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Javascript Do object literals have prototype property? [duplicate]

This question already has answers here:
__proto__ VS. prototype in JavaScript
(34 answers)
Closed 2 years ago.
When understanding about the 3 broad type of objects in Javascript i.e. Literal, Function objects and Objects from functions,
I had read somewhere that literal objects do not have prototype property like function objects (or constructors).
Would that be a true statement ?
Reason I am asking this is since we can do;
var person = {
fn: 'ABC'
}
person.__proto__ = someObj;
Not sure if this setting using "proto" is really allowed across all browsers, which is why the statement is not true ?
Also is the same statement true for "objects from functions" ? i.e. when we do "new SomeFunctionObject()"

I had read somewhere that literal objects do not have prototype property like function objects (or constructors).
This sounds like it's referring to the property named .prototype, which is true. A function and a class will automatically receive a .prototype property, which contains one property (constructor, pointing to the class/function itself) and inheriting from Object.prototype.
The statement in your question is true. Only callable class-like objects - classes and functions - automatically receive these sorts of properties. Other objects do not:
class Foo {
}
function Foo2() {
}
const obj = {};
console.log(
Foo.prototype,
Foo2.prototype,
obj.prototype
);
Regarding the code in your question, using __proto__ is permitted, but it's deprecated. As MDN says:
Warning: While Object.prototype.__proto__ is supported today in most browsers, its existence and exact behavior has only been standardized in the ECMAScript 2015 specification as a legacy feature to ensure compatibility for web browsers. For better support, it is recommended that Object.getPrototypeOf() be used instead.
Object.getPrototypeOf should be preferred nowdays.
i.e. when we do "new SomeFunctionObject()"
When you create an instance with new, the internal prototype of the new instance will (usually) be same object as the .prototype property of the constructor. That is, with the following code:
class Foo {
// anything
}
const f = new Foo();
the internal prototype of the f instance will be the same object as Foo.prototype.
class Foo {
// anything
}
const f = new Foo();
console.log(
Object.getPrototypeOf(f) === Foo.prototype,
f.__proto__ === Foo.prototype,
);
The only time where the internal prototype of an instance will not be the same as the constructor's .prototype would be when the constructor explicitly returns an object, which is somewhat unusual.
class Foo {
constructor() {
return {};
}
}
const f = new Foo();
console.log(
Object.getPrototypeOf(f) === Foo.prototype,
f.__proto__ === Foo.prototype,
);

functions have a prototype property, which inherits from Object.
function Person(last, first, middle){
// constructors allow private variables that can be shared across methods without the use of `this.` - API developers may accidentally access properties meant to be private in classes, so use constructors
const proto = Person.prototype;
if(last !== undefined){
this.last = last; this.first = first; this.middle = middle;
}
this.getFullName = (lastFirst = false)=>{
const m = this.middle ? ' '+this.middle : ''
if(lastFirst){
return this.last+', '+this.first+m;
}
return this.first+m+' '+this.last;
}
this.setFullName = (last, first, middle)=>{
proto.last = this.last = last; proto.first = this.first = first;
proto.middle = this.middle = middle;
return this;
}
}
console.log(typeof Person);
const bob = new Person('Smith', 'Bob', 'Gene');
console.log(bob.getFullName());
const james = new Person('Brown', 'James');
console.log(james.getFullName(true));
bob.setFullName('Static', 'Method', 'like');
const person = new Person;
console.log(person.getFullName()+' -- sort of');
console.log(typeof person);
Note that a new instance of a constructor returns a new Object literal.

You can set prototype using this:
Object.setPrototypeOf(obj, prototype)
You can also read docs here.

If you explicitly assign a value to an object's proto, of course now this object has prototype property. When you just assign a value to an object, currently it has no prototype.This property has some problems on compatibility across browsers.
An object built by new Con() certainly has prototype property natively.

