It's common to see CommonJS modules defined using the following idiom:
(function() {
var logThis = function() { console.log(this); }
module.exports = logThis;
}).call(this);
Underscore.js, for example, does this.
I just spend half an hour discussing with a colleague why they invoke the closure with call(this). This will cause the value of this inside the closure to be inherited from the caller, rather than being set to the global object. However, when I tested this in Node.js, the value of this inside the module was always the global object, even when I loaded and ran it like this:
var bar = {};
bar.foo = function() { var foo = require("./foo"); foo(); }
I was really expecting to see the bar object in the console, but actually I see the global object. It then occurred to me that this might be because modules like Underscore.js are also used in a web context. But in that case it would be loaded with a <script> tag so this will always be equal to the global object anyway.
What gives? I'm sure there is a reason for using this construct but I can't see the practical difference in this particular case whether the module is being used in Node.js or in a webpage.
Update: Just to clarify, I can think of a number of cases where this could make a difference. For example, if I say:
var bar = {}
var foo = require("./foo");
bar.foo = foo;
bar.foo();
(Thanks to #Pointy for correcting my original example.)
I would expect the closure in the module to be evaluated when require() is called, which means that the value of this inside it would be bound to the global object, which would be written to the console even though foo() is then invoked as a member of the "bar" object. However, I am seeing the "bar" object in the console even in this example. I guess that this is not being bound to the closure as I expected?
In a nutshell, I'm looking for one example where a module like Underscore.js will have different behavior due to being wrapped in a closure invoked with fn.call(this) instead of just fn(), either in Node.js or in a web page.
Your call to "foo" inside "bar.foo" is made without any context, so the global context is used. The fact that it's inside a function where this refers to "bar" is not relevant; that's just not how JavaScript works. The only thing that matters is how the function is invoked, not where it's invoked, in other words.
If "bar.foo" looked like this:
bar.foo = function() { require("./foo"); foo.call(this); }
then you'd see "bar" in the console. Or, you could do this:
var bar = {};
require("./foo");
bar.foo = foo;
Then calling bar.foo() would also log the "bar" object. (Does that really work in Node? That is, I thought require() returned an object, and that it didn't just leave things in the global scope. I'm a rank novice at Node however.)
edit — OK thanks for updating. Thus, my example would be corrected as follows. First, I think that your module should look like this:
(function() {
var logThis = function() { console.log(this); }
module.exports.logThis = logThis;
}).call(this);
That is, I think that you want to explort the "logThis" function, so it needs to be bound to the "exports" object as a named property.
Then:
var bar = {};
var foo = require("./foo");
// At this point, foo.logThis is the function
bar.foo = foo.logThis;
// Now the "foo" property of "bar" is a reference to the same function
bar.foo(); // logs the "bar" object
var fee = { fie: foo.logThis };
fee.fie(); // logs the "fee" object
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'm reading "you don't know javascript" and I find some trouble with one example in the book "This & Object Prototypes".
When discussing the different rules for this, specifically in the "Implicit binding" paragraph, the author gives this example:
function foo() {
console.log( this.a );
}
var obj = {
a: 2,
foo: foo
};
var bar = obj.foo; // function reference/alias!
var a = "oops, global"; // `a` also property on global object
bar(); // "oops, global"
However when trying this on JSFiddle I get an undefined output in console instead of the "oops, global".
Conversely, if I define a without var or using window.a I get the output intended by the author regardless of strict mode.
Why is this happening? Did something in ES6 change the way global variables should be declared?
The default settings for JS Fiddle wrap the JS in a function and assign it as an load event handler.
Your tests are not in the global scope.
Unfortunately .bind has been giving me grief when creating more complex closures.
I am quite interested in why .bind seems to work differently once you nest functions.
For example :
function t(){
t = t.bind({}); //correctly assigns *this* to t
function nested_t(){
nested_t = nested_t.bind({}); // fails to assign *this* to nested_t
return nested_t;
}
return nested_t();
}
//CASE ONE
alert(t());
// alerts the whole function t instead of nested_t
//CASE TWO
aleft(t.call(t));
// alerts the global object (window)
In both cases I was expecting a behavior like this:
function t(){
var nested_t = function nested_t(){
return this;
};
return nested_t.call(nested_t);
}
alert(t.call(t));
If someone could explain the behavior of .bind in the first (and/or) second case it would be much appreciated!
So, i'm not entirely reproducing your issue here (both cases return the global object), but i'll try and explain the code as i see it.
function t(){
t = t.bind({}); //correctly assigns *this* to t
function nested_t(){
nested_t = nested_t.bind({}); // fails to assign *this* to nested_t
return this;
}
return nested_t();
}
//CASE ONE
alert(t());
Let's take it step by step.
First, the function t() is defined. Then, upon call, it gets overwritten with a clean context. However, i don't see any usage of this context.
Now, a nested function (nested_t) is defined. Upon call, it is overwritten with a clean context. It then returns the context it had when it was called.
Back to t(). You then return the result of nested_t(), not nested_t itself. In the original function call, nested_t is still being called with global context.
Therefore, when you run t(), it returns the global object.
How your code works
It's very unclear, what your code is trying to do. You can find the documentation for .bind() here. It looks like you might be somehow misunderstanding what this is and how to use it. Anyway, what happens when you run your code is this:
A t function is created in global scope.
[case one] The t function is called.
Global scope t is replaced with a new value (original t bound to a specific context - anonymous empty object), which doesn't affect the current call in any way. Also, while the global t is overwritten, the local t is behaving as read-only. You can check it out by trying the following code: (function foo () { return (function bar () { bar = window.bar = 'baz'; return bar; })(); })() and comparing return value with window.bar.
