What's the difference between
var A = function () {
this.x = function () {
//do something
};
};
and
var A = function () { };
A.prototype.x = function () {
//do something
};
The examples have very different outcomes.
Before looking at the differences, the following should be noted:
A constructor's prototype provides a way to share methods and values among instances via the instance's private [[Prototype]] property.
A function's this is set by how the function is called or by the use of bind (not discussed here). Where a function is called on an object (e.g. myObj.method()) then this within the method references the object. Where this is not set by the call or by the use of bind, it defaults to the global object (window in a browser) or in strict mode, remains undefined.
JavaScript is an object-oriented language, i.e. most values are objects, including functions. (Strings, numbers, and booleans are not objects.)
So here are the snippets in question:
var A = function () {
this.x = function () {
//do something
};
};
In this case, variable A is assigned a value that is a reference to a function. When that function is called using A(), the function's this isn't set by the call so it defaults to the global object and the expression this.x is effective window.x. The result is that a reference to the function expression on the right-hand side is assigned to window.x.
In the case of:
var A = function () { };
A.prototype.x = function () {
//do something
};
something very different occurs. In the first line, variable A is assigned a reference to a function. In JavaScript, all functions objects have a prototype property by default so there is no separate code to create an A.prototype object.
In the second line, A.prototype.x is assigned a reference to a function. This will create an x property if it doesn't exist, or assign a new value if it does. So the difference with the first example in which object's x property is involved in the expression.
Another example is below. It's similar to the first one (and maybe what you meant to ask about):
var A = new function () {
this.x = function () {
//do something
};
};
In this example, the new operator has been added before the function expression so that the function is called as a constructor. When called with new, the function's this is set to reference a new Object whose private [[Prototype]] property is set to reference the constructor's public prototype. So in the assignment statement, the x property will be created on this new object. When called as a constructor, a function returns its this object by default, so there is no need for a separate return this; statement.
To check that A has an x property:
console.log(A.x) // function () {
// //do something
// };
This is an uncommon use of new since the only way to reference the constructor is via A.constructor. It would be much more common to do:
var A = function () {
this.x = function () {
//do something
};
};
var a = new A();
Another way of achieving a similar result is to use an immediately invoked function expression:
var A = (function () {
this.x = function () {
//do something
};
}());
In this case, A assigned the return value of calling the function on the right-hand side. Here again, since this is not set in the call, it will reference the global object and this.x is effective window.x. Since the function doesn't return anything, A will have a value of undefined.
These differences between the two approaches also manifest if you're serializing and de-serializing your Javascript objects to/from JSON. Methods defined on an object's prototype are not serialized when you serialize the object, which can be convenient when for example you want to serialize just the data portions of an object, but not it's methods:
var A = function () {
this.objectsOwnProperties = "are serialized";
};
A.prototype.prototypeProperties = "are NOT serialized";
var instance = new A();
console.log(instance.prototypeProperties); // "are NOT serialized"
console.log(JSON.stringify(instance));
// {"objectsOwnProperties":"are serialized"}
Related questions:
What does it mean that JavaScript is a prototypal language?
What is the scope of a function in JavaScript?
How does the "this" keyword work?
Sidenote: There may not be any significant memory savings between the two approaches, however using the prototype to share methods and properties will likely use less memory than each instance having its own copy.
JavaScript isn't a low-level language. It may not be very valuable to think of prototyping or other inheritance patterns as a way to explicitly change the way memory is allocated.
As others have said the first version, using "this" results in every instance of the class A having its own independent copy of function method "x". Whereas using "prototype" will mean that each instance of class A will use the same copy of method "x".
Here is some code to show this subtle difference:
// x is a method assigned to the object using "this"
var A = function () {
this.x = function () { alert('A'); };
};
A.prototype.updateX = function( value ) {
this.x = function() { alert( value ); }
};
var a1 = new A();
var a2 = new A();
a1.x(); // Displays 'A'
a2.x(); // Also displays 'A'
a1.updateX('Z');
a1.x(); // Displays 'Z'
a2.x(); // Still displays 'A'
// Here x is a method assigned to the object using "prototype"
var B = function () { };
B.prototype.x = function () { alert('B'); };
B.prototype.updateX = function( value ) {
B.prototype.x = function() { alert( value ); }
}
var b1 = new B();
var b2 = new B();
b1.x(); // Displays 'B'
b2.x(); // Also displays 'B'
b1.updateX('Y');
b1.x(); // Displays 'Y'
b2.x(); // Also displays 'Y' because by using prototype we have changed it for all instances
As others have mentioned, there are various reasons to choose one method or the other. My sample is just meant to clearly demonstrate the difference.
