I needed to figure out how to get the value of a field on my form from within a handler function but I didn't know how to reference the field and kept getting errors. I spent time looking at the API, code examples and googling. Finally I found one example which works (I imagine there are others).
Assuming a form named MyForm and a field 'myField'
var myVal = myForm.getForm().findField("myField").getValue();
Maybe I'm just too new at this, but I don't think it's obvious from looking at the API docs.
So my question is, when you're trying to figure something out, what's your approach.
Thanks!
Assuming you have set the id of the field, you can use Ext.getCmp(id) to have the ComponentManager look it up. There's also Ext.getDom(id) which basically acts as a wrapper to getElementById.
In addition, many event handler functions allow setting the scope of the function itself. The documentation for that event should note which object is setting the scope. You may be able to set the form field as the scope object and use this.getValue() but it's hard to say without knowing exactly what you're trying to do.
To answer the question at hand: the more you code, the more you grok. Ext JS has a bit of a learning curve but the example source code provided in the download is a great place to start. There are several errors and omissions in the documentation though, so the most authoritative place to go for answers is straight into the source. Reading up on JavaScript callbacks doesn't hurt either.
Related
Good day all.
I would like to count the js functions present on a given page, and then send this number via ajax (the ajax part is the simple part) do you think is it possible to achieve that in javascript? what should be the best way to do it?
thanks in advance.
explanation:
I'm trying to figure out how to counter measure some fraud attempts on some subscription pages, I suspect that some javascript is injected on the page before the user click, so having the number of functions present at the load event, and then the number of those present on the submit event, should lead me in the right direction.
Well, if someone is injecting code to your site, they could just as easily use that code to turn off your code counting functions. You can never trust anything that happens on the client side and must validate everything on the server.
As for the technical side, you'd use a tool like acorn to traverse the syntax tree and find all FunctionDeclaration and FunctionExpressions (and arrows, concise method definitions and methods). That would not find all functions, but it would find all statically created ones.
Once the code started executing it's impossible since it's easily reducable to the halting problem. You don't know if a code will create a function at some point in the future.
In Backbone.js, there are two ways you can get the value of a single attribute:
var foo = model.get('foo');
var foo = model.attributes.foo;
The .get() approach is the one recommended in the docs, but presumably it's a bit slower, as it involves an extra function call.
Are there any reasons why I shouldn't use the .attributes approach? Are there any cases where it would cause unexpected behaviour?
It's a matter of encapsulation. By using a get method you are saying I don't care how it's implemented I just want that attribute. So in future if you want to override your get method to return something more meaningful, you would not have make a code change everywhere you are using that attribute.
For e.g., If your model has an attribute cost associated with it which is a number.
model.get('cost') //would give you that number say 1999.99
Now in future suppose you want it to return a formatted string, you can override the get method.
model.get('cost') // same method would now give you $1999.99 or EUR 1999,99 in french :)
Also you are reducing the amount of code and increasing the readability.
model.get('foo') reads much better than model.attributes.foo; and it's shorter as well.
As you mentioned, in your question, the .get() approach is the one recommended in the docs. Although calling the attribute directly is going to save you a few nano seconds, you should not do it.
The reason is because it may encourage someone reading your code to use that same method to set an attribute: model.attributes.foo = 'blah';
I probably don't have to tell you that this would be bad. The code may appear to work because you're getting the correct value on the model, but you are bypassing all the change tracking logic built in to backbone. No events will be triggered if you set an attribute that way.
That being said - if no one else is reading your code or changing your code but you, and you care about those extra nanoseconds that much, and you know that you will never make that type of a mistake in the future. Then by all means, use the attributes property directly.
This is quite the vexing question, but please bear with me.
It's a well known feature of the javascript DOM that if you have a form field with name="something", the form's javascript DOM representation will have a something property that is a reference to the field's DOM representation.
Now, if the field's name ("something") is equal to one of the form's native properties, it will take its place in the javascript object. For instance, for a form:
<form id="Form"><input name="submit" /></form>
we'll have:
var s=document.getElementById("Form").submit; // s is the input
instead of
var s=document.getElementById("Form").submit; // s is the native submit function
Is there a way to access the native property at all in a situation like this?
Thanks.
