I have am trying to understand the Javascript/jQuery behind ColorBox. Some forms of syntax are a bit hard to search on Google as they are a bit lengthy to describe. I am having trouble understanding the following line:
publicMethod = $.fn[colorbox] = $[colorbox] = function (options, callback) {
So I assume a new function called publicMethod is being created, but how do we but I don't really understand anything beyond the first equals symbol ("=").
A normal function declaration would look like this:
function publicMethod(options, callback) {
So if anybody could help me understanding the syntax I would greatly appreciate it.
In:
$.fn[colorbox]
$ is an unhelpfully non-descriptive variable name. It contains an object.
$.fn access the fn property of that object.
fn[colorbox] accesses the property of that object which a name that matches the string stored in colorbox
But the right hand side of and = is defined first, So before it assigns that value to publicMethod it assigns the value of $[colorbox] to $.fn[colorbox].
… and before it does that it assigns (to there) a function.
function () {} defines an anonymous function and passes it left (so it gets stored in whatever is on the other side of =)
In JavaScript, functions are on the same level as other objects - you can assign them to variables, and pass them as parameters.
Normally, you would declare a function in this way:
function SomeFunc(arg1, arg2) { /* etc etc */ }
An equivalient way would to be:
var SomeFunc = function(arg1, arg2) { /* etc, etc */ }
...because, as above, functions themselves are values that may be assigned or passed.
Many libraries will accept functions as arguments to their own functions, running the passed function at a time which suits them (or passing them on elsewhere, a la any other variable). Often this is for callbacks. When passing functions as arguments, there isn't really a need to give them a name of their own, thus the following does the job:
SomeLibrary.doSomethingThenCallback(function(arg1, arg2) {
// the doSomethingThenCallback function will decide when, if ever,
// to run this, or pass it on somewhere else, or whatever else would
// be done with any other argument value.
});
This function publicMethod(options, callback) {} is nearly but not completely the same as var publicMethod = function(){options, callback}. In first case you create function named publicMethod and in the second you create anonymous function and assign it to publicMethod variable. Other to assignations just save this function for the further use as API method.
Related
I had a question when reading a source code. The code example is as below:
// ... some code omitted
function p() {var u=new i();this. $Arbiter0=new s(); this.$Arbiter3=[];}
p.prototype.subscribe = function() { ... }
p.call(p) // <-- what is the purpose of this statement?
I'm new at JavaScript. I read from textbook that, when you use Function.call(Function), it usually means borrow inganother function constructor, in order to do some code-reuse/inherit stuff. BUT, I'm not sure what the purpose of doing it in this code example, i mean, the function is calling the function itself?
Clarify:
I know the use of Function.call(). I just want to know, what is the benefit of doing foobar.call(foobar)?
====
The complete source code is as below:
https://fbstatic-a.akamaihd.net/rsrc.php/v2/y6/r/USEL5meM70H.js
Search 'p.call(p)' in that source code. There is only one occurrence in that file.
====
Thank you.
p.call(p) has the effect of calling the function p, such that the value of this, within the function body, is the function itself.
This allows you to add properties to the function from within the function.
Relevant documentation for Function.call can be found here.
Since this is minified code, it's possible that this construct is an artifact of the minification process. It seems that the intent here is to initialize p; it's probably setting up data structures for storing subscribers.
The first argument for the .call is the element that will be used as this in the executed function.
See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call
You know in JavaScript, functions are also Objects. That means you can have method and properties associated to functions.
For example :
I can create a function called foo and add a property to it like below
function Foo(){};
Foo.bar = [1,2,3,4,5];
Foo.sayName = function(){ alert("hi Tom")};
This methods are called as static methods ( Classical OOP pattern: function constructor act like a class in javascript ), so that you don't need to instantiate an Object of Foo to access sayName method.
Coming back to the actual question. What happens when you do Foo.call(Foo);
The call method expect an object as the first parameter, and invoke the function on which it's called. When the function is invoked, this pointer will be pointed to the object passed. In this case, it's the Foo function ( Object ) itself.
Assume that if Foo function is written this way
function Foo(){
this.bar = [1,2,3,4,5];
this.sayName = function(){ alert("hi")};
}
After executing Foo.call(Foo), you can see that the actual Foo function ( Object ) will have the properties bar and method sayName got added.
I guess the script which you have sent is doing that to reduce code duplication.
I know I can define properties on functions, which can then be accessed from within the function. Right now, the only syntax I can work out involves two statements. Is there a more concise way to express the following:
function myFunc() {
// do some work
}
myFunc.myProp = 0;
I'm not looking for a solution that is fewer characters -- this isn't code golf. I'm asking something more along the lines of "are there different patterns of function declaration that have this other desirable merit?" This is almost about ways to use closures (because I suspect the answer lies there).