Method Inheritance in JavaScript

JavaScript uses a Prototype system, which is fundamentally different than a Class system. This is my first serious encounter with the language. I had fooled around with it previously, but this is the first time I built a system with proper OO, inheritance, polymorphism, etc.
From what I read there seems to be a few common methods to do member function inheritance in Javascript. Assuming you have a parent foo as following
foo = function(){ this.one = 1; }
foo.prototype.func1 = function(){return this.one;}
The MDN Introduction to JavaScript Inheritance suggests the naive approach of invoking the parent's method in the context of the child, as shown below.
bar = function(){ foo.call(this); }
bar.prototype = Object.create(foo.prototype);
bar.prototype.func1 = function(){ return this.one + foo.prototype.func1();}
This has the advantage of being simple to understand, but can become cumbersome as pointed out in this Salsify Blog post. The blog post outlines an alternate method where a super property is defined in the child prototype, and the name of each member function is attached as a property to the method. This method, however, relies on the caller property of a method, which the article points out will soon be deprecated. Rather than duplicate the entire post, I believe a summary of the important points are these
Object.defineProperty(bar.prototype, "super", {
get: function get() {
...
// methodName is set as a property on each method in an omitted code segment
methodName = get.caller.methodName;
...
Object.getPrototypeOf(this.prototype)[methodName]
}
}
Which is to say that you find the method with the same name in your prototype's prototype. I was wondering if this can be done in a simpler manner, without having to attach the method name as a parameter and without the Function.caller.
foo.prototype.super = function(method) {
superMethod = Object.getPrototypeOf(this.constructor.prototype)[method];
return superMethod.call(this, Array.prototype.slice.call(arguments, 1));
}
bar.prototype.func1 = function(){ return this.one + super('func1'); }
I'm making a number of assumptions in the above, I'd like to verify some assumptions.
new bar().constructor.prototype === Object.getPrototypeOf(new bar())
If the above is always true, is one preferable over the other?
The Parent's member function will always live in the child's prototype's prototype (assuming that neither of the prototypes were mutated after object creation)
That Object.getPrototypeOf() is not the "language support for accessing super methods" that the blog refers to as being added in ES6
If Object.getPrototypeOf() isn't that language support, what is?
After seeing the error of using this, which does not change throughout the execution and always refers to the instance of the subclass, I've revisited and am thinking I need something like this
Grandfather = function(){};
Grandfather.prototype.function1 = function(){console.log("I am the Grandfather");};
Father = function(){Grandfather.apply(this);};
Father.prototype = Object.create(Grandfather.prototype);
Father.prototype.function1 = function f(){ f.super(); console.log("I am the Father");};
Father.prototype.function1.super = Grandfather.prototype.function1;
Child = function(){Father.apply(this);}
Child.prototype = Object.create(Father.prototype);
Child.prototype.function1 = function f(){ f.super(); console.log("I am the Child");};
Child.prototype.function1.super = Father.prototype.function1;
c = new Child();
c.function1();
// I am the Grandfather
// I am the Father
// I am the Child
And so the question becomes, how to set the super property on to each function in some automatic way?
One such way to do this is shown below, it has the benefit that functions added to the prototype after objects are instantiated still receive the benefit of being able to call superFunc, whereas an approach that sets a super property at class extension time would not set such a property if functions are added to the prototype later.
The downsides of this approach are that it only works in single threaded environment and that it requires functionality inherited from a common base class. It is not threadsafe since some state is held in a what is effectively a static variable of the function. This is fine for my purposes since browsers still have single threaded JavaScript. The requirement that all classes inherit from some base class containing this method isn't a huge blocker (especially if you do a "bad thing" and insert this into Object's prototype).
Grandfather.prototype.superFunc = function f(funcName){
currentPrototype = Object.getPrototypeOf(f.startingPrototype || Object.getPrototypeOf(this));
f.startingPrototype = currentPrototype;
return currentPrototype[funcName]();
}
Child.prototype.function2 = function(){this.superFunc('function2'); console.log("Still in the Child");};
Father.prototype.function2 = function(){this.superFunc('function2'); console.log("Still in the Father");};
GrandFather.prototype.function2 = function(){console.log("Still in the Grandfather");};
c = new Child();
c.function2();
// Still in the Grandfather
// Still in the Father
// Still in the Child

Question 1
new Bar().constructor.prototype should equal Object.getPrototypeOf(new Bar()), provided you haven't overrided Bar.prototype.constructor or Bar.prototype, or return a different object in the Bar constructor. Here's an example:
function Bar() {}
var foo = new Bar();
foo.constructor.prototype === Object.getPrototypeOf(foo); // true
function Bar2() {}
var foo2 = new Bar2();
Bar2.prototype = {};
foo2.constructor.prototype === Object.getPrototypeOf(foo2); // false
function Bar3() {}
var foo3 = new Bar3();
Bar3.prototype.constructor = function NotBar3() {};
foo3.constructor.prototype === Object.getPrototypeOf(foo3); // false
Question 2
If you're looking to get the actual prototype of an object, use Object.getPrototypeOf, as that's unaffected by any of the changes shown above.
Question 3
No, you will not be able to access Foo from new Bar(). In your example, new Bar() would not inherit from Foo.prototype and as a result, there's no way to access Foo unless you make it inherit from Foo.prototype or assign Foo to a property of new Bar() or Bar.prototype.
Question 4/5
No, that's not what they're referring to. ES6 will introduce a separate class contruct, where super takes on a special meaning (similar to how super works in other languages with classes). Here's an example of how classes work in ES6:
class Foo {
constructor() {
this.one = 1;
}
func1() {
return this.one;
}
}
class Bar extends Foo {
func1() {
return this.one + super();
}
}

