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Javascript 1.9.3 / ECMAScript 5 introduces Object.create, which Douglas Crockford amongst others has been advocating for a long time. How do I replace new in the code below with Object.create?
var UserA = function(nameParam) {
this.id = MY_GLOBAL.nextId();
this.name = nameParam;
}
UserA.prototype.sayHello = function() {
console.log('Hello '+ this.name);
}
var bob = new UserA('bob');
bob.sayHello();
(Assume MY_GLOBAL.nextId exists).
The best I can come up with is:
var userB = {
init: function(nameParam) {
this.id = MY_GLOBAL.nextId();
this.name = nameParam;
},
sayHello: function() {
console.log('Hello '+ this.name);
}
};
var bob = Object.create(userB);
bob.init('Bob');
bob.sayHello();
There doesn't seem to be any advantage, so I think I'm not getting it. I'm probably being too neo-classical. How should I use Object.create to create user 'bob'?
With only one level of inheritance, your example may not let you see the real benefits of Object.create.
This methods allows you to easily implement differential inheritance, where objects can directly inherit from other objects.
On your userB example, I don't think that your init method should be public or even exist, if you call again this method on an existing object instance, the id and name properties will change.
Object.create lets you initialize object properties using its second argument, e.g.:
var userB = {
sayHello: function() {
console.log('Hello '+ this.name);
}
};
var bob = Object.create(userB, {
'id' : {
value: MY_GLOBAL.nextId(),
enumerable:true // writable:false, configurable(deletable):false by default
},
'name': {
value: 'Bob',
enumerable: true
}
});
As you can see, the properties can be initialized on the second argument of Object.create, with an object literal using a syntax similar to the used by the Object.defineProperties and Object.defineProperty methods.
It lets you set the property attributes (enumerable, writable, or configurable), which can be really useful.
There is really no advantage in using Object.create(...) over new object.
Those advocating this method generally state rather ambiguous advantages: "scalability", or "more natural to JavaScript" etc.
However, I have yet to see a concrete example that shows that Object.create has any advantages over using new. On the contrary there are known problems with it. Sam Elsamman describes what happens when there are nested objects and Object.create(...) is used:
var Animal = {
traits: {},
}
var lion = Object.create(Animal);
lion.traits.legs = 4;
var bird = Object.create(Animal);
bird.traits.legs = 2;
alert(lion.traits.legs) // shows 2!!!
This occurs because Object.create(...) advocates a practice where data is used to create new objects; here the Animal datum becomes part of the prototype of lion and bird, and causes problems as it is shared. When using new the prototypal inheritance is explicit:
function Animal() {
this.traits = {};
}
function Lion() { }
Lion.prototype = new Animal();
function Bird() { }
Bird.prototype = new Animal();
var lion = new Lion();
lion.traits.legs = 4;
var bird = new Bird();
bird.traits.legs = 2;
alert(lion.traits.legs) // now shows 4
Regarding, the optional property attributes that are passed into Object.create(...), these can be added using Object.defineProperties(...).
Object.create is not yet standard on several browsers, for example IE8, Opera v11.5, Konq 4.3 do not have it. You can use Douglas Crockford's version of Object.create for those browsers but this doesn't include the second 'initialisation object' parameter used in CMS's answer.
For cross browser code one way to get object initialisation in the meantime is to customise Crockford's Object.create. Here is one method:-
Object.build = function(o) {
var initArgs = Array.prototype.slice.call(arguments,1)
function F() {
if((typeof o.init === 'function') && initArgs.length) {
o.init.apply(this,initArgs)
}
}
F.prototype = o
return new F()
}
This maintains Crockford prototypal inheritance, and also checks for any init method in the object, then runs it with your parameter(s), like say new man('John','Smith'). Your code then becomes:-
MY_GLOBAL = {i: 1, nextId: function(){return this.i++}} // For example
var userB = {
init: function(nameParam) {
this.id = MY_GLOBAL.nextId();
this.name = nameParam;
},
sayHello: function() {
console.log('Hello '+ this.name);
}
};
var bob = Object.build(userB, 'Bob'); // Different from your code
bob.sayHello();
So bob inherits the sayHello method and now has own properties id=1 and name='Bob'. These properties are both writable and enumerable of course. This is also a much simpler way to initialise than for ECMA Object.create especially if you aren't concerned about the writable, enumerable and configurable attributes.
For initialisation without an init method the following Crockford mod could be used:-
Object.gen = function(o) {
var makeArgs = arguments
function F() {
var prop, i=1, arg, val
for(prop in o) {
if(!o.hasOwnProperty(prop)) continue
val = o[prop]
arg = makeArgs[i++]
if(typeof arg === 'undefined') break
this[prop] = arg
}
}
F.prototype = o
return new F()
}
This fills the userB own properties, in the order they are defined, using the Object.gen parameters from left to right after the userB parameter. It uses the for(prop in o) loop so, by ECMA standards, the order of property enumeration cannot be guaranteed the same as the order of property definition. However, several code examples tested on (4) major browsers show they are the same, provided the hasOwnProperty filter is used, and sometimes even if not.
MY_GLOBAL = {i: 1, nextId: function(){return this.i++}}; // For example
var userB = {
name: null,
id: null,
sayHello: function() {
console.log('Hello '+ this.name);
}
}
var bob = Object.gen(userB, 'Bob', MY_GLOBAL.nextId());
Somewhat simpler I would say than Object.build since userB does not need an init method. Also userB is not specifically a constructor but looks like a normal singleton object. So with this method you can construct and initialise from normal plain objects.
TL;DR:
new Computer() will invoke the constructor function Computer(){} for one time, while Object.create(Computer.prototype) won't.
All the advantages are based on this point.
Sidenote about performance: Constructor invoking like new Computer() is heavily optimized by the engine, so it may be even faster than Object.create.