Take these 2 examples:
var A = function() { this.hey = function() { alert('from A') } };
vs.
var A = function() {}
A.prototype.hey = function() { alert('from prototype') };
Most people here (especially the top-rated answers) tried to explain how they are different without explaining WHY. I think this is wrong and if you understand the fundamentals first, the difference will become obvious. Let's try to explain the fundamentals first...
a) A function is an object in JavaScript. EVERY object in JavaScript gets an internal property (meaning, you can't access it like other properties, except maybe in browsers like Chrome), often referred to as __proto__ (you can actually type anyObject.__proto__ in Chrome to see what it references. This is just that, a property, nothing more. A property in JavaScript = a variable inside an object, nothing more. What do variables do? They point to things.
So what does this __proto__ property points to? Well, usually another object (we'll explain why later). The only way to force JavaScript for the __proto__ property to NOT point to another object is to use var newObj = Object.create(null). Even if you do this, the __proto__ property STILL exists as a property of the object, just it doesn't point to another object, it points to null.
Here's where most people get confused:
When you create a new function in JavaScript (which is an object as well, remember?), the moment it is defined, JavaScript automatically creates a new property on that function called prototype. Try it:
var A = [];
A.prototype // undefined
A = function() {}
A.prototype // {} // got created when function() {} was defined
A.prototype is TOTALLY DIFFERENT from the __proto__ property. In our example, 'A' now has TWO properties called 'prototype' and __proto__ . This is a big confusion for people. prototype and __proto__ properties are in no way related, they're separate things pointing to separate values.
You may wonder: Why does JavaScript has __proto__ property created on every single object? Well, one word: delegation. When you call a property on an object and the object doesn't have it, then JavaScript looks for the object referenced by __proto__ to see if it maybe has it. If it doesn't have it, then it looks at that object's __proto__ property and so on...until the chain ends. Thus the name prototype chain. Of course, if __proto__ doesn't point to an object and instead points to null, well tough luck, JavaScript realizes that and will return you undefined for the property.
You may also wonder, why does JavaScript creates a property called prototype for a function when you define the function? Because it tries to fool you, yes fool you that it works like class-based languages.
Let's go on with our example and create an "object" out of A:
var a1 = new A();
There's something happening in the background when this thing happened. a1 is an ordinary variable which was assigned a new, empty object.
The fact that you used the operator new before a function invocation A() did something ADDITIONAL in the background. The new keyword created a new object which now references a1 and that object is empty. Here's what happening additionally:
We said that on each function definition there's a new property created called prototype (which you can access it, unlike with the __proto__ property) created? Well, that property is being used now.
So we're now at the point where we have a freshly baked empty a1 object. We said that all objects in JavaScript have an internal __proto__ property which points to something (a1 also has it), whether it's null or another object. What the new operator does is that it sets that __proto__ property to point to the function's prototype property. Read that again. It's basically this:
a1.__proto__ = A.prototype;
We said that A.prototype is nothing more than an empty object (unless we change it to something else before defining a1). So now basically a1.__proto__ points to the same thing A.prototype points to, which is that empty object. They both point to the same object which was created when this line happened:
A = function() {} // JS: cool. let's also create A.prototype pointing to empty {}
Now, there's another thing happening when var a1 = new A() statement is processed. Basically A() is executed and if A is something like this:
var A = function() { this.hey = function() { alert('from A') } };
All that stuff inside function() { } is going to execute. When you reach the this.hey.. line, this is changed to a1 and you get this:
a1.hey = function() { alert('from A') }
I won't cover why this changes to a1 but this is a great answer to learn more.
So to summarize, when you do var a1 = new A() there are 3 things happening in the background:
A totally new empty object is created and assigned to a1. a1 = {}
a1.__proto__ property is assigned to point at the same thing as A.prototype points to (another empty object {} )
The function A() is being executed with this set to the new, empty object created in step 1 (read the answer I referenced above as to why this changes to a1)
Now, let's try to create another object:
var a2 = new A();
Steps 1,2,3 will repeat. Do you notice something? The key word is repeat. Step 1: a2 will be a new empty object, step 2: its __proto__ property will point to the same thing A.prototype points to and most importantly, step 3: function A() is AGAIN executed, which means that a2 will get hey property containing a function. a1 and a2 have two SEPARATE properties named hey which point to 2 SEPARATE functions! We now have duplicate functions in same two different objects doing the same thing, oops...You can imagine the memory implications of this if we have 1000 objects created with new A, after all functions declarations take more memory than something like the number 2. So how do we prevent this?