When you use super in the way you do it'll break when inheritance is more than 2 levels.
Assuming you'd use it the following way:
//changed super to this.super since super is not shown to exist in global scope
bar.prototype.func1(){ return this.one + this.super('func1'); }
See the following example:
function GrandFather(){
this.i = 0;
};
GrandFather.prototype.test = function(){
console.log('test in GrandFather');
};
function Father(){
GrandFather.call(this);
};
Father.prototype = Object.create(GrandFather.prototype);
Father.prototype.constructor = Father;
Father.prototype.super = GrandFather.prototype;
Father.prototype.test = function(){
console.log('test in Father');
//prevent too many recursions
this.i++;
if(this.i>5){
return;
}
this.super.test.call(this);//because test in child was called
// with Child instance as invoking object this will be Child
// and this.super will be Father.prototype
};
function Child(){
Father.call(this);
}
Child.prototype = Object.create(Father.prototype);
Child.prototype.constructor = Child;
Child.prototype.super = Father.prototype;
Child.prototype.test = function(){
console.log('test in Child');
this.super.test.call(this);//because invoking object is Child
//this.super in Father is Child
};
var c = new Child();
c.test();
It's also common practice to start a constructor function with a capital so it's better to use Foo and Bar for constructor function names.
If you want to go through all the trouble of simulating super in JavaScript then the following way would be slightly more robust: http://ejohn.org/blog/simple-javascript-inheritance/

Adding a prototype to an object literal

I have some object, say son, which I'd like to inherit from another object father.
Of course I can make a constructor function for father, like
Father = function() {
this.firstProperty = someValue;
this.secondProperty = someOtherValue;
}
And then use
var son = new Father();
son.thirdProperty = yetAnotherValue;
but this is not exactly what I want. Since son is going to have many properties, it would be more readable to have son declared as an object literal. But then I don't know how to set its protoype.
Doing something like
var father = {
firstProperty: someValue;
secondProperty: someOtherValue;
};
var son = {
thirdProperty: yetAnotherValue
};
son.constructor.prototype = father;
will not work, as the prototype chain seems to be hidden and not care about the change of constructor.prototype.
I think I can use the __proto__ property in Firefox, like
var father = {
firstProperty: someValue;
secondProperty: someOtherValue;
};
var son = {
thirdProperty: yetAnotherValue
__proto__: father
};
son.constructor.prototype = father;
but, as far as I understand, this is not a standard feature of the language and it is better not to use it directly.
Is there a way to specify the prototype for an object literal?

You're right, __proto__ is a non-standard property, and the only two standard ways you have to set a new object's [[Prototype]], are:
Through the use of a constructor and the new operator (as you already mention).
Using the ECMAScript 5 Object.create method.
Object.create is not widely supported yet (works on IE9Pre3+, Firefox 3.7Alpha+, Chrome 5+ Safari 5+, Rhino 1.7), but at some point all the implementations will conform the ES5 spec.
It can take two arguments, the first one is the object that will be used as the [[Prototype]] of the new object, and the second one, is another object where the own properties can be described (in the same structure that you would use Object.defineProperties).
For example:
var father = {
firstProperty: 1,
secondProperty: 2
};
var son = Object.create(father, {
thirdProperty: {
value: 'foo'
}
});
father.isPrototypeOf(son); // true
son.firstProperty; // 1
The son internal [[Prototype]] property will refer to father, and it will contain a value property named thirdProperty.

That's incorrect jmar777. If for example you have
var X = function() {};
X.prototype = {
protoFunc1: function() { console.log('p1');},
protoFunc2: function() { console.log('p2');}
};
X.protoFunc1(); // is not a function
That means that what you're doing:
X.prototype = {}
is just creating an object called prototype. Not the actual prototype. To use prototype you have to use constructor functions.
if however you modify it to this (constructor method)
function X(){};
X.prototype.protoFunc1 = function() {
console.log('p1');
}
X.prototype.protoFunc2 = function() {
console.log('p2');
}
var x = new X();
x.protoFunc1(); //'p1'
It would work.
Either go the object literal method without using prototype or use the contructor method using prototype.