You could make the init method return this, and then chain the calls together, like this:
var userB = {
init: function(nameParam) {
this.id = MY_GLOBAL.nextId();
this.name = nameParam;
return this;
},
sayHello: function() {
console.log('Hello '+ this.name);
}
};
var bob = Object.create(userB).init('Bob');
Another possible usage of Object.create is to clone immutable objects in a cheap and effective way.
var anObj = {
a: "test",
b: "jest"
};
var bObj = Object.create(anObj);
bObj.b = "gone"; // replace an existing (by masking prototype)
bObj.c = "brand"; // add a new to demonstrate it is actually a new obj
// now bObj is {a: test, b: gone, c: brand}
Notes: The above snippet creates a clone of an source object (aka not a reference, as in cObj = aObj). It benefits over the copy-properties method (see 1), in that it does not copy object member properties. Rather it creates another -destination- object with it's prototype set on the source object. Moreover when properties are modified on the dest object, they are created "on the fly", masking the prototype's (src's) properties.This constitutes a fast an effective way of cloning immutable objects.
The caveat here is that this applies to source objects that should not be modified after creation (immutable). If the source object is modified after creation, all the clone's unmasked properties will be modified, too.
Fiddle here(http://jsfiddle.net/y5b5q/1/) (needs Object.create capable browser).
I think the main point in question - is to understand difference between new and Object.create approaches. Accordingly to this answer and to this video new keyword does next things:
Creates new object.
Links new object to constructor function (prototype).
Makes this variable point to the new object.
Executes constructor function using the new object and implicit perform return this;
Assigns constructor function name to new object's property constructor.
Object.create performs only 1st and 2nd steps!!!
In code example provided in question it isn't big deal, but in next example it is:
var onlineUsers = [];
function SiteMember(name) {
this.name = name;
onlineUsers.push(name);
}
SiteMember.prototype.getName = function() {
return this.name;
}
function Guest(name) {
SiteMember.call(this, name);
}
Guest.prototype = new SiteMember();
var g = new Guest('James');
console.log(onlineUsers);
As side effect result will be:
[ undefined, 'James' ]
because of Guest.prototype = new SiteMember();
But we don't need to execute parent constructor method, we need only make method getName to be available in Guest.
Hence we have to use Object.create.
If replace Guest.prototype = new SiteMember();
to Guest.prototype = Object.create(SiteMember.prototype); result be:
[ 'James' ]
Sometimes you cannot create an object with NEW but are still able to invoke the CREATE method.
For example: if you want to define a Custom Element it must derive from HTMLElement.
proto = new HTMLElement //fail :(
proto = Object.create( HTMLElement.prototype ) //OK :)
document.registerElement( "custom-element", { prototype: proto } )
The advantage is that Object.create is typically slower than new on most browsers
In this jsperf example, in a Chromium, browser new is 30 times as fast as Object.create(obj) although both are pretty fast. This is all pretty strange because new does more things (like invoking a constructor) where Object.create should be just creating a new Object with the passed in object as a prototype (secret link in Crockford-speak)
Perhaps the browsers have not caught up in making Object.create more efficient (perhaps they are basing it on new under the covers ... even in native code)
Summary:
Object.create() is a Javascript function which takes 2 arguments and returns a new object.
The first argument is an object which will be the prototype of the newly created object
The second argument is an object which will be the properties of the newly created object
Example:
const proto = {
talk : () => console.log('hi')
}
const props = {
age: {
writable: true,
configurable: true,
value: 26
}
}
let Person = Object.create(proto, props)
console.log(Person.age);
Person.talk();
Practical applications:
The main advantage of creating an object in this manner is that the prototype can be explicitly defined. When using an object literal, or the new keyword you have no control over this (however, you can overwrite them of course).
If we want to have a prototype The new keyword invokes a constructor function. With Object.create() there is no need for invoking or even declaring a constructor function.
It can Basically be a helpful tool when you want create objects in a very dynamic manner. We can make an object factory function which creates objects with different prototypes depending on the arguments received.
You have to make a custom Object.create() function. One that addresses Crockfords concerns and also calls your init function.
This will work:
var userBPrototype = {
init: function(nameParam) {
this.name = nameParam;
},
sayHello: function() {
console.log('Hello '+ this.name);
}
};
function UserB(name) {
function F() {};
F.prototype = userBPrototype;
var f = new F;
f.init(name);
return f;
}
var bob = UserB('bob');
bob.sayHello();
Here UserB is like Object.create, but adjusted for our needs.
If you want, you can also call:
var bob = new UserB('bob');
While Douglas Crockford used to be a zealous advocate of Object.create() and he is basically the reason why this construct actually is in javascript, he no longer has this opinion.
He stopped using Object.create, because he stopped using this keyword altogether as it causes too much trouble. For example, if you are not careful it can easily point to the global object, which can have really bad consequences. And he claims that without using this Object.create does not make sense anymore.
You can check this video from 2014 where he talks at Nordic.js:
https://www.youtube.com/watch?v=PSGEjv3Tqo0
new and Object.create serve different purposes. new is intended to create a new instance of an object type. Object.create is intended to simply create a new object and set its prototype. Why is this useful? To implement inheritance without accessing the __proto__ property. An object instance's prototype referred to as [[Prototype]] is an internal property of the virtual machine and is not intended to be directly accessed. The only reason it is actually possible to directly access [[Prototype]] as the __proto__ property is because it has always been a de-facto standard of every major virtual machine's implementation of ECMAScript, and at this point removing it would break a lot of existing code.
In response to the answer above by 7ochem, objects should absolutely never have their prototype set to the result of a new statement, not only because there's no point calling the same prototype constructor multiple times but also because two instances of the same class can end up with different behavior if one's prototype is modified after being created. Both examples are simply bad code as a result of misunderstanding and breaking the intended behavior of the prototype inheritance chain.
Instead of accessing __proto__, an instance's prototype should be written to when an it is created with Object.create or afterward with Object.setPrototypeOf, and read with Object.getPrototypeOf or Object.isPrototypeOf.
Also, as the Mozilla documentation of Object.setPrototypeOf points out, it is a bad idea to modify the prototype of an object after it is created for performance reasons, in addition to the fact that modifying an object's prototype after it is created can cause undefined behavior if a given piece of code that accesses it can be executed before OR after the prototype is modified, unless that code is very careful to check the current prototype or not access any property that differs between the two.
Given
const X = function (v) { this.v = v };
X.prototype.whatAmI = 'X';
X.prototype.getWhatIAm = () => this.whatAmI;
X.prototype.getV = () => this.v;
the following VM pseudo-code is equivalent to the statement const x0 = new X(1);:
const x0 = {};
x0.[[Prototype]] = X.prototype;
X.prototype.constructor.call(x0, 1);
Note although the constructor can return any value, the new statement always ignores its return value and returns a reference to the newly created object.
