What's the difference between these two function expressions? - javascript

Please correct me if I am wrong. The snippet below is a function statement:
function foo() {}
where as the ones below are all function expressions.
var foo = function() { } // or
var foo = function foo() { } // or
var foo = new function() { }
My question is, what is the difference between these two forms of a function expression?
var foo = function() { } // and
var foo = new function() { }
Is the second one also a constructor expression? And if it is, of which class (I am at a loss of words here, I know JavaScript does not have classes, but by class, here, I mean template or function or prototype)
Update
Those who have provided links to related questions, thank you very much. I really appreciate it. I am trying to learn this language and at this point, honestly, I am so nascent in my judgement that I cannot tell if they're all the same question. I do know that the more I read, although I find it all very fascinating, the more it confuses me at this stage. I will probably take time to fully understand the beauty of this wonderful language. Meanwhile, please do keep mentioning other related threads.

This
var foo = function() { }
instantiates a function and assigns to foo a reference to that function.
This
var foo = new function() { }
instantiates a function, calls it with a new object as its context, and assigns the newly-created object (or the object return value from the function) as the value of foo. The function itself is discarded unless it by some means manages to return itself.
The second piece of code is, or should be, fairly rare in code not written by a confused person.

A Function Expression defines a function as a part of a larger expression syntax (typically a variable assignment ). Functions defined via Functions Expressions can be named or anonymous.
The following are examples of function expression that create a function and assign to the variable foo:
var foo = function() { }
var foo = function foo() { }
When you use the new keyword it does not create a function, it just creates an object and assigns it to the variable foo, the instantiated function gets called as well.
var foo = new function() { }

Related

Property of object not returning the expected value in Javascript [duplicate]

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

Why does "this" change when passing a function as a call back? [duplicate]

This question already has answers here:
How does the "this" keyword work, and when should it be used?
(22 answers)
Closed 1 year ago.
I have these 2 objects:
var Foo = function () {
this.name = 'foo';
};
Foo.prototype.sayName = function () {
console.log(this.name);
};
var Bar = function () {
this.cb = null;
};
Bar.prototype.setCb = function (cb) {
this.cb = cb;
};
Bar.prototype.callCb = function () {
this.cb();
};
And I have this result:
var foo = new Foo();
var bar = new Bar();
foo.sayName(); // "foo"
bar.setCb(foo.sayName);
bar.callCb(); // undefined
This happens because when we call foo.sayName inside Bar, this is pointing to the Bar object. Can someone explain what's happening here?
I also noticed that I can circumvent this by instead passing in an anonymous function bar.setCb(function() {foo.sayName();});. Are there other/better ways around this problem?
This is the part of Javascript that most throws people coming from another language.
Basically, the this pointer is set at the time of the call. When you pull a function off an object, you just get the function, not the context in which it came from (the object it came from). This is unlike for example Python with its bound method objects.
In fact, if you did something like:
let sayName = foo.sayName
sayName()
that would also throw with an "undefined" exception, since calling it that way you lose which object you're calling the method on.
So how do you work around this? There's a couple of options, both of which involve capturing that context.
The first you can do, as the comment suggested, would be to use an arrow function:
bar.setCb(() => foo.sayName())
This "captures" the object to call the function on.
Another option would be to use the Function.bind function:
bar.setCb(foo.sayName.bind(foo))
This creates a new function instance that has it's this pointer set to foo.

How do i redefine myObject's main function, e. g. what myObject() does, without damaging myObject's properties, e. g. myObject.foo?