Thanks!
Especially if you want to access properties of the function from inside the function itself, you're better off doing this:
var theFunction = function() {
function theRealFunction() {
// the code
if (theRealFunction.something == "not whatever")
// do something
// more code
}
theRealFunction.something = "whatever";
return theRealFunction;
}();
What that does is wrap your function declaration up in an anonymous function. The problem with accessing function properties via the function name is that it must do that by finding the function's name in the surrounding scope. That's kind-of icky, but at least this way it involves a scope that's essentially private. It'll work whether or not the resulting function (returned as the return value of the anonymous function) is assigned to a different variable, passed to a function as a handler function, etc.
This really, really all depends. If you're looking for private variables, then you can easily return a function from the function -- an inner-function will contain access to its parent's scope.
var outer_function = (function () {
var private_var = "secret",
public_var = "public",
inner_function = function () {
return private_var;
};
inner_function.public_var = public_var;
return inner_function;
}());
outer_function now equals inner_function, with the benefit of having access to the enclosed data. Any properties attached to the inner (in the way you did) will now be accessible as public properties of outer.
To this end, you can return, say, a constructor for a class, with public-static properties, with the enclosed vars acting as private-static properties, shared between every instance of the "class" you build.
Not exactly the answer to your question, but if you ever want to read up some different design patterns that can be used when defining a javascript function, this is one of the best articles I've ever read on the topic:
http://www.klauskomenda.com/code/javascript-programming-patterns/
I study JavaScript Proxy Pattern, but I still do not get, where I can benefit from it. I would therefore like to provide you with two examples and kindly ask you to point at the difference between them.
Please, take a look at the code below:
What is the difference between the two addEventListener calls? One of them calls handleDrop in regular way. The other uses Proxy Pattern.
What will I gain using Proxy pattern approach?
I tested both functions, and they both call handleDrop successfully.
DndUpload.prototype.buildDropZone = function ()
{
var self = this,
this.dropZone.addEventListener('drop', function (e) { self.handleDrop.call(self, e) }, false);
this.dropZone.addEventListener('drop', self.handleDrop, false);
DndUpload.prototype.handleDrop = function (e)
{
alert("test");
...
};
}
You can provide me with good reference which contains very clear explanation of Proxy Pattern in JavaScript.
So what you're describing in your example isn't so much a demonstration of the Proxy pattern as much as a demonstration of confusion regarding the "calling object" and how it works in JavaScript.
In JavaScript, functions are "first-class." This essentially means that functions are data just like any other data. So let's consider the following situation:
var fn = (function () { return this.x; }),
a = {
x : 1,
fn : fn,
},
x = 2,
nothing = (function (z) { return z; });
So, we have an object a, which has two properties: fn and x. We also have variables x, fn (which is a function returning this.x), and nothing (which returns whatever it gets passed).
If we evaluate a.x, we get 1. If we evaluate x, we get 2. Pretty simple, eh? Now, if we evaluate nothing(a.x), then we get 1. That's also very simple. But it's important to realize that the value 1 associated with the property a.x is not in any way connected to the object a. It exists independently and can be passed around simply as a value.
In JavaScript, functions work the same way. Functions that are properties (often called "methods") can be passed as simple references. However, in doing so, they can become disconnected from their object. This becomes important when you use the this keyword inside a function.
The this keyword references the "calling object." That's the object that is associated with a function when that function is evaluated. There are three basic ways to set the calling object for a function:
If the function is called using the dot operator (e.g. a.fn()), the relevant object (in the example, a) is set as the calling object.
If the function is called using the function's call or apply properties, then you can explicitly set the calling object (we'll see why this is useful in a second).
If no calling object is set through method 1 or method 2, the global object is used (in a browser, this is typically called window).
So, back to our code. If we call a.fn(), it will evaluate as 1. That's expected because the this keyword in the function will be set to a due to the use of the dot operator. However, if we call simply fn(), it will return 2 because it is referencing the x property of the global object (meaning our global x is used).
Now, here's where things get tricky. What if you called: nothing(a.fn)()? You might be surprised that the result is 2. This is because passing a.fn into nothing() passes a reference to fn, but does not retain the calling object!
This is the same concept as what's going on in your coding example. If your function handleDrop were to use the this keyword, you would find it has a different value depending on which handler form you use. This is because in your second example, you're passing a reference to handleDrop, but as with our nothing(a.fn)() example, by the time it gets called, the calling object reference is lost.
