class Aa {
methodA() {
console.log('welcome to method a')
}
methodB() {
console.log('welcome to method b')
// methodA() // Fails!! Uncaught ReferenceError: methodA is not defined
this.methodA() // Works because of this.
}
}
let myclass = new Aa()
myclass.methodB()
What is the best way to concisely explain why you need to use 'this' when calling another method of the class that you are already in?
Intuition might say, well if JS knows that I'm using a method of a class, then it knows about that class...because I'm using it's method.....so why can't it find another method of that same class without me telling it...yeah 'this' same place!
By way of contrast, functions can do it with no problem:
function a() {
console.log('welcome to function a')
}
function b() {
console.log('welcome to function b')
a() // works without any sort of 'scope help'
}
b()
I want to be able to explain it without getting people distracted with needing to know the deepest reasons for it. If possible, ha!
Part of me just wants to say, "That's how JS classes work. You gotta 'this'-ify things."
In order to understand why we need to explicitly reference this in Javascript classes, we first need to understand what the this reference points to in ordinary functions.
This
In Javascript the this keyword is always a reference to the object that called the function.
Consider the following:
const obj1 = {
foo() { console.log(this); }
}
obj1.foo(); // "this" will reference "obj1"
Nothing strange here, this is a reference to the object where it was defined obj1.
Now think about what would happen if we took the function foo and "removed" it from the object. Since this is a reference to the object that called the function, what should this be if the function doesn't belong to an object?
const obj1 = {
foo() { console.log(this); }
}
const foo = obj1.foo;
foo(); // "this" will reference "window"
This is were things start to become strange, this is actually a reference to the global object. This is because everything in Javascript is an object, even the root level of a file. In the browser this global object is called the window object.
Now consider what happens if we reattache the method to another object?
const obj1 = {
foo() { console.log(this); }
}
const foo = obj1.foo;
const obj2 = {};
obj2.foo = foo;
obj2.foo(); // "this" will reference "obj2"
The same rules apply here, since the function now belongs to an object again the this reference points to obj2.
Classes
Fist of all Javascript doesn't actually have classes. A js class in just a different way of writing a prototype.
Lets start by writing a simple class.
class Foo {
constructor() { console.log("I'm a Class constructor") }
log() { console.log("I'm a Class method") }
}
const obj1 = new Foo(); // I'm a Class constructor
obj1.log(); // I'm a Class method
Now lets write the same "class" as a prototype.
function Bar() {
console.log("I'm a Prototype constructor")
}
Bar.prototype = {
log() {
console.log("I'm a Prototype method")
}
}
const obj2 = new Bar(); // I'm a Prototype constructor
obj2.log(); // I'm a Prototype method
These two ways of writing inheritance (classes & prototypes) are the same thing in Javascript.
So as we can more clearly see in the prototype implementation the "class methods" are actually just an object assigned to the prototype of a function.
Now that we know about this and about classes / prototypes we can see that a class method is actually just a function on an object and that this refers to the the object that called the function. So if we want to reference another function on the same object we should do so by referencing it through the this reference.
class Aa {
methodA() {
console.log('welcome to method a')
}
methodB() {
console.log('welcome to method b')
this.methodA()
}
}
const myclass = new Aa()
myclass.methodB()
It is confusing mostly because Javascript has a long history of trying to fit the square peg of prototypes into the round hole of classes. A lot of things that make sense in traditional object-oriented languages only lead to confusion in javascript.
Your (very reasonable) intuition goes off the rails here:
Intuition might say, well if JS knows that I'm using a method of a class, then it knows about that class...because I'm using it's method
methodA() and methodA() are just functions. They don't know anything about where they were defined. What they know is how they were called. this is the key to tying how the function is called to the rest of the class. Consider this example:
class Aa {
methodA() {console.log('welcome to method a')}
methodB() {
console.log('welcome to method b')
this.methodA() // Works because of this.