And the following pseudo-code is equivalent to the statement const x1 = Object.create(X.prototype);:
const x0 = {};
x0.[[Prototype]] = X.prototype;
As you can see, the only difference between the two is that Object.create does not execute the constructor, which can actually return any value but simply returns the new object reference this if not otherwise specified.
Now, if we wanted to create a subclass Y with the following definition:
const Y = function(u) { this.u = u; }
Y.prototype.whatAmI = 'Y';
Y.prototype.getU = () => this.u;
Then we can make it inherit from X like this by writing to __proto__:
Y.prototype.__proto__ = X.prototype;
While the same thing could be accomplished without ever writing to __proto__ with:
Y.prototype = Object.create(X.prototype);
Y.prototype.constructor = Y;
In the latter case, it is necessary to set the constructor property of the prototype so that the correct constructor is called by the new Y statement, otherwise new Y will call the function X. If the programmer does want new Y to call X, it would be more properly done in Y's constructor with X.call(this, u)
new Operator
This is used to create object from a constructor function
The new keywords also executes the constructor function
function Car() {
console.log(this) // this points to myCar
this.name = "Honda";
}
var myCar = new Car()
console.log(myCar) // Car {name: "Honda", constructor: Object}
console.log(myCar.name) // Honda
console.log(myCar instanceof Car) // true
console.log(myCar.constructor) // function Car() {}
console.log(myCar.constructor === Car) // true
console.log(typeof myCar) // object
Object.create
You can also use Object.create to create a new object
But, it does not execute the constructor function
Object.create is used to create an object from another object
const Car = {
name: "Honda"
}
var myCar = Object.create(Car)
console.log(myCar) // Object {}
console.log(myCar.name) // Honda
console.log(myCar instanceof Car) // ERROR
console.log(myCar.constructor) // Anonymous function object
console.log(myCar.constructor === Car) // false
console.log(typeof myCar) // object
I prefer a closure approach.
I still use new.
I don't use Object.create.
I don't use this.
I still use new as I like the declarative nature of it.
Consider this for simple inheritance.
window.Quad = (function() {
function Quad() {
const wheels = 4;
const drivingWheels = 2;
let motorSize = 0;
function setMotorSize(_) {
motorSize = _;
}
function getMotorSize() {
return motorSize;
}
function getWheelCount() {
return wheels;
}
function getDrivingWheelCount() {
return drivingWheels;
}
return Object.freeze({
getWheelCount,
getDrivingWheelCount,
getMotorSize,
setMotorSize
});
}
return Object.freeze(Quad);
})();
window.Car4wd = (function() {
function Car4wd() {
const quad = new Quad();
const spareWheels = 1;
const extraDrivingWheels = 2;
function getSpareWheelCount() {
return spareWheels;
}
function getDrivingWheelCount() {
return quad.getDrivingWheelCount() + extraDrivingWheels;
}
return Object.freeze(Object.assign({}, quad, {
getSpareWheelCount,
getDrivingWheelCount
}));
}
return Object.freeze(Car4wd);
})();
let myQuad = new Quad();
let myCar = new Car4wd();
console.log(myQuad.getWheelCount()); // 4
console.log(myQuad.getDrivingWheelCount()); // 2
console.log(myCar.getWheelCount()); // 4
console.log(myCar.getDrivingWheelCount()); // 4 - The overridden method is called
console.log(myCar.getSpareWheelCount()); // 1
Feedback encouraged.
Alright, just looking at the question, it seems like it would matter, but lets look at a test I did.
So I created 3 constructor functions. I did the usual Car, then a Mazda, then a MX8 constructor. All of them inherit based on the code in the JSFiddle. Here is a shell of what I did, more detail can be found in the JSFiddle.
By the way I'm a big fan of Object.create which doesn't need any babysitting of anything.
var Car = function () {
//code here
}
var Mazda = function (type) {
this.type = type;
//more code here including a simple method test
}
var MX8 = function () {
//more code here
}
Mazda.prototype = new Car();
MX8.prototype = new Mazda("MX8");
var z = new MX8();
//refer to my JSFiddle
z.interior; // got correct interior
z.type; // got correct type ie model
z.warrantyInfo(); // method was correctly inherited
z.speed; // got correct speed
z.constructor === MX8; // false (unless I specify the constructor in the code of which is commented out in my JSFiddle)
short answer:
You need to explicitly set the constructor.
function Base() {}
function Derived() {}
// old prototype object is gone, including constructor property
// it will get however all the properties attached by Base constructor
Derived.prototype = new Base();
Derived.prototype.constructor = Derived;
Does it matter if my Object returns the wrong constructor?
Well, it depends on whether you're using that property or not. I'd say it's a good practice to keep the correct constructor.
Possible use case:
function getType (target) {
// name is empty unless you use the function declaration syntax
// (read-only property)
return target.constructor.name;
}
getType(new Derived()) // "Derived"
getType(new Base()) // "Base"
side note:
There are better ways of implementing inheritance in JS.
My favorite pattern is the following:
function Base (x) { this.x = x; }
function Derived (x, y) { Base.call(this, x); this.y = y; }
// creates an empty object whose internal "[[Prototype]]" property
// points to Base.prototype
Derived.prototype = Object.create(Base.prototype);
Derived.prototype.constructor = Derived;
The ideea behind Object.create is based on:
function create (proto) {
function f () {}
f.prototype = proto;
return new f();
}
The actual function Object.create is better because you can pass null as prototype, which doesn't work using the above code.
Anyway, you should watch this excellent playlist: Crockford on JavaScript.
I found different ways that seem to work.
Mostly recommended way in textbooks and the internet:
var Person = function() {
this.age = 23;
}
Tony = new Person();
This also seems to work:
function Person() {
this.age = 23;
}
Tony = new Person();
Is there a difference? And an additional question: Usually you cannot simply leave out parentheses. Here it is possible (new Person instead of new Person()). This is because of using the new keyword, right?
A third odd way that I just tried out looks like this:
function Person() {
return {age: 2};
}
Tony = new Person();
Tony = Person(); // both ways work! It seems that you can leave out 'new' here.
Here I don't get an object with the name of my class, but my property is also accessible and it seems like it was quite similar to both above approaches.