So let's add something else to the puzzle:
var b = {
x : 3
};
You'll note that while b has an x property (and therefore satisfies the requirements for fn's use of this), it doesn't have a property referencing fn. So if we wanted to call the fn function with its this set to b, it might seem we need to add a new property to b. But instead we can use the aforementioned apply method on fn to explicitly set b as the calling object:
fn.apply(b); //is 3
This can be used to "permanently" bind a calling object to a function by creating a new function "wrapper." It's not really permanently binding, it's just creating a new function that calls the old function with the desired calling object. Such a tool is often written like so:
Function.prototype.bind = function (obj) {
var self = this;
return function() {
return self.apply(obj, arguments);
};
};
So after executing that code, we could do the following:
nothing(a.fn.bind(a))(); //is 1.
It's nothing tricky. In fact, the bind() property is built into ES5 and works pretty much like the simple code above. And our bind code is actually a really complicated way to do something that we can do more simply. Since a has fn as a property, we can use the dot operator to call it directly. We can skip all the confusing use of call and apply. We just need to make sure when the function gets called, it gets called using the dot operator. We can see how to do it above, but in practice, it's far simpler and more intuitive:
nothing(function () { return a.fn(); })(); //is 1
Once you have an understanding of how data references can be stored in closure scope, how functions are first-class objects, and how the calling object works, this all becomes very simple to understand and reasonably intuitive.
As for "proxies," those also exploit the same concepts to hook into functions. So, let's say that you wanted to count the number of times a.fn gets called. You can do that by inserting a proxy, like so (making use of some concepts from our bind code from above):
var numCalls = (function () {
var calls = 0, target = a.fn;
a.fn = (function () {
calls++;
return target.apply(a, arguments);
});
return (function () {
return calls;
});
}());
So now, whenever you call numCalls(), it will return the number of times a.fn() was called without actually modifying the functionality of a.fn! which is pretty cool. However, you must keep in mind that you did change the function referenced by a.fn, so looking way back to the beginning of our code, you'll notice that a.fn is no longer the same as fn and can't be used interchangeably anymore. But the reasons should now be pretty obvious!
I know that was basically a week of JavaScript education in a couple pages of text, but that's about as simple as it gets. Once you understand the concepts, the functionality, usefulness, and power of many JavaScript patterns become very simple to understand.
Hope that made things clearer!
UPDATE: Thanks to #pimvdb for pointing out my unnecessary use of [].slice.call(arguments, 0). I have removed it because it's, well, unnecessary.
Basically, passing self.handleDrop directly is functionally equivalent to passing the following function:
function() {
return self.handleDrop.apply(this, arguments);
}
because everything is passed through to the original function:
The this value
The arguments
The return value
With this in mind, compare your functions as follows:
function(e) { self.handleDrop.call(self, e) }
function() { return self.handleDrop.apply(this, arguments); }
The difference with your proxy way is:
It doesn't pass the return value through.
It doesn't pass all arguments through (only the first, e)
It doesn't pass the this value through, but uses a predefined one: self.
Now, the first two items don't make a difference here, because addEventListener doesn't care about the return value, and it also only passes one argument anyway.
But the third item is important: it sets a different this value in the function. By default, this is the element you bind the event to (it it set by the browser). Using the proxy way, you can set another this value.
Now, in your snippet it is not fully clear why you're setting a prototype function each time buildDropZone is called. Usually you define prototype functions only once. But when your handler handleDrop is called using the proxy way, this refers to the DndUpload instance, which is consistent with prototype functions in general.
Consider the code below:
function printThis() {
console.log(this);
}
var someObject = {
performTest : function() {
var self = this;
someOtherObject.higherOrderFunction(printThis);
someOtherObject.higherOrderFunction(function(){printThis.call(self)});
}
}
var someOtherObject = {
higherOrderFunction : function(f) {
f();
}
}
What will someOtherObject.higherOrderFunction(printThis) return?
How about someOtherObject.higherOrderFunction(function(){printThis.call(self)})
The answer to the first question depends on who and how you call someObject.performTest(). If I just call someObject.performTest() from a global context, it will probably print Window.
The second one will always print the someObject instance no matter what.
The closures or 'proxy pattern' as you call it comes in handy when you want to control exactly the execution context of a function.
Note: this in javascript does not behave like it does in other languages(in Java for example).
I have the following javascript method:
function 123_test_function(){
}
The function is generated by java and sent to the client. The 123 is the id of the component so it could change. i.e I can have another function called 111_test_function()
I want to pass this function as a reference.
So I need to create the reference
var 123_test_function = function 123_test_function(){
}
In another js file inside an object I have a function that needs to use the 123_test_function reference like so:
useFunction(123_test_function);
The problem I'm having is which the 123 part of the function.