I've got a legit JS question which is kinda hard to explain. Please forgive my unintelligible manner of speaking and try to grasp the idea. :P
In JS objects can have properties. You can assign functions to properties. You can redefine properties of an object without altering its other properties.
var myObject = {};
myObject.myFunction = function() { return 1; };
myObject.bar = "Bar";
myObject.myFunction = function() { return "Another function here"; }
console.log( myObject.bar ); // This is still "bar"
But objects can themselves be functions.
Question 1: What is the correct term for the function of such a function-object? I'll call it "function-object's main function" below.
I understand that in JS all funcitons are function-objects, so it's incorrect to say "function of a function-object". But when an object 'foo' can execute two functions: foo() and foo.bar(), there should be an unambiguous way to distinguish the two in speech.
As function-objects are objects, they can have properties.
Question 2: How do i redefine the function-object's main function while leaving other function-object's properties intact?
Example of what i want:
myFunctionObject = function () { return 1; }
myFunctionObject.bar = "Bar";
// How do i do this...
myFunctionObject = function () { return "Another function here"; }
// ...so that this still returns "Bar"?
console.log( myFunctionObject.bar );
The main function of the function-object should be stored somewhere... Some hidden property maybe, e. g. myFunctionObject.__function__ = function () { return 1; }?
PS My goal is to understand JS internals. I don't mean any practical use case for this, other than solving some crazy JS-kungfu exercises on [CodeWars](http://codewars.com .
What is the correct term for the function of such a function-object? I'll call it "function-object's main function" below.
It's called a "function". The properties of the (function) object that are functions themselves are called "methods". If you're talking about methods on a constructor function, you also call them "static methods" (of the "class").
How do i redefine the function-object's main function while leaving other function-object's properties intact?
You cannot. The call behaviour of a function is immutable in javascript.
All you can do is create a new function, overwrite the variable in which the old one was stored with it, and copy over all properties.
Question 1: Adding user defined properties to functions is not recommended, because you don't control what properties functions have, so you can't know for sure if you are going to shadow another property. Since it is not recommended, I don't think there is a correct term for that. Personally, I would call them functions with user defined properties.
Question 2: The code myFunctionObject = function () { return "Another function here"; } sets myFunctionObject to a different object (function () { return "Another function here"; }). So, you need to pass the properties of your old object to the new one. One way of doing that is by using functions that add properties instead of adding properties with assignment.
function addBar(o){
o.bar = "Bar";
return o;
}
myFunctionObject = addBar(function () { return 1; });
myFunctionObject = addBar(function () { return "Another function here"; });
console.log(myFunctionObject.bar);

Performance disadvantages of using this javascript pattern?

I am trying to duplicate private prototype methods in javascript and found this bit of code, but I do not fully understand how it implements itself.
The code is...
var Foo = function () {};
Foo.prototype = (function () {
// Private attributes
var somePrivateAttribute = 'Hello world';
// Private methods
function somePrivateMethod(val) {
alert(val);
}
// Public attributes and methods
return {
somePublicMethod: function () {
somePrivateMethod(somePrivateAttribute);
}
};
})();
What I do not understand is when this object is initiated new Foo() does that create a new object for the prototype every time as it is in a self executing function? Could anyone please explain the benefits and disadvantages of using this for a project of mine.
How would this type of pattern work for objects that could have 500 instances or just 1. I am trying to understand and would greatly appreciate if anyone could enlighten me?
Is there any performance benefits to using this method?
What I do not understand is when this object is initiated new Foo() does that create a new object for the prototype every time as it is in a self executing function?
No. That's the point of prototypes, they are not duplicated - every of your Foo instances will inherit from the same Foo.prototype object.
Could anyone please explain the benefits and disadvantages of using this for a project of mine.
It looks like you're having a little misunderstanding, regarding the naming: Those "private" things are no "attributes" or "methods", as they have absolutely nothing to do with instance objects. They are just local variables which are only accessible from the functions you assign to the prototype object.
You might want to read How do JavaScript closures work? and their use in IIFEs.
How would this type of pattern work for objects that could have 500 instances or just 1.
It's perfectly fine. However, using the constructor pattern for just 1 instance might be a little odd - singletons can be defined in a simpler way.
If you want to read more on that pattern, search for revealing prototype pattern - it's the revealing module pattern applied to create prototype objects.
I haven't seen that pattern before, using a module (that's what it's called) for the prototype... but here's what I'd recommend; it's similar but the module encloses everything:
var Foo = (function FooClass() {
var privateVar = 'Hello world';
function privateFunc() {
...
}
function Foo() {}
Foo.prototype = {
publicMethod: function() {
...
}
};
return Foo;
}());
var foo = new Foo();
What I do not understand is when this object is initiated new Foo()
does that create a new object for the prototype every time as it is in
a self executing function?
The self executing function runs once, returns the object, and it won't run again; it keeps a reference to the variables via closure.
What I do not understand is when this object is initiated new Foo() does that create a new object for the prototype every time as it is in a self executing function?
No, the assignment to Foo's prototype only happens once. Nothing wrong with it at all, just a little unconventional.
You could also write it like this (also unconventional, but accomplishes the same thing):
var Foo = function () {};
Foo.prototype = new function() {
// Private attributes
var somePrivateAttribute = 'Hello world';
// Private methods
function somePrivateMethod(val) {
alert(val);
}
// Public attributes and methods
this.constructor = Foo;
this.somePublicMethod = function () {
somePrivateMethod(somePrivateAttribute);
}
};
As Bergi pointed out, this adds an extra object to the prototype chain; if you want to avoid that, you can write it like this instead:
var Foo = function () {};
(function() {
// Private attributes
var somePrivateAttribute = 'Hello world';
// Private methods
function somePrivateMethod(val) {
alert(val);
}
// Public attributes and methods
this.somePublicMethod = function () {
somePrivateMethod(somePrivateAttribute);
}
}).call(Foo.prototype)
By modifying the prototype instead of replacing it, the constructor property also stays intact.