What shall I use and what are the technical differences? Thank you!
JavaScript is a classless language. Classes don't exist, but objects may inherit properties from each other by using prototypes. This means you are not limited to implementing inheritance in a class-like manner. Personally, I like to use a BackboneJS-inspired method (code requires UnderscoreJS):
var BaseObject = function(){}; //Create a function so that we may use the new operator.
//There may be code in the constructor
BaseObject.extend = function(obj) { //Add a static function to the BaseObject to extend it
var base = this; //Save a reference for later
//Create the constructor for the sub object. We need to extend it, so we can't use the base constructor. AFAIK, this is the only way to clone the base constructor, i.e. by creating a new function that calls it
var SubObject = _.extend(function(){
base.apply(this, arguments); //Call base constructor
}, this);
SubObject.prototype= _.extend({}, this.prototype, obj); //Create new prototype that extends the super prototype, but does not overwrite it.
return SubObject; //Return the new constructor + prototype
};
This allows you to do cool class-like stuff like this:
var Car = BaseObject.extend({
speed: 0,
acceleration: 5,
accelerate: function(){
this.speed += this.acceleration;
}
});
var RaceCar = Car.extend({
acceleration: 10,
});
var car = new Car();
var raceCar = new RaceCar();
car.accelerate();
raceCar.accelerate();
if(raceCar.speed > car.speed){
console.log('raceCar won');
}else{
console.log('car won');
}
For more information on inheritance in JavaScript, I strongly recommend reading JavaScript: The Good Parts by Douglas Crockford.
Regarding your examples:
The difference between 1 and 2 is minimal. For more information see this question.
In 3, you are just returning an object literal. The new keyword only has influence on the this keyword within the function, which you are not using, and so using new has no effect. For more information, see this quesion
1 and 2 (var x = function vs function x) are very similar. 3 is a completely different thing.
The difference between 1 and 2 has nothing to do with classes and has been asked before on SO (several times). I think the most complete answer is this one:
https://stackoverflow.com/a/338053/1669279
In short, the first x points to an anonymous function and some debugging tools might have problems with that. The first one is available from the line it was defined on while the second is available in the entire scope. Read the linked answer for details.
The 3rd "solution" is not a class at all. It is simply a function that returns an object (might be called a Factory method).
It is not a good idea to return things from your constructors, especially return this. You should only return things if you want to override the normal process of creating objects (like implementing the singleton pattern for example).
As a side-note, you should always use new when you instantiate a class. Here is what happens when you try to be smart and save characters:
function X () {
return this;
}
console.log(X()); // outputs the window object
The parenthesis after calling the constructor with no parameters are optional, but it is frowned upon to avoid them because it results in slightly more confusing code.
To sum it up, i usually use pattern 1. Pattern 2 is also ok.
One problem with pattern 2 can be this one:
var x = new X(); // works
console.log(x.message); // works, I am X
x.method(); // doesn't work, method hasn't been defined yet
function X() {
this.message = 'I am X';
}
X.prototype.method = function() {
console.log(this.message);
};
this is how i do mine:
;(function (window) {
"use strict";
//-- Private Vars
var opt, obj, rm, Debug;
//-- construtor
function App(requestedMethod) {
//-- set up some vars
if(typeof requestedMethod !== 'undefined') {
rm = requestedMethod;
}
opt = {
rMethod: (typeof rm !== 'undefined') ? (rm != null) ? rm : false : false
}
//-- containe resulable objects
obj = {}
//-- call the init method
this.init();
}
/** Public Methods **/
/**
* The Init method called on every page load
*/
App.prototype.init = function () {
var om = opt.rMethod;
//-- Once all init settings are performed call the requested method if required
if(om) {(typeof App.prototype[om] == 'function') ? App.prototype[om]() : _DB('Call to Method [' + om + '] does not exsist.');}
};
/**
* testmethod
*/
App.prototype.testmethod = function () {
};
/** Private Methods **/
function PrivateMethod(){
}
/**
* A console output that should enable to remain enable js to work in IE
* just incase i forget to remove it when checking on those pesky IE browsers....
*/
function _DB(msg){
if(window.console && window.console.log){
var logDate = new Date();
window.console.log('------------------- ' + logDate + ' ----------------------------------');
window.console.log(msg);
}
};
window.App = App;
})(window);
then call it like:
<script src="ptha/to/your/app.js"></script>
<script>$(function() { new App('testmethod'); });</script>
When the code is loaded the new App() will then run once all page load data has completed
Hope this helps.To get access to it outside add the new to a var
var App = new App('testmethod);
then you can access things like
App.testmethod()...
var Person = function() {
this.age = 23;
}
Person is a variable that contains(is referenced) an anonymous function
function Person() {
this.age = 23;
}
but here you declare a function called "Person"
function Person() {
return {age: 2};
}
and so you declare a function that returns a new static object.
The best way depends on the needs, if you want to declare classes use the second, while to create the modules uses the third. For the first method look here: http://helephant.com/2008/08/23/javascript-anonymous-functions/
In John Resig's post on Simple "Class" Instantiation in Javascript, he states:
"...if you have a frequently-accessed function (returning an object) that you want people to interact with, then it's to your advantage to have the object properties be in the prototype chain and instantiate it. Here it is, in code:"
// Very fast
function User(){}
User.prototype = { /* Lots of properties ... */ };
// Very slow
function User(){
return { /* Lots of properties */ };
}
I would like to apply this to a function like the following (which happens to live inside a "class" declaration)
//...
this.get_subscription = function(args)
{
return {
property_one: "value",
property_two: {
sub_prop: args.something
}
}
}
but have no idea where to put the args. If I do
this.get_subscription = function(args) {}
this.get_subscription.prototype = {
property_one: "value"
//...
}
it'll say args are undefined. I've tried several variants, none of which work. How should I properly do this, in a way that doesn't put args in the parent class's scope?
You seem to misunderstand the point of prototype. Prototype holds methods and fields that should be common to all instances of the object. Therefore, if property_two is based upon args passed to the constructor, then it does not belong to the prototype!
After all, in this code
this.get_subscription = function(args) {}
this.get_subscription.prototype = {
property_one: "value"
//...
}
first, the function get_subscription is created, then its prototype is set to the object literal. There is no args until you call the constructor, so it does not make sense to do something with the args in the prototype.