In this object I have a variable(uniqueID) which has the number at the beginning of the function.
I need the function call to be something like:
useFunction(uniqueID+"_test_function");
This doesn't seem to pass a function instead it passes a string.
Am I doing something wrong?
For one, identifiers (such as function names) cannot begin with a digit.
To solve your problem, use an object, like this:
// 1. define an object to hold all your functions
var allFunctions = {};
// 2. store any function with a unique string as the ID
allFunctions['123_test_function'] = function () {
// whatever
};
// 3. call the function
allFunctions['123_test_function']();
allFunctions[uniqueID + '_test_function']();
Objects are associative arrays. They store key/values pairs, so they do exactly what you want here.
Note that functions don't need a name in JavaScript, so I did not use on in step 2.
If the function is defined as global one, it will be a member of global object (window in case of browsers). Hence you can just do window['id_'+uniqueID+'_test_function'] to access your function
useFunction(window['id_'+uniqueID+'_test_function'])
(Identifiers cannot begin with numbers in JavaScript so I added the 'id_' prefix. You can of course change it to your liking.)
function test_function(number){
if(number == 1)
{
return function() {}
}
if(number == 2)
{
return function() {}
}
}
call the function like this
var func = test_function(1)
func();
As a couple of people have correctly pointed out, a function (or indeed variable) name cannot begin with a numeric. Also this syntax is wrong:
var 123_test_function = function 123_test_function(){
}
The correct syntax would be:
var 123_test_function = function() {
};
...although it should also be noted that the effect of this is exactly the same as a "traditional"
function 123_test_function() {
}
...declaration, in the context of the window object - since window is effectively the global scope of a JS environment in a browser, it doesn't matter how you define the functions, they will always be accessible from anywhere. Understanding exactly what each method of declaring a function means in Javascript is important - luckily, Douglas Crockford to the rescue once again...
People have suggested various methods for calling your named functions from the context of a string, which is basically attempting to use "variable variable" syntax, a subject that has been discussed on SO and elsewhere at length. The eval() approach should be avoided wherever possible - if you find yourself needing an eval() chances are you went wrong somewhere a while back. #Tomalak has the right idea with a collection of functions held in an object, but this still needs the slightly messy string approach to reference things that are actually being accessed by a numeric ID. The collection approach has the advantage of not cluttering up the window object with what are likely to be single/zero use members.
But the way I see it, all you actually need here is an indexed array of functions, where all you need is the numeric index in order to access them. I suggest you create your functions like this:
// Do this once at the top of your JS
var test_functions = [];
// Now, for each function you define:
test_functions[123] = function() {
// Do stuff here
};
// And when you need to call the functions:
var funcId = 123;
test_functions[funcId]();
Update2:
What I really wanted to ask was already argued in a different page. Please check the following entry. (Thanks to BobS.)
How can I access local scope dynamically in javascript?
Hello.
I've started using jQuery and am wondering how to call functions in an anonymous function dynamically from String.
Let's say for instance, I have the following functions:
function foo() {
// Being in the global namespace,
// this function can be called with window['foo']()
alert("foo");
}
jQuery(document).ready(function(){
function bar() {
// How can this function be called
// by using a String of the function's name 'bar'??
alert("bar");
}
// I want to call the function bar here from String with the name 'bar'
}
I've been trying to figure out what could be the counterpart of 'window', which can call functions from the global namespace such as window["foo"].
In the small example above, how can I call the function bar from a String "bar"?
Thank you for your help.
Update:
Here's what I want:
Define functions that are only used in the closure.
Avoid creating an Object in the closure that holds those functions in order to be accessed as obj['bar'].
Avoid eval (if possible) in order to write code more simply in a straightforward manner (if exists).
Decide function's name dynamically via the URI parameter or anything variable.
Being a newbie of Javascript, I thought 'this' would be the counterpart of 'window' in the closure, and tried writing:
// in the closure
name = 'bar';
this[name]; // undefined ...
and failed (of course...).
All of these are for pursuit of further laziness. Javascript is kind of new to me and currently I've been trying to write code as lazy as possible.
As Kobi wrote, eval might be a good option. Alternatively, is there any reason not to do
$(function(){
var localNamespace = {};
function bar() {
alert("bar");
}
localNamespace['bar'] = bar;
// Now bar() can be called by, well, localNamespace['bar']
}
Update:
Similar SO entries, such as How can I access local scope dynamically in javascript?, seem to indicate you're out of luck without using one of these two approaches or something even uglier.
Inside your ready function:
window.bar = function bar() {
// ...
}
Then, you can access window['bar'].