I just got started in Javascript, backbone, and jquery. My coworker was trying to explain the this concept and contexts in Javascript. He was saying this type of pattern is common, but there might be better ways to do it. I was wondering if it is in fact the "standard" pattern or if there was a better way.
Basically, the pattern is, when I'm in my class that is handling what's being viewed on screen (for us, it's a combination of MVC and backbone), I need to fetch my data to populate the page. But before we fetch, we do
var _this = this;
Since the ajax call we do is asynch, he was saying that if in our success handler, if we did
this.model.property = // set some property from the callback
this would not be what we want and we need to do
_this.model.property = // something
I guess I was wondering if this is pretty standard. I feel like after a week of Javascript, I'm doing it every time I fetch data from the server, and sometimes before I try to format my page with underscore's _.each since I guess 'this' changes in my _.each block as well. Thanks!
Scope is one of the most tricky parts of javascript, basically, that is what your friend is describing. Fully understanding scope is the key to being awesome at javascript. Check out some of these links for a full understanding of what is occuring:
Mozilla Article on Functions and Scope
Another good article
Anyways, I hope you like javascript, its personally my favorite language, especially after node.js found its way into my life :)
Edit:
To actually answer your question: What is happening is that the context is becoming lost in the async mode, and so the scope of this changes. Saving it off allows you to reference it later. Hope this helps!
You can bind function to context, so during async call you don't have to use annoying
var _this = this
//or
var self = this;
You can use underscore.js bind function:
//this - current context
var func = function ({ this.doSomething(); });
var bindedFunction = _.bind(func, this);
So when you call bindedFunction it will be bound to the currentContext.
If you are using backbone, usually I bind callback functions, so they'r bound to the object:
var MyView = Backbone.View.extend({
initialize: function (args) {
_.bindAll(this, 'onDataFetched');
},
fetchData: function () {
$.ajax({
//... some code
success: this.onDataFetched
});
},
onDataFetched: function(result) {
//Do something usefull here
//this = MyView instance
}
});
Related
I have tried searching through a lot of S.O. pages but nothing has touched EXACTLY on this top while also NOT USING JQUERY.... I am trying to stick to pure JavaScript as I want to learn it 115% before advancing my current knowledge of JQuery.
I have an object called ScreenResizeTool like this...
function ScreenResizeTool(currImg) {
window.addEventHandler('resize', function() {
listen(currImg);
}, true);
}
and a method like this...
ScreenResizeTool.prototype.listen = function(currImg) {
//Random Code For Resizing
};
My trouble is probably obvious to an experienced JavaScript user but I am having trouble not making this into a messy dirty awful OOP set. I have done various tests to show and prove to myself that the this inside the addEventHandler changes when it becomes bound to the window. This much I assumed before testing but I was able to see that once window.resize event happens the listen method is gone and not a part of the global window variable....
I have also tried adding a this capture such as this.me = this inside the object constructor however it also couldn't see the me variable once it ran. Once the window took the function over it no longer knew anything about the me variable or any reference to my class methods....
I am aware that I could separate this differently but my goal here is to learn how to fully encapsulate and use as many clean OOP structures as possible as I just came from the .NET world and I need it in my life.
I am also aware that I could make messy calls and or store this object or access to the methods inside the window variable but that seems outright wrong to me. I should be able to fully encapsulate this object and have its events and methods all implemented in this class structure.
I also know that the currImg variable is not going to be seen either but lets start small here. I assume once I figure out my incorrect train of thought on scope for JavaScript I should be fine to figure out the currImg problem.
I know there's 1000 JavaScript programmers out there waiting to rip me a new one over asking this simple question but I gotta know...
Thoughts anyone?
this inside a function bound to a DOM Object (like window) will always refer to that object.
this inside a constructor function will always refer to the prototype.
A common practice to circumvent the this issue, as you mentioned, is to cache it in a variable, often called self. Now you want the variables and properties of your object available after instantiation, so what you need is the return keyword, more specifically to return the parent object itself. Let's put that together:
function ScreenResizeTool() {
var self = this;
// method to instantiate the code is often stored in init property
this.init = function() {
window.addEventListener('resize', function() {
self.listen(); // self will refer to the prototype, not the window!