So, your usual javascript object should look like this example - a 2D point class.
function Point(x, y) {
/* In constructor:
* initialize fields that are related to the constructor parameters
* and are different for different instances
*/
this.x = x;
this.y = y;
}
// in prototype, define fields that are common to all instances
Point.prototype.RED = '#FF0000';
/* in prototype, also define the methods; as they can use the "this" keyword,
* they can reference their own instance, so you don't have to include them
* in each instance manually
*/
Point.prototype.toString = function() { return '[' + this.x + ', ' + this.y + ']'; };
Yes, that's the drawback of the "everything-to-prototype"-idea: You don't have access to the constructor arguments - they are only available inside the function body (exept you put them into a public property there). Everything that needs to use them does not belong to the prototype; you only will fill "default properties" on the prototype (usually methods that are common to all instances).
I think that you don't need that anyway in here. The pattern is only useful for real constructor functions, which you don't seem to have here (I then would expect this.Subscription). Also, the promised "performance boost" is negligible.
If you really would want to use it in here, it would be:
function Class() {
this.Subscription = function(args) {
this.propertyTwo = {
subprop: args.something
};
};
this.Subscription.prototype = {
property_one: "value",
property_two: {}
};
}
// a method callable without the "new" operator:
Class.prototype.get_subscription = function(args) {
return new this.Subscription(args);
}
usage:
var object = new Class();
var sub = new object.Subscription();
var sub2 = new object.Subscription();
// the promised boost is less memory usage for:
sub.propertyOne === object.Subscription.prototype.propertyOne === sub2.propertyOne;
Also care for Crockford's Prototypal inheritance - Issues with nested objects when you attempt to set/change/delete properties on the propertyTwo-Object(s).
I wonder about what the best way is to create an JavaScript object that has properties and methods.
I have seen examples where the person used var self = this and then uses self. in all functions to make sure the scope is always correct.
Then I have seen examples of using .prototype to add properties, while others do it inline.
Can someone give me a proper example of a JavaScript object with some properties and methods?
There are two models for implementing classes and instances in JavaScript: the prototyping way, and the closure way. Both have advantages and drawbacks, and there are plenty of extended variations. Many programmers and libraries have different approaches and class-handling utility functions to paper over some of the uglier parts of the language.
The result is that in mixed company you will have a mishmash of metaclasses, all behaving slightly differently. What's worse, most JavaScript tutorial material is terrible and serves up some kind of in-between compromise to cover all bases, leaving you very confused. (Probably the author is also confused. JavaScript's object model is very different to most programming languages, and in many places straight-up badly designed.)
Let's start with the prototype way. This is the most JavaScript-native you can get: there is a minimum of overhead code and instanceof will work with instances of this kind of object.
function Shape(x, y) {
this.x= x;
this.y= y;
}
We can add methods to the instance created by new Shape by writing them to the prototype lookup of this constructor function:
Shape.prototype.toString= function() {
return 'Shape at '+this.x+', '+this.y;
};
Now to subclass it, in as much as you can call what JavaScript does subclassing. We do that by completely replacing that weird magic prototype property:
function Circle(x, y, r) {
Shape.call(this, x, y); // invoke the base class's constructor function to take co-ords
this.r= r;
}
Circle.prototype= new Shape();
before adding methods to it:
Circle.prototype.toString= function() {
return 'Circular '+Shape.prototype.toString.call(this)+' with radius '+this.r;
}
This example will work and you will see code like it in many tutorials. But man, that new Shape() is ugly: we're instantiating the base class even though no actual Shape is to be created. It happens to work in this simple case because JavaScript is so sloppy: it allows zero arguments to be passed in, in which case x and y become undefined and are assigned to the prototype's this.x and this.y. If the constructor function were doing anything more complicated, it would fall flat on its face.
So what we need to do is find a way to create a prototype object which contains the methods and other members we want at a class level, without calling the base class's constructor function. To do this we are going to have to start writing helper code. This is the simplest approach I know of:
function subclassOf(base) {
_subclassOf.prototype= base.prototype;
return new _subclassOf();
}
function _subclassOf() {};
This transfers the base class's members in its prototype to a new constructor function which does nothing, then uses that constructor. Now we can write simply:
function Circle(x, y, r) {
Shape.call(this, x, y);
this.r= r;
}
Circle.prototype= subclassOf(Shape);
instead of the new Shape() wrongness. We now have an acceptable set of primitives to built classes.
There are a few refinements and extensions we can consider under this model. For example here is a syntactical-sugar version:
Function.prototype.subclass= function(base) {
var c= Function.prototype.subclass.nonconstructor;
c.prototype= base.prototype;
this.prototype= new c();
};
Function.prototype.subclass.nonconstructor= function() {};
...
function Circle(x, y, r) {
Shape.call(this, x, y);
this.r= r;
}
Circle.subclass(Shape);
Either version has the drawback that the constructor function cannot be inherited, as it is in many languages. So even if your subclass adds nothing to the construction process, it must remember to call the base constructor with whatever arguments the base wanted. This can be slightly automated using apply, but still you have to write out:
function Point() {
Shape.apply(this, arguments);
}
Point.subclass(Shape);
So a common extension is to break out the initialisation stuff into its own function rather than the constructor itself. This function can then inherit from the base just fine:
function Shape() { this._init.apply(this, arguments); }
Shape.prototype._init= function(x, y) {
this.x= x;
this.y= y;
};
function Point() { this._init.apply(this, arguments); }
Point.subclass(Shape);
// no need to write new initialiser for Point!
Now we've just got the same constructor function boilerplate for each class. Maybe we can move that out into its own helper function so we don't have to keep typing it, for example instead of Function.prototype.subclass, turning it round and letting the base class's Function spit out subclasses:
Function.prototype.makeSubclass= function() {
function Class() {
if ('_init' in this)
this._init.apply(this, arguments);
}
Function.prototype.makeSubclass.nonconstructor.prototype= this.prototype;
Class.prototype= new Function.prototype.makeSubclass.nonconstructor();
return Class;
};
Function.prototype.makeSubclass.nonconstructor= function() {};
...
