Simpler "prose-like" syntax with Object.create()
And the true prototypial nature of Javascript
*This example is updated for ES6 classes and TypeScript.
Firstly, Javascript is a prototypal language, not class-based. Its true nature is expressed in the prototypial form below, which you may come to see that is very simple, prose-like, yet powerful.
TLDR;
Javascript
const Person = {
name: 'Anonymous', // person has a name
greet: function() { console.log(`Hi, I am ${this.name}.`) }
}
const jack = Object.create(Person) // jack is a person
jack.name = 'Jack' // and has a name 'Jack'
jack.greet() // outputs "Hi, I am Jack."
TypeScript
In TypeScript, you will need to set up interfaces, which will be extended as you create descendents of the Person prototype. A mutation politeGreet shows an example of attaching new method on the descendent jack.
interface IPerson extends Object {
name: string
greet(): void
}
const Person: IPerson = {
name: 'Anonymous',
greet() {
console.log(`Hi, I am ${this.name}.`)
}
}
interface IPolitePerson extends IPerson {
politeGreet: (title: 'Sir' | 'Mdm') => void
}
const PolitePerson: IPolitePerson = Object.create(Person)
PolitePerson.politeGreet = function(title: string) {
console.log(`Dear ${title}! I am ${this.name}.`)
}
const jack: IPolitePerson = Object.create(Person)
jack.name = 'Jack'
jack.politeGreet = function(title): void {
console.log(`Dear ${title}! I am ${this.name}.`)
}
jack.greet() // "Hi, I am Jack."
jack.politeGreet('Sir') // "Dear Sir, I am Jack."
This absolves the sometimes convoluted constructor pattern. A new object inherits from the old one, but is able to have its own properties. If we attempt to obtain a member from the new object (#greet()) which the new object jack lacks, the old object Person will supply the member.
In Douglas Crockford's words: "Objects inherit from objects. What could be more object-oriented than that?"
You don't need constructors, no new instantiation. You simply create Objects and then extend or morph them.
This pattern also offers immutability (partial or full), and getters/setters.
Clean and clear. It's simplicity does not compromise features. Read on.
Creating an descendant/copy of Person prototype (technically more correct than class).
*Note: Below examples are in JS. To write in Typescript, just follow the example above to set up interfaces for typing.
const Skywalker = Object.create(Person)
Skywalker.lastName = 'Skywalker'
Skywalker.firstName = ''
Skywalker.type = 'human'
Skywalker.greet = function() { console.log(`Hi, my name is ${this.firstName} ${this.lastName} and I am a ${this.type}.`
const anakin = Object.create(Skywalker)
anakin.firstName = 'Anakin'
anakin.birthYear = '442 BBY'
anakin.gender = 'male' // you can attach new properties.
anakin.greet() // 'Hi, my name is Anakin Skywalker and I am a human.'
Person.isPrototypeOf(Skywalker) // outputs true
Person.isPrototypeOf(anakin) // outputs true
Skywalker.isPrototypeOf(anakin) // outputs true
If you feel less safe throwing away the constructors in-lieu of direct assignments, one common way is to attach a #create method:
Skywalker.create = function(firstName, gender, birthYear) {
let skywalker = Object.create(Skywalker)
Object.assign(skywalker, {
firstName,
birthYear,
gender,
lastName: 'Skywalker',
type: 'human'
})
return skywalker
}
const anakin = Skywalker.create('Anakin', 'male', '442 BBY')
Branching the Person prototype to Robot
When you branch the Robot descendant from Person prototype, you won't affect Skywalker and anakin:
// create a `Robot` prototype by extending the `Person` prototype:
const Robot = Object.create(Person)
Robot.type = 'robot'
Attach methods unique to Robot
Robot.machineGreet = function() {
/*some function to convert strings to binary */
}
// Mutating the `Robot` object doesn't affect `Person` prototype and its descendants
anakin.machineGreet() // error
Person.isPrototypeOf(Robot) // outputs true
Robot.isPrototypeOf(Skywalker) // outputs false
In TypeScript you would also need to extend the Person interface:
interface Robot extends Person {
machineGreet(): void
}
const Robot: Robot = Object.create(Person)
Robot.machineGreet = function() { console.log(101010) }
And You Can Have Mixins -- Because.. is Darth Vader a human or robot?
const darthVader = Object.create(anakin)
// for brevity, property assignments are skipped because you get the point by now.