The same thing happens with nested_t in the nested context (instead of global context).
Result of nested_t call is returned. nested_t returns the context it was called with, which was window, as no context was specified. Specifically, it was not called with an empty object context, because the .bind() inside didn't affect the call itself.
[case two] The exact same thing happens once again. Now you're just calling t with itself as context. Since t doesn't use its context (this) anywhere in its code, nothing really changes.
What your misconceptions might be
Basically, you're mixing up two things - function instance and function call context. A function is a "first-class citizen" in JavaScript - it's an object and you can assign values to its properties.
function foo () {
foo.property = 'value';
}
foo(); // foo.property is assigned a value
This has nothing to do with function call context. When you call a function a context is assigned to that call, which can be accessed using this (inside function body)
function foo () {
this.property = 'value';
}
var object = {};
foo.call(object); // object.property is assigned a value
When you use .bind(), you just create a new function with the same code, that is locked to a specific context.
function foo () {
this.property = 'value';
}
var fixedContext = {},
object = {};
bar = foo.bind(fixedContext);
foo.call(object); // fixedContext.property is set instead of object.property
But in this case, there are also function instances foo and bar, which can also be assigned properties, and which have nothing to do with contexts of calls of those functions.
Let's look at how bind works. First, one level of nesting:
var foo = function() { return this.x; };
alert(foo()); // undefined
alert(foo.bind({x: 42})()); // 42
Now we can add the next level of nesting:
var bar = function() { return foo.bind(this)(); };
alert(bar()); // undefined
alert(bar.bind({x: 42})());
We pass our this context to foo with - guess what? - bind. There is nothing different about the way bind works between scopes. The only difference is that we've already bound this within bar, and so the body of bar is free to re-bind this within foo.
As a couple of commenters have noted, functions that overwrite themselves are a huge code smell. There is no reason to do this; you can bind context to your functions when you call them.
I highly, highly recommend reading the documentation on bind, and trying to understand it to the point where you can write a basic version of Function.prototype.bind from scratch.
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.
I am namespacing some JavaScript code, and I want to know if I have rewritten all my function calls and references to global variables correctly. The current code is this:
var x = 10;
function foo() {
console.log('foo');
bar();
},
function bar() {
console.log('bar', x);
}
foo();
I have rewritten it as follows:
var namespace = {
x : 10,
foo: function() {
console.log('foo');
// Is this the correct way to call bar?
this.bar();
},
bar: function() {
// Is this the correct way to refer to x?
console.log('bar', this.x);
}
};
namespace.foo();
Is this correct, i.e. do I need to rewrite all function calls (EDIT: by which I mean all inside the namespace variable) to this.functionname() and all global variables to this.variablename?
If you like, you can make yourself some "real" private variables and methods by incorporating a closure. I favour the module pattern.
var ns = (function () {
var x = 10;
return {
foo: function () {
console.log('foo');
this.bar();
},
bar: function () {
console.log('bar', x);
},
increment: function () {
x++;
}
}
}());
ns.foo();
ns.increment();
ns.foo();
http://jsfiddle.net/DjYue/
No, in some places (code that calls those functions from outside of your namespace object) you would have to call namespace.foo(), namespace.bar(), namespace.varName
Actually, if you use namespace.bar() from the function in the object itself, you get the added benefit that your functions don't depend on context. That is, the following code doesn't work with what you have
var fun = namespace.foo;
fun(); // This would break, since the this value is going to be the global object
I usually don't like to use this from literal objects because of the problem mentioned above. It's definitely a preference, I just prefer to avoid problems with this
No, you shouldn't rewrite to this.funcname() but rather namespace.funcname(). The this keyword is reserved to refer to the current context object (or whatever it's called), meaning that you'd use it within your "namespaced" code to refer to the namespace object.
And generally, it's good to know that you aren't actually working with namespaces as you'd know it from more classical OOP. JavaScript is a prototype-based language, which means that you don't have namespaces - rather, what you're creating is an object containing several properties and methods. As previously mentioned, when you're working with methods of the object, the this keyword refers to the object itself. You'd need a reference to the object when outside of it's context.
Currently, the approach of using objects to contain all of your code as a sort of namespaces and classes in disguise is diminishing in favor of closures which attaches the methods and properties that should be publicly available to a function (which, in JavaScript, is a first class object), and either returns that function or attaches it to the global object. You may also, alternatively, use a function to contain the properties and methods instead of an object.
I disagree with the people who say No to your question. It really depends on what you want to accomplish here. Based the rewritten code, I don't see anything wrong with it. As long as you call namespace.foo(), the execution context of each function should be namespace. Thus, it is not wrong to use this.
There is a situation that invoking the function foo() will not work:
var foo1 = namespace.foo;
foo1();
Although foo1 is same as the function namespace.foo, foo1's execution context has changed. this will refer to window, not namespace.
Their is no need to rewrite that much because you can have more than one reference to a function - a private and a public one.
var namespace = (function() {
var ns = {}; // the 'namespace'
var x = 10;
var foo = // 'private' reference: bar();
ns.foo = // 'public' reference: ns.bar() -> namespace.bar();
function foo() { // the name is optional because it's a function expression
console.log('foo');
bar();
}; // don't forget the semicolon
var bar =
ns.bar =
function() {
console.log('bar', x);
};
return ns; // return the 'namespace'
})();
namespace.foo();
var foo = namespace.foo;
foo();
delete namespace.bar;
// namespace.bar(); // Error
foo(); // bar is called