Javascript add method to object

Suppose I have a Foo object
How can I extend this object by adding a bar() method and also ensure that future instances of Foo have this method?
you need to add it to Foo's prototype:
function Foo(){}
Foo.prototype.bar = function(){}
var x = new Foo()
x.bar()
This all depends on how you're creating Foo, and how you intend to use .bar().
First, are you using a constructor-function for your object?
var myFoo = new Foo();
If so, then you can extend the Foo function's prototype property with .bar, like so:
function Foo () { /*...*/ }
Foo.prototype.bar = function () { /*...*/ };
var myFoo = new Foo();
myFoo.bar();
In this fashion, each instance of Foo now has access to the SAME instance of .bar.
To wit: .bar will have FULL access to this, but will have absolutely no access to variables within the constructor function:
function Foo () { var secret = 38; this.name = "Bob"; }
Foo.prototype.bar = function () { console.log(secret); };
Foo.prototype.otherFunc = function () { console.log(this.name); };
var myFoo = new Foo();
myFoo.otherFunc(); // "Bob";
myFoo.bar(); // error -- `secret` is undefined...
// ...or a value of `secret` in a higher/global scope
In another way, you could define a function to return any object (not this), with .bar created as a property of that object:
function giveMeObj () {
var private = 42,
privateBar = function () { console.log(private); },
public_interface = {
bar : privateBar
};
return public_interface;
}
var myObj = giveMeObj();
myObj.bar(); // 42
In this fashion, you have a function which creates new objects.
Each of those objects has a .bar function created for them.
Each .bar function has access, through what is called closure, to the "private" variables within the function that returned their particular object.
Each .bar still has access to this as well, as this, when you call the function like myObj.bar(); will always refer to myObj (public_interface, in my example Foo).
The downside to this format is that if you are going to create millions of these objects, that's also millions of copies of .bar, which will eat into memory.
You could also do this inside of a constructor function, setting this.bar = function () {}; inside of the constructor -- again, upside would be closure-access to private variables in the constructor and downside would be increased memory requirements.
So the first question is:
Do you expect your methods to have access to read/modify "private" data, which can't be accessed through the object itself (through this or myObj.X)?
and the second question is:
Are you making enough of these objects so that memory is going to be a big concern, if you give them each their own personal function, instead of giving them one to share?
For example, if you gave every triangle and every texture their own .draw function in a high-end 3D game, that might be overkill, and it would likely affect framerate in such a delicate system...
If, however, you're looking to create 5 scrollbars per page, and you want each one to be able to set its position and keep track of if it's being dragged, without letting every other application have access to read/set those same things, then there's really no reason to be scared that 5 extra functions are going to kill your app, assuming that it might already be 10,000 lines long (or more).
There are many ways to create re-usable objects like this in JavaScript. Mozilla have a nice introduction here:
https://developer.mozilla.org/en-US/docs/JavaScript/Introduction_to_Object-Oriented_JavaScript
The following will work in your example:
function Foo(){
this.bar = function (){
alert("Hello World!");
}
}
myFoo = new Foo();
myFoo.bar(); // Hello World​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​
You can make bar a function making it a method.
Foo.bar = function(passvariable){ };
As a property it would just be assigned a string, data type or boolean
Foo.bar = "a place";

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