Remember why the __proto__ property exists on every object? So that if you retrieve the yoMan property on a1 (which doesn't exist), its __proto__ property will be consulted, which if it's an object (and is most cases it is), it will check if it contains yoMan, and if it doesn't, it will consult that object's __proto__ etc. If it does, it will take that property value and display it to you.
So someone decided to use this fact + the fact that when you create a1, its __proto__ property points to the same (empty) object A.prototype points to and do this:
var A = function() {}
A.prototype.hey = function() { alert('from prototype') };
Cool! Now, when you create a1, it again goes through all of the 3 steps above, and in step 3, it doesn't do anything, since function A() has nothing to execute. And if we do:
a1.hey
It will see that a1 does not contain hey and it will check its __proto__ property object to see if it has it, which is the case.
With this approach we eliminate the part from step 3 where functions are duplicated on each new object creation. Instead of a1 and a2 having a separate hey property, now NONE of them has it. Which, I guess, you figured out yourself by now. That's the nice thing...if you understand __proto__ and Function.prototype, questions like these will be pretty obvious.
NOTE: Some people tend to not call the internal Prototype property as __proto__, I've used this name through the post to distinguish it clearly to the Functional.prototype property as two different things.
In most cases they are essentially the same, but the second version saves memory because there is only one instance of the function instead of a separate function for each object.
A reason to use the first form is to access "private members". For example:
var A = function () {
var private_var = ...;
this.x = function () {
return private_var;
};
this.setX = function (new_x) {
private_var = new_x;
};
};
Because of javascript's scoping rules, private_var is available to the function assigned to this.x, but not outside the object.
The first example changes the interface for that object only. The second example changes the interface for all object of that class.
The ultimate problem with using this instead of prototype is that when overriding a method, the constructor of the base class will still refer to the overridden method. Consider this:
BaseClass = function() {
var text = null;
this.setText = function(value) {
text = value + " BaseClass!";
};
this.getText = function() {
return text;
};
this.setText("Hello"); // This always calls BaseClass.setText()
};
SubClass = function() {
// setText is not overridden yet,
// so the constructor calls the superclass' method
BaseClass.call(this);
// Keeping a reference to the superclass' method
var super_setText = this.setText;
// Overriding
this.setText = function(value) {
super_setText.call(this, "SubClass says: " + value);
};
};
SubClass.prototype = new BaseClass();
var subClass = new SubClass();
console.log(subClass.getText()); // Hello BaseClass!
subClass.setText("Hello"); // setText is already overridden
console.log(subClass.getText()); // SubClass says: Hello BaseClass!
versus:
BaseClass = function() {
this.setText("Hello"); // This calls the overridden method
};
BaseClass.prototype.setText = function(value) {
this.text = value + " BaseClass!";
};
BaseClass.prototype.getText = function() {
return this.text;
};
SubClass = function() {
// setText is already overridden, so this works as expected
BaseClass.call(this);
};
SubClass.prototype = new BaseClass();
SubClass.prototype.setText = function(value) {
BaseClass.prototype.setText.call(this, "SubClass says: " + value);
};
var subClass = new SubClass();
console.log(subClass.getText()); // SubClass says: Hello BaseClass!
If you think this is not a problem, then it depends on whether you can live without private variables, and whether you are experienced enough to know a leak when you see one. Also, having to put the constructor logic after the method definitions is inconvenient.
var A = function (param1) {
var privateVar = null; // Private variable
// Calling this.setPrivateVar(param1) here would be an error
this.setPrivateVar = function (value) {
privateVar = value;
console.log("setPrivateVar value set to: " + value);
// param1 is still here, possible memory leak
console.log("setPrivateVar has param1: " + param1);
};
// The constructor logic starts here possibly after
// many lines of code that define methods
this.setPrivateVar(param1); // This is valid
};
var a = new A(0);
// setPrivateVar value set to: 0
// setPrivateVar has param1: 0
a.setPrivateVar(1);
//setPrivateVar value set to: 1
//setPrivateVar has param1: 0
versus:
var A = function (param1) {
this.setPublicVar(param1); // This is valid
};
A.prototype.setPublicVar = function (value) {
this.publicVar = value; // No private variable
};
var a = new A(0);
a.setPublicVar(1);
console.log(a.publicVar); // 1
Every object is linked to a prototype object. When trying to access a property that does not exist, JavaScript will look in the object's prototype object for that property and return it if it exists.