Shape= Object.makeSubclass();
Shape.prototype._init= function(x, y) {
this.x= x;
this.y= y;
};
Point= Shape.makeSubclass();
Circle= Shape.makeSubclass();
Circle.prototype._init= function(x, y, r) {
Shape.prototype._init.call(this, x, y);
this.r= r;
};
...which is starting to look a bit more like other languages, albeit with slightly clumsier syntax. You can sprinkle in a few extra features if you like. Maybe you want makeSubclass to take and remember a class name and provide a default toString using it. Maybe you want to make the constructor detect when it has accidentally been called without the new operator (which would otherwise often result in very annoying debugging):
Function.prototype.makeSubclass= function() {
function Class() {
if (!(this instanceof Class))
throw('Constructor called without "new"');
...
Maybe you want to pass in all the new members and have makeSubclass add them to the prototype, to save you having to write Class.prototype... quite so much. A lot of class systems do that, eg:
Circle= Shape.makeSubclass({
_init: function(x, y, z) {
Shape.prototype._init.call(this, x, y);
this.r= r;
},
...
});
There are a lot of potential features you might consider desirable in an object system and no-one really agrees on one particular formula.
The closure way, then. This avoids the problems of JavaScript's prototype-based inheritance, by not using inheritance at all. Instead:
function Shape(x, y) {
var that= this;
this.x= x;
this.y= y;
this.toString= function() {
return 'Shape at '+that.x+', '+that.y;
};
}
function Circle(x, y, r) {
var that= this;
Shape.call(this, x, y);
this.r= r;
var _baseToString= this.toString;
this.toString= function() {
return 'Circular '+_baseToString(that)+' with radius '+that.r;
};
};
var mycircle= new Circle();
Now every single instance of Shape will have its own copy of the toString method (and any other methods or other class members we add).
The bad thing about every instance having its own copy of each class member is that it's less efficient. If you are dealing with large numbers of subclassed instances, prototypical inheritance may serve you better. Also calling a method of the base class is slightly annoying as you can see: we have to remember what the method was before the subclass constructor overwrote it, or it gets lost.
[Also because there is no inheritance here, the instanceof operator won't work; you would have to provide your own mechanism for class-sniffing if you need it. Whilst you could fiddle the prototype objects in a similar way as with prototype inheritance, it's a bit tricky and not really worth it just to get instanceof working.]
The good thing about every instance having its own method is that the method may then be bound to the specific instance that owns it. This is useful because of JavaScript's weird way of binding this in method calls, which has the upshot that if you detach a method from its owner:
var ts= mycircle.toString;
alert(ts());
then this inside the method won't be the Circle instance as expected (it'll actually be the global window object, causing widespread debugging woe). In reality this typically happens when a method is taken and assigned to a setTimeout, onclick or EventListener in general.
With the prototype way, you have to include a closure for every such assignment:
setTimeout(function() {
mycircle.move(1, 1);
}, 1000);
or, in the future (or now if you hack Function.prototype) you can also do it with function.bind():
setTimeout(mycircle.move.bind(mycircle, 1, 1), 1000);
if your instances are done the closure way, the binding is done for free by the closure over the instance variable (usually called that or self, though personally I would advise against the latter as self already has another, different meaning in JavaScript). You don't get the arguments 1, 1 in the above snippet for free though, so you would still need another closure or a bind() if you need to do that.
There are lots of variants on the closure method too. You may prefer to omit this completely, creating a new that and returning it instead of using the new operator:
function Shape(x, y) {
var that= {};
that.x= x;
that.y= y;
that.toString= function() {
return 'Shape at '+that.x+', '+that.y;
};
return that;
}
function Circle(x, y, r) {
var that= Shape(x, y);
that.r= r;
var _baseToString= that.toString;
that.toString= function() {
return 'Circular '+_baseToString(that)+' with radius '+r;
};
return that;
};
var mycircle= Circle(); // you can include `new` if you want but it won't do anything
Which way is “proper”? Both. Which is “best”? That depends on your situation. FWIW I tend towards prototyping for real JavaScript inheritance when I'm doing strongly OO stuff, and closures for simple throwaway page effects.
But both ways are quite counter-intuitive to most programmers. Both have many potential messy variations. You will meet both (as well as many in-between and generally broken schemes) if you use other people's code/libraries. There is no one generally-accepted answer. Welcome to the wonderful world of JavaScript objects.
[This has been part 94 of Why JavaScript Is Not My Favourite Programming Language.]
I use this pattern fairly frequently - I've found that it gives me a pretty huge amount of flexibility when I need it. In use it's rather similar to Java-style classes.
var Foo = function()
{
var privateStaticMethod = function() {};
var privateStaticVariable = "foo";
var constructor = function Foo(foo, bar)
{
var privateMethod = function() {};
this.publicMethod = function() {};
};
constructor.publicStaticMethod = function() {};
return constructor;
}();
This uses an anonymous function that is called upon creation, returning a new constructor function. Because the anonymous function is called only once, you can create private static variables in it (they're inside the closure, visible to the other members of the class). The constructor function is basically a standard Javascript object - you define private attributes inside of it, and public attributes are attached to the this variable.
Basically, this approach combines the Crockfordian approach with standard Javascript objects to create a more powerful class.
You can use it just like you would any other Javascript object:
Foo.publicStaticMethod(); //calling a static method
var test = new Foo(); //instantiation
test.publicMethod(); //calling a method
Douglas Crockford discusses that topic extensively in The Good Parts. He recommends to avoid the new operator to create new objects. Instead he proposes to create customized constructors. For instance:
var mammal = function (spec) {
var that = {};
that.get_name = function ( ) {
return spec.name;
};
that.says = function ( ) {
return spec.saying || '';
};
return that;
};
var myMammal = mammal({name: 'Herb'});
In Javascript a function is an object, and can be used to construct objects out of together with the new operator. By convention, functions intended to be used as constructors start with a capital letter. You often see things like:
function Person() {
this.name = "John";
return this;
}
var person = new Person();
alert("name: " + person.name);**
In case you forget to use the new operator while instantiating a new object, what you get is an ordinary function call, and this is bound to the global object instead to the new object.
To continue off of bobince's answer
In es6 you can now actually create a class
So now you can do:
class Shape {
constructor(x, y) {
this.x = x;
this.y = y;
}
toString() {
return `Shape at ${this.x}, ${this.y}`;
}
}
So extend to a circle (as in the other answer) you can do:
class Circle extends Shape {
constructor(x, y, r) {
super(x, y);
this.r = r;
}
toString() {
let shapeString = super.toString();
return `Circular ${shapeString} with radius ${this.r}`;
}
}
Ends up a bit cleaner in es6 and a little easier to read.