}
}
let obj = {
methodA() { console.log("welcome to imposter A")}
}
let myclass = new Aa()
obj.methodB = myclass.methodB // take a reference to methodB and add it a different obj
console.log("Same?",obj.methodB === myclass.methodB)
// identical function but it has no idea
// it "belongs" to myclass. Called from a different context
// it acts like a normal function (because it is)
obj.methodB()
The function obj.methodB doesn't know it belongs to class Aa. The only thing the relates it to the class is the way it's called and the way it's called determines the value of this. So this (and the prototype chain) is the glue that holds the whole scheme together.
Related
In C++, the language I'm most comfortable with, usually one declares an object like this:
class foo
{
public:
int bar;
int getBar() { return bar; }
}
Calling getBar() works fine (ignoring the fact that bar might be uninitialized). The variable bar within getBar() is in the scope of class foo, so I don't need to say this->bar unless I really need to make it clear that I'm referring to the class' bar instead of, say, a parameter.
Now, I'm trying to get started with OOP in Javascript. So, I look up how to define classes and try the same sort of thing:
function foo()
{
this.bar = 0;
this.getBar = function() { return bar; }
}
And it gives me bar is undefined. Changing the bar to this.bar fixes the issue, but doing that for every variable clutters up my code quite a bit. Is this necessary for every variable? Since I can't find any questions relating to this, it makes me feel like I'm doing something fundamentally wrong.
EDIT: Right, so, from the comments what I'm getting is that this.bar, a property of an object, references something different than bar, a local variable. Can someone say why exactly this is, in terms of scoping and objects, and if there's another way to define an object where this isn't necessary?
JavaScript has no classes class-based object model. It uses the mightier prototypical inheritance, which can mimic classes, but is not suited well for it. Everything is an object, and objects [can] inherit from other objects.
A constructor is just a function that assigns properties to newly created objects. The object (created by a call with the new keyword) can be referenced trough the this keyword (which is local to the function).
A method also is just a function which is called on an object - again with this pointing to the object. At least when that function is invoked as a property of the object, using a member operator (dot, brackets). This causes lots of confusion to newbies, because if you pass around that function (e.g. to an event listener) it is "detached" from the object it was accessed on.
Now where is the inheritance? Instances of a "class" inherit from the same prototype object. Methods are defined as function properties on that object (instead of one function for each instance), the instance on which you call them just inherits that property.
Example:
function Foo() {
this.bar = "foo"; // creating a property on the instance
}
Foo.prototype.foo = 0; // of course you also can define other values to inherit
Foo.prototype.getBar = function() {
// quite useless
return this.bar;
}
var foo = new Foo; // creates an object which inherits from Foo.prototype,
// applies the Foo constructor on it and assigns it to the var
foo.getBar(); // "foo" - the inherited function is applied on the object and
// returns its "bar" property
foo.bar; // "foo" - we could have done this easier.
foo[foo.bar]; // 0 - access the "foo" property, which is inherited
foo.foo = 1; // and now overwrite it by creating an own property of foo
foo[foo.getBar()]; // 1 - gets the overwritten property value. Notice that
(new Foo).foo; // is still 0
So, we did only use properties of that object and are happy with it. But all of them are "public", and can be overwritten/changed/deleted! If that doesn't matter you, you're lucky. You can indicate "privateness" of properties by prefixing their names with underscores, but that's only a hint to other developers and may not be obeyed (especially in error).
So, clever minds have found a solution that uses the constructor function as a closure, allowing the creating of private "attributes". Every execution of a javascript function creates a new variable environment for local variables, which may get garbage collected once the execution has finished. Every function that is declared inside that scope also has access to these variables, and as long as those functions could be called (e.g. by an event listener) the environment must persist. So, by exporting locally defined functions from your constructor you preserve that variable environment with local variables that can only be accessed by these functions.