The prototype property of a function constructor refers to the prototype object of all instances created with that function when using new.
In your first example, you are adding a property x to each instance created with the A function.
var A = function () {
this.x = function () {
//do something
};
};
var a = new A(); // constructor function gets executed
// newly created object gets an 'x' property
// which is a function
a.x(); // and can be called like this
In the second example you are adding a property to the prototype object that all the instances created with A point to.
var A = function () { };
A.prototype.x = function () {
//do something
};
var a = new A(); // constructor function gets executed
// which does nothing in this example
a.x(); // you are trying to access the 'x' property of an instance of 'A'
// which does not exist
// so JavaScript looks for that property in the prototype object
// that was defined using the 'prototype' property of the constructor
In conclusion, in the first example a copy of the function is assigned to each instance. In the second example a single copy of the function is shared by all instances.
What's the difference? => A lot.
I think, the this version is used to enable encapsulation, i.e. data hiding.
It helps to manipulate private variables.
Let us look at the following example:
var AdultPerson = function() {
var age;
this.setAge = function(val) {
// some housekeeping
age = val >= 18 && val;
};
this.getAge = function() {
return age;
};
this.isValid = function() {
return !!age;
};
};
Now, the prototype structure can be applied as following:
Different adults have different ages, but all of the adults get the same rights.
So, we add it using prototype, rather than this.
AdultPerson.prototype.getRights = function() {
// Should be valid
return this.isValid() && ['Booze', 'Drive'];
};
Lets look at the implementation now.
var p1 = new AdultPerson;
p1.setAge(12); // ( age = false )
console.log(p1.getRights()); // false ( Kid alert! )
p1.setAge(19); // ( age = 19 )
console.log(p1.getRights()); // ['Booze', 'Drive'] ( Welcome AdultPerson )
var p2 = new AdultPerson;
p2.setAge(45);
console.log(p2.getRights()); // The same getRights() method, *** not a new copy of it ***
Hope this helps.
I know this has been answered to death but I'd like to show an actual example of speed differences.
Function directly on object:
function ExampleFn() {
this.print = function() {
console.log("Calling print! ");
}
}
var objects = [];
console.time('x');
for (let i = 0; i < 2000000; i++) {
objects.push(new ExampleFn());
}
console.timeEnd('x');
//x: 1151.960693359375ms
Function on prototype:
function ExampleFn() {
}
ExampleFn.prototype.print = function() {
console.log("Calling print!");
}
var objects = [];
console.time('y');
for (let i = 0; i < 2000000; i++) {
objects.push(new ExampleFn());
}
console.timeEnd('y');
//x: 617.866943359375ms
Here we're creating 2,000,000 new objects with a print method in Chrome. We're storing every object in an array. Putting print on the prototype takes about 1/2 as long.
Prototype is the template of the class; which applies to all future instances of it. Whereas this is the particular instance of the object.
Let me give you a more comprehensive answer that I learned during a JavaScript training course.
Most answers mentioned the difference already, i.e. when prototyping the function is shared with all (future) instances. Whereas declaring the function in the class will create a copy for each instance.
In general there is no right or wrong, it's more a matter of taste or a design decision depending on your requirements. The prototype however is the technique that is used to develop in an object oriented manner, as I hope you'll see at the end of this answer.
You showed two patterns in your question. I will try to explain two more and try to explain the differences if relevant. Feel free to edit/extend.
In all examples it is about a car object that has a location and can move.
Object Decorator pattern
Not sure if this pattern is still relevant nowadays, but it exists. And it is good to know about it.
You simply pass an object and a property to the decorator function. The decorator returns the object with property and method.
var carlike = function(obj, loc) {
obj.loc = loc;
obj.move = function() {
obj.loc++;
};
return obj;
};
var amy = carlike({}, 1);
amy.move();
var ben = carlike({}, 9);
ben.move();
Functional Classes
A function in JavaScript is a specialised object. In addition to being invoked, a function can store properties like any other object.