Specifying the prototype for an object literal is a little "wonky", since you'll primarily want the prototype on objects that you create using the constructor syntax (e.g., new X()). Not saying this isn't possible... but it's strange. A similar pattern that is well proved out (used by jQuery, for example), is to instead define the prototype as an object literal. For example:
var X = function() {};
X.prototype = {
protoFunc1: function() {},
protoFunc2: function() {}
};

Using "Object.create" instead of "new"

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.

JavaScript: Inheritance and Constant declaration

Help,
I have this class
var jMath = {
pi2: Math.PI,
foo: function() {
return this.pi2;
}
}
I want to make the pi2 constant and i want jMath to inherit from Math object. How do I do that?

Oh amusing, scratch all that, this is the correct version:
function JMath() {
this.foo = function() {
return this.PI;
}
}
JMath.prototype = Math;
var jMath = new JMath();
alert(jMath.foo());
(which matches what the other answer is here)
(I originally tried to set the prototype using "JMath.prototype = new Math()" which is how I've seen it other places, but the above works)
Edit
Here's one way to do it as a singleton
// Execute an inline anon function to keep
// symbols out of global scope
var jMath = (function()
{
// Define the JMath "class"
function JMath() {
this.foo = function() {
return this.PI;
}
}
JMath.prototype = Math;
// return singleton
return new JMath();
})();
// test it
alert( jMath.PI );
// prove that JMath no longer exists
alert( JMath );

Consider using prototype:
function JMath() {};
JMath.prototype = {
pi2: Math.PI,
foo: function() {
return this.pi2;
}
}
var j = new JMath();
j.pi2=44; j.foo(); // returns 44
delete j.pi2; j.foo(); // now returns Math.PI
The difference between this and #altCognito's answer is that here the fields of the object are shared and all point to the same things. If you don't use prototypes, you create new and unlinked instances in the constructor. You can override the prototype's value on a per-instance basis, and if you override it and then decide you don't like the override value and want to restore the original, use delete to remove the override which merely "shadows" the prototype's value.
Edit: if you want to inherit all the methods and fields of the Math object itself, but override some things without affecting the Math object, do something like this (change the name "Constructor1" to your liking):
function Constructor1() {};
Constructor1.prototype = Math;
function JMath() {};
JMath.prototype = new Constructor1();
JMath.prototype.pi2 = JMath.prototype.PI;
JMath.prototype.foo = function() { return this.pi2; }
var j = new JMath();
j.cos(j.foo()); // returns -1
edit 3: explanation for the Constructor1 function: This creates the following prototype chain:
j -> JMath.prototype -> Math
j is an instance of JMath. JMath's prototype is an instance of Constructor1. Constructor1's prototype is Math. JMath.prototype is where the overridden stuff "lives". If you're only implementing a few instances of JMath, you could make the overridden stuff be instance variables that are setup by the constructor JMath, and point directly to Math, like #altCognito's answer does. (j is an instance of JMath and JMath's prototype is Math)
There are 2 downsides of augmenting-an-object-in-the-constructor. (Not actually downsides necessarily) One is that declaring instance fields/methods in the constructor creates separate values for each instance. If you create a lot of instances of JMath, each instance's JMath.foo function will be a separate object taking up additional memory. If the JMath.foo function comes from its prototype, then all the instances share one object.
In addition, you can change JMath.prototype.foo after the fact and the instances will update accordingly. If you make the foo function in the constructor as a per-instance method, then once JMath objects are created, they are independent and the only way to change the foo function is by changing each one.
edit 2: as far as read-only properties go, you can't really implement them from within Javascript itself, you need to muck around under the surface. However you can declare so-called "getters" which effectively act as constants:
JMath.prototype.__defineGetter__("pi2", function() { return Math.PI; });
JMath.prototype.__defineSetter__("pi2", function(){}); // NOP
var j = new JMath();
j.pi2 = 77; // gee, that's nice
// (if the setter is not defined, you'll get an exception)
j.pi2; // evaluates as Math.PI by calling the getter function
Warning: The syntax for defining getters/setters apparently is not something that IE doesn't implement nicely.

User-defined object properties can't be constant. Math (and a few other objects) is a special built-in - it has read-only properties and functions. But it's not a constructor - it's just a static object (Math.constructor === Object).
And because JavaScript has prototypal inheritance, and not classical, you can't inherit from Math. (Read more here)
What you can do, however, is define a prototype. When a property isn't found locally, the JS parser looks for that property on the current object's prototype. altCognito's current solutions shows this very well.
I'm curious about exactly what it is you're trying to achieve. Perhaps something like this is what you want?
var jMath = function()
{
const pi2 = Math.PI;
this.getPi2 = function()
{
return pi2;
}
}
var j = new jMath;
alert( j.getPi2() );