}, true);
};
return this;
}
ScreenResizeTool.prototype.listen = function() { // Dummy function
var h = window.innerHeight, w = window.innerWidth;
console.log('Resized to ' + w + ' x ' + h + '!');
};
Pretty easy huh? So we have our prototype now, but prototypes can't do anything if there's not an instance. So we create an instance of ScreenResizeTool and instantiate it with its init method:
var tool = new ScreenResizeTool();
tool.init();
// every time you resize the window now, a result will be logged!
You could also simply store the listen & init methods as private functions inside your constructor, and return them in an anonymous object:
function ScreenResizeTool() {
var listen = function() { ... };
var init = function() { ... };
// in this.init you can now simply call listen() instead of this.listen()
return {
listen: listen,
init: init
}
}
Check out the fiddle and make sure to open your console. Note that in this case I'd rather use the first function than the second (it does exactly the same) because prototypes are only useful if you have multiple instances or subclasses
The whole concept of this in JavaScript is a nightmare for beginners and in my code I usually try to avoid it as it gets confusing fast and makes code unreadable (IMHO). Also, many people new to JavaScript but experienced in object-oriented programming languages try to get into the whole this and prototype stuff directly though the don't actually need to (google JS patterns like IIFE for example as alternatives).
So looking at your original code:
function ScreenResizeTool(currImg) {
window.addEventHandler('resize', function() {
listen(currImg); // global function listen?
}, true);
}
ScreenResizeTool.prototype.listen = function(currImg) {
//Random Code For Resizing
};
First off, you probably mean addEventListener instead. In its callback you refer to listen but as a global variable which would look for it as window.listen - which doesn't exit. So you could think to do this:
function ScreenResizeTool(currImg) {
window.addEventHandler('resize', function() {
this.listen(currImg); // what's this?
}, true);
}
As you want to use the prototype.listen function of ScreenResizeTool. But this won't work either as the event listener's callback function is called with a different this and not the this that is your function scope.
This is where something comes in which makes most programmers cringe, you have to cache this, examples from code I've seen:
var _this = this;
var that = this;
var _self = this;
Let's just use the latter to be able to refer to the function within the event callback:
function ScreenResizeTool(currImg) {
var _self = this;
window.addEventListener('resize', function() {
_self.listen();
}, true);
}
Now this will actually work and do what you want to achieve: invoke the prototype.listen function of ScreenResizeTool.
See this JSFiddle for a working example: http://jsfiddle.net/KNw6R/ (check the console for output)
As a last word, this problem did not have anything to do with using jQuery or not. It's a general problem of JS. And especially when having to deal with different browser implementations you should be using jQuery (or another such library) to make your own code clean and neat and not fiddle around with multiple if statements to find out what feature is supported in what way.
is it possible to pass the questions variable into the view render?
Ive attempted calling this.render inside the success on the fetch however I got an error, presumably it's because this. is not at the correct scope.
app.AppView = Backbone.View.extend({
initialize: function(){
var inputs = new app.Form();
inputs.fetch({
success: function() {
var questions = inputs.get(0).toJSON().Questions;
app.validate = new Validate(questions);
app.validate.questions();
}, // End Success()
error: function(err){
console.log("Couldn't GET the service " + err);
}
}); // End Input.fetch()
this.render();
}, // End Initialize
render: function(){
el: $('#finder')
var template = _.template( $("#form_template").html(), {} );
this.$el.html(template);
}
The success callback is called with a different this object than your View instance.
The easiest way to fix it is to add something like this before you call inputs.fetch:
var self = this;
And then inside the success callback:
self.render();
I'm not quite sure what you're trying to achieve, but if your problem is calling render from the success callback, you have two options, Function#bind or assigning a self variable.
For more information about "self" variable, see var self = this? . An example:
var self = this;
inputs.fetch({
success: function () {
self.render();
},
...