In this case Car is a function (also think object) that can be invoked as you are used to do. It has a property methods (which is an object with a move function). When Car is invoked the extend function is called, which does some magic, and extends the Car function (think object) with the methods defined within methods.
This example, though different, comes closest to the first example in the question.
var Car = function(loc) {
var obj = {loc: loc};
extend(obj, Car.methods);
return obj;
};
Car.methods = {
move : function() {
this.loc++;
}
};
var amy = Car(1);
amy.move();
var ben = Car(9);
ben.move();
Prototypal Classes
The first two patterns allow a discussion of using techniques to define shared methods or using methods that are defined inline in the body of the constructor. In both cases every instance has its own move function.
The prototypal pattern does not lend itself well to the same examination, because function sharing via a prototype delegation is the very goal for the prototypal pattern. As others pointed out, it is expected to have a better memory footprint.
However there is one point interesting to know:
Every prototype object has has a convenience property constructor, which points back to the function (think object) it came attached to.
Concerning the last three lines:
In this example Car links to the prototype object, which links via constructor to Car itself, i.e. Car.prototype.constructor is Car itself. This allows you to figure out which constructor function built a certain object.
amy.constructor's lookup fails and thus is delegated to Car.prototype, which does have the constructor property. And so amy.constructor is Car.
Furthermore, amy is an instanceof Car. The instanceof operator works by seeing if the right operand's prototype object (Car) can be found anywhere in the left operand's prototype (amy) chain.
var Car = function(loc) {
var obj = Object.create(Car.prototype);
obj.loc = loc;
return obj;
};
Car.prototype.move = function() {
this.loc++;
};
var amy = Car(1);
amy.move();
var ben = Car(9);
ben.move();
console.log(Car.prototype.constructor);
console.log(amy.constructor);
console.log(amy instanceof Car);
Some developers can be confused in the beginning. See below example:
var Dog = function() {
return {legs: 4, bark: alert};
};
var fido = Dog();
console.log(fido instanceof Dog);
The instanceof operator returns false, because Dog's prototype cannot be found anywhere in fido's prototype chain. fido is a simple object that is created with an object literal, i.e. it just delegates to Object.prototype.
Pseudoclassical patterns
This is really just another form of the prototypal pattern in simplified form and more familiar to do those who program in Java for example, since it uses the new constructor.
It does the same as in the prototypal pattern really, it is just syntactic sugar overtop of the prototypal pattern.
However, the primary difference is that there are optimizations implemented in JavaScript engines that only apply when using the pseudoclassical pattern. Think of the pseudoclassical pattern a probably faster version of the prototypal pattern; the object relations in both examples are the same.
var Car = function(loc) {
this.loc = loc;
};
Car.prototype.move = function() {
this.loc++;
};
var amy = new Car(1);
amy.move();
var ben = new Car(9);
ben.move();
Finally, it should not be too difficult to realize how object oriented programming can be done. There are two sections.
One section that defines common properties/methods in the prototype (chain).
And another section where you put the definitions that distinguish the objects from each other (loc variable in the examples).
This is what allows us to apply concepts like superclass or subclass in JavaScript.
Feel free to add or edit. Once more complete I could make this a community wiki maybe.
I believe that #Matthew Crumley is right. They are functionally, if not structurally, equivalent. If you use Firebug to look at the objects that are created using new, you can see that they are the same. However, my preference would be the following. I'm guessing that it just seems more like what I'm used to in C#/Java. That is, define the class, define the fields, constructor, and methods.
var A = function() {};
A.prototype = {
_instance_var: 0,
initialize: function(v) { this._instance_var = v; },
x: function() { alert(this._instance_var); }
};
EDIT Didn't mean to imply that the scope of the variable was private, I was just trying to illustrate how I define my classes in javascript. Variable name has been changed to reflect this.
As discussed in other answers, it's really a performance consideration because the function in the prototype is shared with all of the instantiations - rather than the function being created for each instantiation.
I put together a jsperf to show this. There is a dramatic difference in the time it takes to instantiate the class, although it is really only relevant if you are making many instances.
http://jsperf.com/functions-in-constructor-vs-prototype
Think about statically typed language, things on prototype are static and things on this are instance related.