Here is a good example of it in action:
class Shape {
constructor(x, y) {
this.x = x;
this.y = y;
}
toString() {
return `Shape at ${this.x}, ${this.y}`;
}
}
class Circle extends Shape {
constructor(x, y, r) {
super(x, y);
this.r = r;
}
toString() {
let shapeString = super.toString();
return `Circular ${shapeString} with radius ${this.r}`;
}
}
let c = new Circle(1, 2, 4);
console.log('' + c, c);
You can also do it this way, using structures :
function createCounter () {
var count = 0;
return {
increaseBy: function(nb) {
count += nb;
},
reset: function {
count = 0;
}
}
}
Then :
var counter1 = createCounter();
counter1.increaseBy(4);
Another way would be http://jsfiddle.net/nnUY4/
(i dont know if this kind of handling object creation and revealing functions follow any specific pattern)
// Build-Reveal
var person={
create:function(_name){ // 'constructor'
// prevents direct instantiation
// but no inheritance
return (function() {
var name=_name||"defaultname"; // private variable
// [some private functions]
function getName(){
return name;
}
function setName(_name){
name=_name;
}
return { // revealed functions
getName:getName,
setName:setName
}
})();
}
}
// … no (instantiated) person so far …
var p=person.create(); // name will be set to 'defaultname'
p.setName("adam"); // and overwritten
var p2=person.create("eva"); // or provide 'constructor parameters'
alert(p.getName()+":"+p2.getName()); // alerts "adam:eva"
Creating an object
The easiest way to create an object in JavaScript is to use the following syntax :
var test = {
a : 5,
b : 10,
f : function(c) {
return this.a + this.b + c;
}
}
console.log(test);
console.log(test.f(3));
This works great for storing data in a structured way.
For more complex use cases, however, it's often better to create instances of functions :
function Test(a, b) {
this.a = a;
this.b = b;
this.f = function(c) {
return this.a + this.b + c;
};
}
var test = new Test(5, 10);
console.log(test);
console.log(test.f(3));
This allows you to create multiple objects that share the same "blueprint", similar to how you use classes in eg. Java.
This can still be done more efficiently, however, by using a prototype.
Whenever different instances of a function share the same methods or properties, you can move them to that object's prototype. That way, every instance of a function has access to that method or property, but it doesn't need to be duplicated for every instance.
In our case, it makes sense to move the method f to the prototype :
function Test(a, b) {
this.a = a;
this.b = b;
}
Test.prototype.f = function(c) {
return this.a + this.b + c;
};
var test = new Test(5, 10);
console.log(test);
console.log(test.f(3));
Inheritance
A simple but effective way to do inheritance in JavaScript, is to use the following two-liner :
B.prototype = Object.create(A.prototype);
B.prototype.constructor = B;
That is similar to doing this :
B.prototype = new A();
The main difference between both is that the constructor of A is not run when using Object.create, which is more intuitive and more similar to class based inheritance.
You can always choose to optionally run the constructor of A when creating a new instance of B by adding adding it to the constructor of B :
function B(arg1, arg2) {
A(arg1, arg2); // This is optional
}
If you want to pass all arguments of B to A, you can also use Function.prototype.apply() :
function B() {
A.apply(this, arguments); // This is optional
}
If you want to mixin another object into the constructor chain of B, you can combine Object.create with Object.assign :
B.prototype = Object.assign(Object.create(A.prototype), mixin.prototype);
B.prototype.constructor = B;
Demo
function A(name) {
this.name = name;
}
A.prototype = Object.create(Object.prototype);
A.prototype.constructor = A;
function B() {
A.apply(this, arguments);
this.street = "Downing Street 10";
}
B.prototype = Object.create(A.prototype);
B.prototype.constructor = B;
function mixin() {
}
mixin.prototype = Object.create(Object.prototype);
mixin.prototype.constructor = mixin;
mixin.prototype.getProperties = function() {
return {
name: this.name,
address: this.street,
year: this.year
};
};
function C() {
B.apply(this, arguments);
this.year = "2018"
}
C.prototype = Object.assign(Object.create(B.prototype), mixin.prototype);
C.prototype.constructor = C;
var instance = new C("Frank");
console.log(instance);
console.log(instance.getProperties());
Note
Object.create can be safely used in every modern browser, including IE9+. Object.assign does not work in any version of IE nor some mobile browsers. It is recommended to polyfill Object.create and/or Object.assign if you want to use them and support browsers that do not implement them.
You can find a polyfill for Object.create here
and one for Object.assign here.
When one uses the trick of closing on "this" during a constructor invocation, it's in order to write a function that can be used as a callback by some other object that doesn't want to invoke a method on an object. It's not related to "making the scope correct".
Here's a vanilla JavaScript object:
function MyThing(aParam) {
var myPrivateVariable = "squizzitch";
this.someProperty = aParam;
this.useMeAsACallback = function() {
console.log("Look, I have access to " + myPrivateVariable + "!");
}
}
// Every MyThing will get this method for free:
MyThing.prototype.someMethod = function() {
console.log(this.someProperty);
};
You might get a lot out of reading what Douglas Crockford has to say about JavaScript. John Resig is also brilliant. Good luck!
Closure is versatile. bobince has well summarized the prototype vs. closure approaches when creating objects. However you can mimic some aspects of OOP using closure in a functional programming way. Remember functions are objects in JavaScript; so use function as object in a different way.
Here is an example of closure:
function outer(outerArg) {
return inner(innerArg) {
return innerArg + outerArg; //the scope chain is composed of innerArg and outerArg from the outer context
}
}
A while ago I came across the Mozilla's article on Closure. Here is what jump at my eyes: "A closure lets you associate some data (the environment) with a function that operates on that data. This has obvious parallels to object oriented programming, where objects allow us to associate some data (the object's properties) with one or more methods". It was the very first time I read a parallelism between closure and classic OOP with no reference to prototype.
How?