});
You should probably do some reading on JavaScript scopes, for example "Effective Javascript" or search the topic ( for example this MDN article ) online to get a better idea what happens there.
For Function#bind(), see the MDN article about it. With Backbone I suggest you use Underscore/LoDash's _.bind instead though, to make sure it works even where Function#bind() is not supported.
As for more high-level concepts, the fetching operation looks like it belongs to the model or router level and instead you should assign the questions variable as the model of your view. Ideally views don't do data processing / fetching, they're just given a model that has the methods necessary to perform any data transformations you might need.
The views shouldn't even need to worry about where the model comes from, this is normally handled by a router in case of single page applications or some initialization code on the page.
I'm ("still", for those who read my previous posts) working on an ICEFaces web application.
This question can be interpreted as general Javascript question, so read on if you don't know much about ICEFaces
I need to extend the behaviour of the classes created by ICEFaces Javascript framework, in particular ToolTipPanelPopup.
I cannot modify the source code of the library (otherwise I would have achieved my goal).
This is how ICEFaces defines the class (much like jQuery and other Javascript frameworks).
ToolTipPanelPopup = Class.create({
[...]
showPopup: function() {
[...]
},
updateCordinate: function(event) {
[...]
},
[...]
});
My question is very simple
How do I extend the behaviour of showPopup() function in order to run my custom function at the end of it?
I mean something like following Java example code that supposes inheritance
public void ShowPopup()
{
super.ShowPopup();
customMethod();
}
Something like this should work:
var original = ToolTipPanel.showPopup;
ToolTipPanel.showPopup = function() {
original(); //this is kind of like the call to super.showPopup()
//your code
};
I tried out this trivial example in Firebug, and it seems to work:
var obj = {
func: function() {
console.log("foo");
}
};
obj.func();
var original = obj.func;
obj.func = function() {
original();
console.log("bar");
};
obj.func();
Firebug output:
foo
foo
bar
So what's happening here is that you're saving a reference to the original showPopup function. Then you're creating a closure and assigning it back to showPopup. The original showPopup is not lost, because you still have a reference to it in original. In the closure, you call the function that original references, and then you have your own code. Just swap around the order if you want to do something first before you call original. Since you're using a closure, original is lexically bound to the current scope and should be available every time the new showPopup is called (if I'm wrong about this, someone please correct me).
Let me know if this works out for you.
I like to organize my javascript in namespace style like below. What I want to know : is there another (shorter?) way to call myFirstFunction() from mySecondFunction()? I tried this.myFirstFunction() and it's not working so maybe there's some kind of mysterious trick here that I don't know.
var myNameSpace = {
myFirstFunction: function(){
alert("Hello World!");
},
mySecondFunction: function(){
myNameSpace.myFirstFunction();
}
}
Thanks for your help as usual, people of SO! :)
As written in your example code, this.myFirstFunction() would work. Your code is likely simplified to illustrate your problem, so it would probably help to see the actual code to tell why it doesn't work with this.
One possible reason that it fails would be if the code where you call this.myFirstFunction() is inside a closure. If so, this would be a reference to the closing function, not your namespace and would therefore fail. See here for a contrived example based on your code to see what I mean. Again, having a look at the actual code would probably be helpful to diagnose what's going on.
Your suggestion to use 'this' should work. i.e.:
var myNameSpace = {
myFirstFunction: function(){
alert("Hello World!");
},
mySecondFunction: function(){
this.myFirstFunction();
}
}
Result:
myNameSpace.mySecondFunction() // "Hello World!".
If you want it to be shorter maybe you should consider the following pattern:
Javascript Design Pattern Suggestion
basically for your example:
var myNameSpace = (function()
{
function _myFirstFunction(){
alert("Hello World!");
}
function _mySecondFunction(){
_myFirstFunction();
}
return {
MyFirstFunction : _myFirstFunction,
MySecondFunction : _mySecondFunction
};
})();
I find this to be the cleanest pattern, also providing "private/public" variables in javascript that's otherwise pretty much impossible
In some cases the this keyword should work fine. If you explicitly call myNameSpace.mySecondFunction() then this.myFirstFunction() will execute as intended.