When you use prototype, the function will only be loaded only once into memory (independently on the amount of objects you create) and you can override the function whenever you want.
Having this:
sillyObject = {
init: function init(sillySettings) {
name = sillySettings.name
}
};
sillyObject.showAlert = function(x) {
return alert(x);
};
When I run this code:
var sillyvar = new sillyObject()
sillyvar.init(mySettings);
silly.showAlert("silly!");
I get an error but instead if I run the same thing using Object.create it runs..
var sillyvar = Object.create(sillyObject);
sillyvar.init(mySettings);
silly.showAlert("silly!");
Any (silly) help will be appreciated.
new and Object.create are two fundamentally different things.
new is going to expect to be followed by a function, and if not (as you saw) it will give you an error. That is because new expects to call a constructor function and then use that as a basis for a new execution context. During that context, the function has this bound to the scope of the execution context. Once the function is done executing it returns the this value which usually has had some data attached to it. In your example, that would look like this:
function sillyObject() {}
sillyObject.prototype.init = function(sillySettings) {
//perhaps you wanted to attach this name to the sillyObject?
name = sillySettings.name;
//which would look like this
this.name = sillySettings.name;
//because `this` here refers to the object context (remember?)
};
sillyObject.prototype.showAlert = function(x){
return alert(x);//returning alert simply returns undefined (not sure why this is used here)
};
and then you could use new, it would create the execution context using the constructor and then attach the prototype and you would end up with a new instance of sillyObject (all instances would be different).
var sO = new sillyObject();
sO.init(mySettings);
sO.showAlert("silly!");
Object.create() on the other hand is expecting an object as an argument (which is why your version worked here). It will create a new object using that object argument as a template basically. Or as MDN explains it "The Object.create() method creates a new object with the specified prototype object and properties". This basically creates a copy if nothing else is done with the Object, and that is why the alert worked here but not in the new version.
If you tried doing new sillyObject(), the error you get is Uncaught TypeError: object is not a function since sillyObject is an object and not a function.
This answer gives a good overview on the new keyword.
Object.create does not do the same thing as new. It will create a new object with sillyObject as the prototype.
sillyObject is a Object not a function. new will create a new instance of a function. You probably want to use this or just .prototype
var sillyObject = function () {
this.sillySettings = {};
}
sillyObject.prototype = {
init : function (name) {
this.sillySettings.name = name;
},
showAlert: function (x) {
return alert(x);
}
};
var silly = new sillyObject();
silly.init('foo');
silly.showAlert('bar');
this.sillySettings isn't a function so we don't keep it in the prototype. We can keep init and showAlert in the prototype. We use prototype because when using new we use sillyObject() so imagine the variable silly being replaced this sillyObject() showing why we use prototype because silly is instantiated as a function.
I'm not understanding why in the following code, obj.BaseMethod doesn't return the method defined in BaseClass constructor. In other words, why is it that
SubClass.prototype.BaseMethod
is defined, but
new SubClass().prototype.BaseMethod
is undefined.
http://jsfiddle.net/HvxJ4/4/
I'm obviously missing something important here.
function BaseClass() {
var privatestuff = "blah";
return {
BaseMethod: function() {
console.log('BaseMethod called');
}
}
}
function SubClass() {
var moreprivates = "stuff";
return {
SubClassMethod: function() {
console.log('SubClassMethod called');
}
}
}
SubClass.prototype = new BaseClass();
var obj = new SubClass();
obj.SubClassMethod(); // SubClassMethod called
obj.BaseMethod(); // Uncaught TypeError: undefined is not a function
UPDATE
I actually understood how to get my code working using
this.method = function() { }
in my constructor function. I just didn't understand why the code above didn't do the same thing.
The answer is that if you return an object in a constructor function, you are no longer using "protoypal" inheritance.
The thing that makes this most clear to me was this answer
https://stackoverflow.com/a/2118831/834770
Quoting Douglas Crockford in Chapter 5, Inheritance, of JavaScript:
(...)