Suppose you want to calculate the VAT of some items. The VAT is likely to stay stable during the lifetime of an application. One way to do it in OOP (pseudo code):
public class Calculator {
public property VAT { get; private set; }
public Calculator(int vat) {
this.VAT = vat;
}
public int Calculate(int price) {
return price * this.VAT;
}
}
Basically you pass a VAT value into your constructor and your calculate method can operate upon it via closure.
Now instead of using a class/constructor, pass your VAT as an argument into a function. Because the only stuff you are interested in is the calculation itself, returns a new function, which is the calculate method:
function calculator(vat) {
return function(item) {
return item * vat;
}
}
var calculate = calculator(1.10);
var jsBook = 100; //100$
calculate(jsBook); //110
In your project identify top-level values that are good candidate of what VAT is for calculation. As a rule of thumb whenever you pass the same arguments on and on, there is a way to improve it using closure. No need to create traditional objects.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Closures
A Pattern That Serves Me Well
var Klass = function Klass() {
var thus = this;
var somePublicVariable = x
, somePublicVariable2 = x
;
var somePrivateVariable = x
, somePrivateVariable2 = x
;
var privateMethod = (function p() {...}).bind(this);
function publicMethod() {...}
// export precepts
this.var1 = somePublicVariable;
this.method = publicMethod;
return this;
};
First, you may change your preference of adding methods to the instance instead of the constructor's prototype object. I almost always declare methods inside of the constructor because I use Constructor Hijacking very often for purposes regarding Inheritance & Decorators.
Here's how I decide where which declarations are writ:
Never declare a method directly on the context object (this)
Let var declarations take precedence over function declarations
Let primitives take precedence over objects ({} and [])
Let public declarations take precedence over private declarations
Prefer Function.prototype.bind over thus, self, vm, etc
Avoid declaring a Class within another Class, unless:
It should be obvious that the two are inseparable
The Inner class implements The Command Pattern
The Inner class implements The Singleton Pattern
The Inner class implements The State Pattern
The Inner Class implements another Design Pattern that warrants this
Always return this from within the Lexical Scope of the Closure Space.
Here's why these help:
Constructor Hijacking
var Super = function Super() {
...
this.inherited = true;
...
};
var Klass = function Klass() {
...
// export precepts
Super.apply(this); // extends this with property `inherited`
...
};
Model Design
var Model = function Model(options) {
var options = options || {};
this.id = options.id || this.id || -1;
this.string = options.string || this.string || "";
// ...
return this;
};
var model = new Model({...});
var updated = Model.call(model, { string: 'modified' });
(model === updated === true); // > true
Design Patterns
var Singleton = new (function Singleton() {
var INSTANCE = null;
return function Klass() {
...
// export precepts
...
if (!INSTANCE) INSTANCE = this;
return INSTANCE;
};
})();
var a = new Singleton();
var b = new Singleton();
(a === b === true); // > true
As you can see, I really have no need for thus since I prefer Function.prototype.bind (or .call or .apply) over thus. In our Singleton class, we don't even name it thus because INSTANCE conveys more information. For Model, we return this so that we can invoke the Constructor using .call to return the instance we passed into it. Redundantly, we assigned it to the variable updated, though it is useful in other scenarios.
Alongside, I prefer constructing object-literals using the new keyword over {brackets}:
Preferred
var klass = new (function Klass(Base) {
...
// export precepts
Base.apply(this); //
this.override = x;
...
})(Super);
Not Preferred
var klass = Super.apply({
override: x
});
As you can see, the latter has no ability to override its Superclass's "override" property.
If I do add methods to the Class's prototype object, I prefer an object literal -- with or without using the new keyword:
Preferred
Klass.prototype = new Super();
// OR
Klass.prototype = new (function Base() {
...
// export precepts
Base.apply(this);
...
})(Super);
// OR
Klass.prototype = Super.apply({...});
// OR
Klass.prototype = {
method: function m() {...}
};
Not Preferred
Klass.prototype.method = function m() {...};
I'd like to mention that we can use either a Title or a String to declare an Object.
There are different ways on calling each type of them. See below:
var test = {
useTitle : "Here we use 'a Title' to declare an Object",
'useString': "Here we use 'a String' to declare an Object",
onTitle : function() {
return this.useTitle;
},
onString : function(type) {
return this[type];
}
}
console.log(test.onTitle());
console.log(test.onString('useString'));
var Person = function (lastname, age, job){
this.name = name;
this.age = age;
this.job = job;
this.changeName = function(name){
this.lastname = name;
}
}
var myWorker = new Person('Adeola', 23, 'Web Developer');
myWorker.changeName('Timmy');
console.log("New Worker" + myWorker.lastname);
In addition to the accepted answer from 2009. If you can can target modern browsers, one can make use of the Object.defineProperty.
The Object.defineProperty() method defines a new property directly on
an object, or modifies an existing property on an object, and returns
the object.
Source: Mozilla
var Foo = (function () {
function Foo() {
this._bar = false;
}
Object.defineProperty(Foo.prototype, "bar", {
get: function () {
return this._bar;
},
set: function (theBar) {
this._bar = theBar;
},
enumerable: true,
configurable: true
});
Foo.prototype.toTest = function () {
alert("my value is " + this.bar);
};
return Foo;
}());
// test instance
var test = new Foo();
test.bar = true;
test.toTest();
To see a desktop and mobile compatibility list, see Mozilla's Browser Compatibility list. Yes, IE9+ supports it as well as Safari mobile.
You can also try this
function Person(obj) {
'use strict';
if (typeof obj === "undefined") {
this.name = "Bob";
this.age = 32;
this.company = "Facebook";
} else {
this.name = obj.name;
this.age = obj.age;
this.company = obj.company;
}
}
Person.prototype.print = function () {
'use strict';
console.log("Name: " + this.name + " Age : " + this.age + " Company : " + this.company);
};
var p1 = new Person({name: "Alex", age: 23, company: "Google"});
p1.print();
Bascially there is no concept of class in JS so we use function as a class constructor which is relevant with the existing design patterns.
//Constructor Pattern
function Person(name, age, job){
this.name = name;
this.age = age;
this.job = job;
this.doSomething = function(){
alert('I am Happy');
}
}
Till now JS has no clue that you want to create an object so here comes the new keyword.
var person1 = new Person('Arv', 30, 'Software');
person1.name //Arv
Ref : Professional JS for web developers - Nik Z