Let's see it in action:
function Foo() {
var bar = "foo"; // a local variable
this.getBar = function getter() {
return bar; // accesses the local variable
}; // the assignment to a property makes it available to outside
}
var foo = new Foo; // an object with one method, inheriting from a [currently] empty prototype
foo.getBar(); // "foo" - receives us the value of the "bar" variable in the constructor
This getter function, which is defined inside the constructor, is now called a "privileged method" as it has access to the "private" (local) "attributes" (variables). The value of bar will never change. You also could declare a setter function for it, of course, and with that you might add some validation etc.
Notice that the methods on the prototype object do not have access to the local variables of the constructor, yet they might use the privileged methods. Let's add one:
Foo.prototype.getFooBar = function() {
return this.getBar() + "bar"; // access the "getBar" function on "this" instance
}
// the inheritance is dynamic, so we can use it on our existing foo object
foo.getFooBar(); // "foobar" - concatenated the "bar" value with a custom suffix
So, you can combine both approaches. Notice that the privileged methods need more memory, as you create distinct function objects with different scope chains (yet the same code). If you are going to create incredibly huge amounts of instances, you should define methods only on the prototype.
It gets even a little more complicated when you are setting up inheritance from one "class" to another - basically you have to make the child prototype object inherit from the parent one, and apply the parent constructor on child instances to create the "private attributes". Have a look at Correct javascript inheritance, Private variables in inherited prototypes, Define Private field Members and Inheritance in JAVASCRIPT module pattern and How to implement inheritance in JS Revealing prototype pattern?
Explicitly saying this.foo means (as you've understood well) that you're interested about the property foo of the current object referenced by this. So if you use: this.foo = 'bar'; you're going to set the property foo of the current object referenced by this equals to bar.
The this keyword in JavaScript doesn't always mean the same thing like in C++. Here I can give you an example:
function Person(name) {
this.name = name;
console.log(this); //Developer {language: "js", name: "foo"} if called by Developer
}
function Developer(name, language) {
this.language = language;
Person.call(this, name);
}
var dev = new Developer('foo', 'js');
In the example above we're calling the function Person with the context of the function Developer so this is referencing to the object which will be created by Developer. As you might see from the console.log result this is comes from Developer. With the first argument of the method call we specify the context with which the function will be called.
If you don't use this simply the property you've created will be a local variable. As you might know JavaScript have functional scope so that's why the variable will be local, visible only for the function where it's declared (and of course all it's child functions which are declared inside the parent). Here is an example:
function foo() {
var bar = 'foobar';
this.getBar = function () {
return bar;
}
}
var f = new foo();
console.log(f.getBar()); //'foobar'
This is true when you use the var keyword. This means that you're defining bar as local variable if you forget var unfortunately bar will became global.
function foo() {
bar = 'foobar';
this.getBar = function () {
return bar;
}
}
var f = new foo();
console.log(window.bar); //'foobar'
Exactly the local scope can help you to achieve privacy and encapsulation which are one of the greatest benefits of OOP.
Real world example:
function ShoppingCart() {
var items = [];
this.getPrice = function () {
var total = 0;
for (var i = 0; i < items.length; i += 1) {
total += items[i].price;
}
return total;
}
this.addItem = function (item) {
items.push(item);
}
this.checkOut = function () {
var serializedItems = JSON.strigify(items);
//send request to the server...
}
}
var cart = new ShoppingCart();
cart.addItem({ price: 10, type: 'T-shirt' });
cart.addItem({ price: 20, type: 'Pants' });
console.log(cart.getPrice()); //30
One more example of the benefits of the JavaScript scope is the Module Pattern.
In Module Pattern you can simulate privacy using the local functional scope of JavaScript. With this approach you can have both private properties and methods. Here is an example:
var module = (function {
var privateProperty = 42;
function privateMethod() {
console.log('I\'m private');
}
return {
publicMethod: function () {
console.log('I\'m public!');
console.log('I\'ll call a private method!');
privateMethod();
},
publicProperty: 1.68,
getPrivateProperty: function () {
return privateProperty;
},
usePublicProperty: function () {
console.log('I\'ll get a public property...' + this.publicProperty);
}
}
}());
module.privateMethod(); //TypeError
module.publicProperty(); //1.68
module.usePublicProperty(); //I'll get a public property...1.68
module.getPrivateProperty(); //42
module.publicMethod();
/*
* I'm public!