Douglas Crockford then explains how the new operator could be
implemented as a JavaScript function. This function makes use of
several other functions defined in the book, so I rewrote it in a
(somewhat) simpler form below:
function createNew(constructor) {
// a function to explain the new operator:
// var object = createNew(constructor);
// is equivalent to
// var object = new constructor();
//
// param: constructor, a function
// return: a new instance of the "constructor" kind of objects
// step 1. create a new empty object instance
// linked to the prototype of provided constructor
var hiddenLink = function(){};
hiddenLink.prototype = constructor.prototype;
var instance = new hiddenLink(); // cheap trick here: using new to implement new
// step 2. apply the constructor the new instance and get the result
var result = constructor.apply(instance); // make this a reference to instance within constructor
// step 3. check the result, and choose whether to return it or the created instance
if (typeof result === 'object') {
return object;
} else {
return instance;
}
}
So, in short, if you return an object in this function, then the inheritance bit is effectively ignored.
Here's a way to think of the statement
new SubClass().prototype.BaseMethod
First, the new keyword tells JavaScript to create a new, empty object, i.e. {}
Then JavaScript sets the context (this) to be equal to that new object and calls the function after new. So in this case JavaScript will look for a function to call, but your syntax doesn't reference a defined function, so the result is undefined.
Contrast that with a typical approach for defining objects in JavaScript:
function ExampleObject() {
// stuff related to the example object
this.exampleProperty = "value";
}
var exObj = new ExampleOject();
In this case new creates the empty object {} as before, but now there is a defined function to call. When this function is called, the newly created object (set equal to this) will have an exampleProperty set equal to "value". The resulting object is then assigned to the variable exObj.
It may sound strange to those coming from a Java background (or similar), but JavaScript doesn't really support the concept of classes. The language made the unfortunate choice of trying to make it's prototypical inheritance look like classical inheritance, but it's really not the same. If you're going to be spending a lot of time in JavaScript, you might want to stop trying to think in terms of classes and subclasses and learn a bit about prototypes instead.
In JavaScript what is the difference between these two examples:
Prerequisite:
function SomeBaseClass(){
}
SomeBaseClass.prototype = {
doThis : function(){
},
doThat : function(){
}
}
Inheritance example A using Object.create:
function MyClass(){
}
MyClass.prototype = Object.create(SomeBaseClass.prototype);
Inheritance example B using the new keyword
function MyClass(){
}
MyClass.prototype = new SomeBaseClass();
Both examples seem to do the same thing. When would you chose one over the other?
An additional question:
Consider code in below link (line 15), where a reference to the the function's own constructor is stored in the prototype. Why is this useful?
https://github.com/mrdoob/three.js/blob/master/src/loaders/ImageLoader.js
Excerpt (if you don't want to open the link):
THREE.ImageLoader.prototype = {
constructor: THREE.ImageLoader
}
In your question you have mentioned that Both examples seem to do the same thing, It's not true at all, because
Your first example
function SomeBaseClass(){...}
SomeBaseClass.prototype = {
doThis : function(){...},
doThat : function(){...}
}
function MyClass(){...}
MyClass.prototype = Object.create(SomeBaseClass.prototype);
In this example, you are just inheriting SomeBaseClass' prototype but what if you have a property in your SomeBaseClass like
function SomeBaseClass(){
this.publicProperty='SomeValue';
}
and if you use it like
var obj=new MyClass();
console.log(obj.publicProperty); // undefined
console.log(obj);
The obj object won't have publicProperty property like in this example.
Your second example
MyClass.prototype = new SomeBaseClass();
It's executing the constructor function, making an instance of SomeBaseClass and inheriting the whole SomeBaseClass object. So, if you use
var obj=new MyClass();
console.log(obj.publicProperty); // SomeValue
console.log(obj);
In this case its publicProperty property is also available to the obj object like in this example.
Since the Object.create is not available in some old browsers, in that case you can use
if(!Object.create)
{
Object.create=function(o){
function F(){}
F.prototype=o;
return new F();
}
}
Above code just adds Object.create function if it's not available so you can use Object.create function and I think the code above describes what Object.create actually does. Hope it'll help in some way.
Both examples seem to do the same thing.
That's true in your case.
When would you chose one over the other?
When SomeBaseClass has a function body, this would get executed with the new keyword. This usually is not intended - you only want to set up the prototype chain. In some cases it even could cause serious issues because you actually instantiate an object, whose private-scoped variables are shared by all MyClass instances as they inherit the same privileged methods. Other side effects are imaginable.
So, you should generally prefer Object.create. Yet, it is not supported in some legacy browsers; which is the reason you see the new-approach much too frequent as it often does no (obvious) harm. Also have a look at this answer.