If you are using myNameSpace.mySecondFunction as an event handler it likely will not. In the case of an event handler you would need some way to refer to the namespace you want to use. A lot of JavaScript frameworks provide a way to define what the this keyword refers to. For example, in MooTools you can do myNameSpace.mySecondFunction.bind(myNameSpace) which will cause this to refer to myNameSpace inside mySecondFunction. If you are not using a framework you could make your event handler an anonymous function like:
document.getElementById('myId').addEventListener('click', function(e) {
myNameSpace.mySecondFunction.call(myNameSpace);
});
For more information on the call method I would refer to the MDC page for the call function or you could use apply which behaves similarly to call but passing an array of arguments for the second paramter rather than having a varargs like approach for additional parameters.
All of these suggestions are predicated on defining your namespace as #Harnish suggested:
var myNameSpace = {
myFirstFunction: function(){
alert("Hello World!");
},
mySecondFunction: function(){
this.myFirstFunction();
}
}
For more information about JavaScript function binding I'd highly suggest reading Justin's article on Function scope and binding in JavaScript
If you are attaching to event:
possible issue could be if you are attaching Namespace's function to event, like:
$(el).on("click", nameSpace.myFunc);
....
nameSpace = {
myFunc: function(){
this.anotherFunc();
}
}
that will throw error.
Solution 1
You may change this.anotherFunc() with nameSpace.anotherFunc()
Solution 2
You might change
$(el).on("click", nameSpace.myFunc);
// to ----->
$(el).on("click", function(){ nameSpace.myFunc(); } );
I want to do something that in a classical object oriented language like Java, C# etc. is very easy to do. I simply want to access a property of an instantiated object. The object is globally scoped in the browser's window object, and provided by the twitter #anywhere API.
For my code examples, assume you have already logged the user in.
If I were using java for instance, I would say (assuming all fields were public:
twttr = new twtter();
String screenName = twtter.currentUser.data('screen_name');
For some reason, this is way hard in Javascript. I've gotten a workaround working where inside the anonymous method that the twitter anywhere API is using, I set the value I want to a DOM element, and fish it out later. This is ugly though. I just want to access it directly.
Here's what I have so far, which doesn't even pass syntax checks in eclipse:
function AnywhereFacade()
{
var twitterReference;
window.twttr.anywhere
(
return function(T)
{
twitterReference = T;
};
)
getValue(propertyToGet)
{
return twitterReference.currentUser.data(propertyToGet);
}
};
var anywhereFacade = AnywhereFacade();
var screen_name = anywhereFacade.getValue("screen_name");
alert("screen name is: " + propertyGetter);
Please help! Why is Javascript so hard to use anyway? What I'm trying to do is use a closure I think.
Thanks!
I have done something similar in my app since I am using the Facebook JavaScript SDK and Twitter SDK and want to provide a consistent interface to access both. So I namespace the variables under App. For twitter anywhere, this is how the variable is captured.
window.App = {};
twttr.anywhere(function(T) {
App.Twitter = {
getValue: function(property) {
return T.currentUser.data(property);
},
getPublicTimeline: function() {
return T.Status.publicTimelime();
}
};
});
We are calling the anywhere function and passing it a callback function. The callback function is needed because the anywhere library might not be loaded at this point. By passing the entire function, we are saying that this function should be executed whenever the anywhere library is loaded.
Now when the library does load, this function will execute, define the App.Twitter property which contains a getValue function. The anywhere or T object is captured in the closure.
If you now call,
App.Twitter.getValue("screen_name");
the actually anywhere object (T), will be used to get the screen_name property.
this is all I needed to do.
document.getElementById('messagePanel').innerHTML = "loading...";
window.twttr.anywhere(function(T)
{
document.getElementById('messagePanel').innerHTML = "screen_name: " + T.currentUser.data('screen_name');
});
this made me realize my issue was just that I had to use a callback for when twitter returned from the async call. that helped me solve my initial problem of how to wrap it for gwt.