* I'll call a private method!
* I'm private
*/
There's a little strange syntax with the parentless wrapping the anonymous functions but forget it for the moment (it's just executing the function after it's being initialized). The functionality can be saw from the example of usage but the benefits are connected mainly of providing a simple public interface which does not engages you with all implementation details. For more detailed explanation of the pattern you can see the link I've put above.
I hope that with this :-) information I helped you to understand few basic topics of JavaScript.
function Foo() {
this.bar = 0;
this.getBar = function () { return this.bar };
}
When you call the function above with the new keyword - like this...
var foo = new Foo();
... - a few things happen:
1) an object is created
2) the function is executed with the this keyword referencing that object.
3) that object is returned.
foo, then, becomes this object:
{
bar: 0,
getBar: function () { return this.bar; }
};
Why not, then, just do this:
var foo = {
bar: 0,
getBar: function () { return this.bar; }
};
You would, if it's just that one simple object.
But creating an object with a constructor (that's how it's called) gives us a big advantage in creating multiple of the "same" objects.
See, in javascript, all functions are created with a prototype property [an object], and all objects created with that function (by calling it with the new keyword) are linked to that prototype object. This is why it's so cool - you can store all common methods (and properties, if you wanted to) in the prototype object, and save a lot of memory. This is how it works:
function Foo( bar, bob ) {
this.bar = bar;
this.bob = bob;
}
Foo.prototype.calculate = function () {
// 'this' points not to the 'prototype' object
// as you could've expect, but to the objects
// created by calling Foo with the new keyword.
// This is what makes it work.
return this.bar - this.bob;
};
var foo1 = new Foo(9, 5);
var foo2 = new Foo(13, 3);
var result1 = foo1.calculate();
var result2 = foo2.calculate();
console.log(result1); //logs 4
console.log(result2); //logs 10
That's it!
To get closer to OOP in JavaScript, you might want to take a look into a Module design pattern (for instance, described here).
Based on the closure effect, this pattern allows emulating private properties in your objects.
With 'private' properties you can reference them directly by its identifier (i.e., no this keyword as in constructors).
But anyway, closures and design patterns in JS - an advanced topic. So, get familiar with basics (also explained in the book mentioned before).
In javascript this always refers to the owner object of the function. For example, if you define your function foo() in a page, then owner is the javascript object windows; or if you define the foo() on html element <body>, then the owner is the html element body; and likewise if you define the function onclick of element <a>, then the owner is the anchor.
In your case, you are assigning a property bar to the 'owner' object at the begining and trying to return the local variable bar.
Since you never defined any local varialbe bar, it is giving you as bar is undefined.
Ideally your code should have defined the variable as var bar; if you want to return the value zero.
this is like a public access modifier of objects(variables or functions), while var is the private access modifier
Example
var x = {};
x.hello = function(){
var k = 'Hello World';
this.m = 'Hello JavaScript';
}
var t = new x.hello();
console.log(t.k); //undefined
console.log(t.m); //Hello JavaScript
I want to call an inner function from a static function that was called without an instance, like so:
Foo.Bar = function (options) {
Autodesk.Viewing.Extension.call(this, options);
...
this.innerFunc = function innerFunc(){
...
}
};
Foo.Bar.prototype.constructor =
Foo.Bar;
Foo.Bar.SomeStaticFunc = function () {
innerFunc();
}
Use: Foo.Bar.SomeStaticFunc();.
But I get SomeStaticFunc is not a function.
The example here uses a variable for the class, like var Foo.Bar = function (options) {... but isn't that the same as making an instance of the class like so and calling an inner function?
let x= new Foo.Bar(options);
x.innerFunc();
Is there another way to do this?
PS: I know about ES6 classes but I prefer not to migrate this class to ES6 for now because it isn't totally straight forward.