The difference becomes obvious if you use Object.create() as it is intended. Actually, it does entirely hideout the prototype word from your code, it'll do the job under the hood. Using Object.create(), we can go like
var base = {
doThis : function(){
},
doThat : function(){
}
};
And then we can extend/inherit other objects from this
var myObject = Object.create( base );
// myObject will now link to "base" via the prototype chain internally
So this is another concept, a more "object oriented" way of inherting. There is no "constructor function" out of the box using Object.create() for instance. But of course you could just create and call a self defined constructor function within those objects.
One argument for using Object.create() is that it might look more natural to mix/*inherit* from other objects, than using Javascripts default way.
I am not an expert in java script but here is a simple example to understand difference between "Object.create" and "new" ..
step 1 : create the parent function with some properties and actions..
function Person() {
this.name = 'venkat';
this.address = 'dallas';
this.mobile='xxxxxxxxxx'
}
Person.prototype.func1 = function () {
return this.name + this.address;
}
step 2 : create a child function (PersonSalary) which extends above Person function using New keyword..
function PersonSalary() {
Person.call(this);
}
PersonSalary.prototype = new Person();
PersonSalary();
step 3 : create second child function (PersonLeaves) which extends above Person function using Object.create keyword..
function PersonLeaves() {
Person.call(this);
}
PersonLeaves.prototype = Object.create(Person.prototype);
PersonLeaves();
// Now check both child functions prototypes.
PersonSalary.prototype
PersonLeaves.prototype
both of these child functions will link to Person(parent function) prototype and can access it's methods but if you create child function using new it will return a brand new object with all parent properties which we don't need and also when you create any object or function using "New" that parent function is executed which we don't want to be.
Here are the takeaways
if you just want to delegate to some methods in parent function and don't want a new object to be created , using Object.create is best way.
A couple of additions to this answer set, mindful that JS obviously now has native classes:
In both Example A and Example B the static inheritance chain is not configured.
In Example B the superclass constructor is run at the "wrong time". It is run before the call to instantiate an instance of the subclass, before any arguments are known and perhaps before you have decided to instantiate an instance of the subclass. Note that constructors can contain any logic they like, including side-effectful logic, so this can be impactful.
Post-ES6 the inheritance chain can be configured in a standardised way using the class and extends keywords (which solve both of these issues).
See also.
I have the below prog
Object.prototype.inherit = function(baseConstructor) {
this.prototype = (baseConstructor.prototype);
this.prototype.constructor = this;
};
Object.prototype.method = function(name, func) {
this.prototype[name] = func;
};
function StrangeArray(){}
StrangeArray.inherit(Array);
StrangeArray.method("push", function(value) {
Array.prototype.push.call(this, value);
});
var strange = new StrangeArray();
strange.push(4);
alert(strange);
and when irun it i get stack overflow? why?
You're setting StrangeArray.prototype to Array.prototype. Later, you're adding a push method to StrangeArray.prototype (which is now the same thing as Array.prototype). So you're effectively setting Array.prototype.push. Your push method calls Array.prototype.push - i.e. itself. So it ends up just calling itself repeatedly and you get a stack overflow.
Jon is right. Here's a way to fix it. Instead of setting StrangeArray.prototype to Array.prototype, this will let you set StrangeArray.prototype to a new instance of Array, so it inherits Array.prototype's properties (without calling Array's constructor).
Object.prototype.inherit = function(baseConstructor) {
var tmp = Function();
tmp.prototype = baseConstructor.prototype;
this.prototype = new tmp();
this.prototype.constructor = this;
};
Edit:
this.prototype = new baseConstructor();
works in this example, but is not good in more complex programs. If the base constructor does some initialization, such as creating DOM elements, incrementing a count, or connecting to a server, then all that initialization would take place when the script loads, instead of when the child object is instantiated.
Another way to deal with this is to differentiate in the constructor whether it is being called for inheritance (to assign to a prototype) or just to instantiate, and then not do the initialization stuff if it is being called for inheritance. But I prefer to just not call the constructor when inheriting, and so use an empty function for the constructor when inheriting.
I'm not very good at explaining this stuff. I recommend Crockford's site and also these two articles on Javascript inheritance.
You actually only have to set the actual object's prototype, to a new instance of the baseConstructor:
Object.prototype.inherit = function(baseConstructor) {
this.prototype = new baseConstructor();
};
I also suggest you to give a look to this Prototypal Inheritance technique that I found very clean.