Object.assign(darthVader, Robot)
Darth Vader gets the methods of Robot:
darthVader.greet() // inherited from `Person`, outputs "Hi, my name is Darth Vader..."
darthVader.machineGreet() // inherited from `Robot`, outputs 001010011010...
Along with other odd things:
console.log(darthVader.type) // outputs robot.
Robot.isPrototypeOf(darthVader) // returns false.
Person.isPrototypeOf(darthVader) // returns true.
Which elegantly reflects the "real-life" subjectivity:
"He's more machine now than man, twisted and evil." - Obi-Wan Kenobi
"I know there is good in you." - Luke Skywalker
Compare to the pre-ES6 "classical" equivalent:
function Person (firstName, lastName, birthYear, type) {
this.firstName = firstName
this.lastName = lastName
this.birthYear = birthYear
this.type = type
}
// attaching methods
Person.prototype.name = function() { return firstName + ' ' + lastName }
Person.prototype.greet = function() { ... }
Person.prototype.age = function() { ... }
function Skywalker(firstName, birthYear) {
Person.apply(this, [firstName, 'Skywalker', birthYear, 'human'])
}
// confusing re-pointing...
Skywalker.prototype = Person.prototype
Skywalker.prototype.constructor = Skywalker
const anakin = new Skywalker('Anakin', '442 BBY')
// #isPrototypeOf won't work
Person.isPrototypeOf(anakin) // returns false
Skywalker.isPrototypeOf(anakin) // returns false
ES6 Classes
Clunkier compared to using Objects, but code readability is okay:
class Person {
constructor(firstName, lastName, birthYear, type) {
this.firstName = firstName
this.lastName = lastName
this.birthYear = birthYear
this.type = type
}
name() { return this.firstName + ' ' + this.lastName }
greet() { console.log('Hi, my name is ' + this.name() + ' and I am a ' + this.type + '.' ) }
}
class Skywalker extends Person {
constructor(firstName, birthYear) {
super(firstName, 'Skywalker', birthYear, 'human')
}
}
const anakin = new Skywalker('Anakin', '442 BBY')
// prototype chain inheritance checking is partially fixed.
Person.isPrototypeOf(anakin) // returns false!
Skywalker.isPrototypeOf(anakin) // returns true
Further reading
Writability, Configurability and Free Getters and Setters!
For free getters and setters, or extra configuration, you can use Object.create()'s second argument a.k.a propertiesObject. It is also available in #Object.defineProperty, and #Object.defineProperties.
To illustrate its usefulness, suppose we want all Robot to be strictly made of metal (via writable: false), and standardise powerConsumption values (via getters and setters).
// Add interface for Typescript, omit for Javascript
interface Robot extends Person {
madeOf: 'metal'
powerConsumption: string
}
// add `: Robot` for TypeScript, omit for Javascript.
const Robot: Robot = Object.create(Person, {
// define your property attributes
madeOf: {
value: "metal",
writable: false, // defaults to false. this assignment is redundant, and for verbosity only.
configurable: false, // defaults to false. this assignment is redundant, and for verbosity only.
enumerable: true // defaults to false
},
// getters and setters
powerConsumption: {
get() { return this._powerConsumption },
set(value) {
if (value.indexOf('MWh')) return this._powerConsumption = value.replace('M', ',000k')
this._powerConsumption = value
throw new Error('Power consumption format not recognised.')
}
}
})
// add `: Robot` for TypeScript, omit for Javascript.
const newRobot: Robot = Object.create(Robot)
newRobot.powerConsumption = '5MWh'
console.log(newRobot.powerConsumption) // outputs 5,000kWh
And all prototypes of Robot cannot be madeOf something else:
const polymerRobot = Object.create(Robot)
polymerRobot.madeOf = 'polymer'
console.log(polymerRobot.madeOf) // outputs 'metal'