Well... It seems that you do not know how JavaScript works internally, so here is a quick recap. :)
JavaScript IS an object-oriented language. Object literals are objects, arrays are objects, functions are objects, etc.
JavaScript IS NOT a class-based language. You could say: "Hey, I've seen class and extends keywords in ES6!". Yes, but this is just syntactic sugar. JavaScript is not based on classes like Java, it is based on prototypes.
With ES5 syntax, there are two ways to create what you call a "class":
An object literal
A constructor function
When you use an object literal, your class looks like this:
var myClass = {
myAttribute: "foo",
myMethod: function () {
return "bar";
}
};
When you use a constructor function, your class looks like this:
function MyClass() {
this.myAttribute = "foo";
this.myMethod = function () {
return "bar";
};
}
There are of course differences between these two approaches. With the object literal, you have a sort of Singleton where properties are all static to some extent. With the constructor function, you can produce instances whose properties will be introduced by the this keyword. Example:
var myInstance = new MyClass();
console.log(myInstance);
This instance will have "myAttribute" and "myMethod" in its own properties. This means that these properties are tied to the instance. If you want to call this method, you must do this:
myInstance.myMethod();
So far so good... But there is something wrong with what we did previously. this.myMethod will be created again and again for each instance of MyClass and it is always the same. A better way to handle this is to put it in the prototype so that it can be shared by all instances:
function MyClass() {
this.myAttribute = "foo";
}
MyClass.prototype.myMethod = function () {
return "bar";
};
This is much better, but myMethod is still tied to MyClass instances...
Now I want to create a static method. By definition, our static method will be tied to the class, not to its instances:
MyClass.myStaticMethod = function () {
return "baz";
};
Nice. Here, for the sake of experimentation, I want to do something like this:
MyClass.myStaticMethod = function () {
myMethod();
};
This does not work. Why? In fact, myMethod does not exist in the given scope nor in the outer scope. myMethod has been declared inside another function (the constructor function) and it is not returned, so it is invisible from the outside. Moreover, this function has been put in the prototype of MyClass. This means that it is not available globally, but only on instances of MyClass. When you think about it, it is pretty logical. For instance, when you want to call array methods (methods in Array.prototype), it does not make sense to do that:
push('test');
reverse();
includes('a');
You must call these methods on an array (instance of Array).
[].push('test');
['foo', 'bar', 'baz'].reverse();
['a', 'b'].includes('a');
some beginner question here but i can't seem to find a working answer for my problem. I want to write a self executing, global javascript class 'foo' whose methods can be called from window context (like window.foo.bar() or just foo.bar() in javascript console), without having to instantiate the class.
At the same time, i want to be able to extend said class with custom functions, e.g.
foo.fn.baz = function() {}
This is how far i have gotten this far:
(function (window) {
var foo = function() {
return this;
};
foo.fn = foo.prototype = {
bar: function (string) {
console.log(string);
}
};
window.foo = foo;
})(window);
When i execute this javascript, the js console now knows the class foo, and i can extend its functions via foo.fn.baz = function(){} but i can't call those functions: foo.bar is undefined.
If i change the code from window.foo = foo; to window.foo = new foo();, then i can call the functions, but i can't extend the class anymore.
How do i do this rigt? Is my code anywhere near the right way to do such a thing? Is it even possible to get both things at the same time?
Anyone with an idea or a hint? Anything would be great.
thanks
foo.bar is undefined
Right. You've put the methods on the prototype property of the function. The object that foo.prototype refers to will get assigned to instances you create via the new operator, as their underlying prototype. So:
var f = new foo();
f.bar();
If you want a singleton (foo is, itself, the one object and you can call foo.bar()), you don't need prototypes at all:
window.foo = {
bar: function(string) {
console.log(string);
}
};
...but as you used the word "class," my guess is that you really do want to create multiple instances using foo, rather than using foo directly, so new foo() would be what you want.
Side note: The overwhelming convention in JavaScript is that if a function is expected to be called via new, it starts with an upper-case letter. So Foo rather than foo.
