPN Golf');
 
// Unsubscribe from a channel
subscriber.unsubscribe(espn, 'ESPN Golf');
 
// Calling get publishers on an existing publisher returns the same publisher instance
const sportsChannel = pubSub.getPublisher('ESPN Network');
console.log(sportsChannel === espn);
// Expect true.
 
// Calling subscriber ALWAYS returns a new instance regardless of the name
const otherSubscriber = pubSub.getSubscriber('Sam Subscriber', {'Fox Network': ['Fox Sports'],'ESPN Network': ['ESPN Ocho']});
console.log(subscriber === otherSubscriber);
// Expect false.
 

Structural

Well known structural patterns

Bridge

A take on the well known bridge pattern. Allows for any two methods of class instances to be joined, resulting in a return of an executable function. The bridge function takes scope to be applied to a function and then returns a to function to join another scope and a function. The to function when called then returns an execute function that accepts the same argument parameters as the initial function passed to bridge. It then calls the first joined function and using its return value as arguments to the second joined function, calls the second, returning its return value.

import { bridge } from 'js-software-design-patterns';
 
class Remote {
    changeChannelTo = (channel) => {
        console.log(`Remote changing channel to ${channel}. `)
        return channel;
    }
}
 
class TV {
    changeChannelTo = (channel) => console.log(`TV set to channel ${channel}.`)
}
 
const remote = new Remote();
const tv = new TV();
 
const to = bridge(remote, remote.changeChannelTo);
const execute = to(tv, tv.changeChannelTo);
execute(20);
// expected output -
//      Remote changing channel to 20.
//      TV set to channel 20.

Changelog

v1.0.0

- adds curry function

v0.5.1

- updates description of how to use momento
- adds TOC to readme

v0.5.0

- fixes export syntax for structural exports
- adds momento pattern