As far as I can tell, there are two main ways of creating functions for an object in javascript. They are:
Method A, make it in the constructor:
function MyObject() {
this.myFunc1 = function() {
...
}
this.myFunc2 = function() {
...
}
...
}
Method B, add it to the prototype:
function MyObject() {
...
}
MyObject.prototype.myFunc1 = function() {
...
}
MyObject.prototype.myFunc2 = function() {
....
}
Obviously if you did:
MyObject.myFunc3 = function() {
....
}
then myFunc3 would become associated with MyObject itself, and not any new objects created with the new keyword. For clarity, we'll call it method C, even though it doesn't work for creating new objects with the new keyword.
So, I would like to know what the differences between the two are. As far as I can tell they have the same effect logically, even if what's happening on the machine is different.
If I were to guess I would say that the only real difference is when you're defining them in the constructor like in method A, it creates a whole new function object for each object that's created, and Method B only keeps one copy of it (in MyObject), that it refers to any time it's called. if this is the case, why would you do it one way over the other. Otherwise, what is the difference between method A and method B.
The advantage of giving a separate function to each object is that you can close over variables in the constructor, essentially allowing for "private data".
function MyObject(a,b) {
var n = a + b; //private variable
this.myFunc1 = function() {
console.log(n);
}
};
vs
function MyObject(a,b) {
this.n = a + b; //public variable
}
MyObject.prototype.myFunc1 = function() {
console.log(this.n);
}
Whether this is a good idea or not depends on who you ask. My personal stance is reserving constructor functions for when I actually use the prototype, as in option #2 and using plain functions (say, make_my_object(a,b)) when using closures, as in option #1.
The idea is that you can modify the prototype at any time and all objects of the type (even those created before the modification) will inherit the changes. This is because, as you mentioned, the prototype is not copied with every new instance.
The MyObject in method A is an instance for inner functions.
You cannot call its functions explicitly outside of it unless object (you can call it a class) was instantiated.
Assume this:
MyObject.MyFunc1(); // will not work
var obj = new MyObject();
obj.MyFunc1(); // will work
so this is the same as any class in other languages. Describing usefulness of classes and their usages goes beyond that question though.
Also to notice:
function MyObject() {
var privateVar = "foo";
this.publicProperty = "bar";
// public function
this.publicFunc = function() {
...
}
// private function
function privateFunc () {
...
}
}
For method B it's same as with method A, the only difference is prototyping is a style of creating object. Some use prototypes for readability or out of preference.
The main advantage in prototypes is that you can extend existing object without touching the original source. You need to be careful with that though.
(as example Prototype framework)
For method C you can call them a static functions. As you said they can be called explicitly by referring through object like:
MyObject.MyFunc1();
So which one to use depends on situation you're handling.
Is it better to declare fields of a class in this way:
function MyClass() {
this.aMethod = function() {
// code
}
}
Or this way:
function MyClass() {
}
MyClass.prototype.aMethod = function() {
// code
}
Any why?
Thanks in advance.
The prototypal method, because it's more efficient as it adds the function to the prototype meaning any objects created with the constructor automatically get that method, while the former/first way those methods have to be created for each and every little object you make individually.
Though really it depends on what you're doing with the method. If you don't need it to be a public method then you may opt with this.method or a function statement.
As #meder said, the prototypal approach is the preferred way, since the the methods are declared in the constructor's prototype, but the first approach you show us is useful to implement private members, for example:
function MyClass() {
var privateValue = 'secret';
function privateMethod() { }
this.aMethod = function() {
// privateValue and privateMethod are accessible here.
return privateValue;
}
}
var foo = new MyClass();
foo.aMethod(); // "secret"
foo.privateValue; // undefined !
That's what Crockford calls a privileged method, because this method has access to all the identifiers created in the lexical scope of the constructor (a closure is created).
It depends on the usage. Use the first method when you are going to use the method frequently; the second, when you are going to make rare use of it. The reason is that when the object is created with the constructor, prototypal methods are not loaded, they are only loaded when called, which will save you some performance.