Edit: Just to clarify two points:
I'd like to be able to access any property, not just functions
I mean this as a general question. A logical solution would be not to provoke name clashes in the first place.
I don't think there is a way to access the native function if you have an input named 'submit.' I just ran across this same topic in Chapter 11 of Secrets of the Javascript Ninja by John Resig and Bear Bibeault. I think if anyone knows the ins and outs of JS and the DOM, it's them.
The following should do the trick:
HTMLFormElement.prototype.submit.call(document.getElementById("Form"))
It will use the native method and invoke it in the context of the element that's on the page.
I tried to post this as an edit to Ivo's answer, but it was rejected. The message is cryptic, but if what they meant was that I added too much to the original, then I think it's wrong, since I was merely trying to complete it. If the reason was different, feel free to comment. With this edit I'd have accepted Ivo's answer, but without it I think it is incomplete.
Ivo's answer works for native functions, but it isn't applicable to attributes. For attributes it seems there is a number of ways, all similar but none of them perfect, to retrieve the value. I've tested the following on IE9, which is all I have available here, but http://www.quirksmode.org/dom/w3c_core.html#attributes indicates the behaviours are generally, if not completely, consistent:
form.attributes['name'] // node representing the 'name' attribute
form.getAttribute('name') // input with name="name"
form.attributes['submit'] // null
form.getAttribute('submit') // input with name="submit"
form.attributes['onsubmit'] // node representing the 'onsubmit' attribute,
// whose value is a string
form.getAttribute('onsubmit') // input with name="submit"
Based on this, it seems that (again, notice I didn't test it thoroughly):
For value attributes, one may use the attributes['attribute name'] approach, which should work down to IE6.
For prototype functions, one may use the prototype.fn.call approach
For function attributes, maybe one can retrieve the source code of the attribute and use that, but I'd rather not have to.
Since getAttribute seems to return the named input instead of the attribute, at least in IE, it seems pretty useless.
Answering my original question, it looks like in the majority of cases there will be ways to access the properties, but it will depend on the property and eventually the browser.
I am learning jquery and started my first attempt at building a form validation script based on what I know so far (which isnt much).
This is really only the Radio button portion of the validation script, but I thought I get on the right track -coding wise- before I went too far. There are some fundamental issues that I know need addressing.
The Script (jsFiddle): http://jsfiddle.net/pkdsleeper/xNt5n/
The Questions:
a. How best to remove the global variables using
b. jsLint recommends "use strict", so I added it, but im not sure what it does.
c. any good refs?
d. Generally, feel free to rip this code apart (cuz I AM trying to learn) but
please explain my errors in noob-speak :)
Thanks In advance
sleeper
a. How best to remove the global variables using
Wrap it in an anonymous function and assign it to the form as a submit listener.
b. jsLint recommends "use strict", so I added it, but im not sure what it does. any good ref's?
Don't bother. It's just a buzz-word for those trying to be hip. You can't use strict mode features on the general web because way too many browsers don't support them. You can use it with otherwise compliant ES 3 code, but it's only useful as a debugging tool for errors that should have been found during testing anyway (e.g. calling constructors without new).
No c?
d. Generally, feel free to rip this code apart (cuz I AM trying to learn) but please explain my errors in noob-speak :)
> $rdoGroup = [], // an empty array which will be used to hold
You seem to be using $ to indicate a variable that references a jQuery object, but $rdoGroup is just an array. That may be confusing later.
> $rdoGroup.push($(this).attr("name"));
The $ function is very expensive, don't use it if you don't need to. Standard HTML attributes are available in all browsers as DOM properties, so:
$rdoGroup.push(this.name);
Is up to 100 times more efficient, depending on the browser.
> for (i = 0; i < $rdoGroup.length; i++) {
> if ($rdoGroup[c].toString() !== $(this).attr("name").toString()) {
The values assigned to $rdoGroup are strings, calling their toString method is redundant.
As above, use this.name. The name property is a string, so no need for toString.
I think the exercise would be easier without jQuery, which seems to be getting in the way far more than helping. If you are trying to learn javascript, I'd suggest that you learn javascript without using any library.
Once you are reasonably confident with using javascript, then you are far better equipped to use a library for the things the library is good with, and not using it for the things it isn't good at.
a. Well, you got rid of the globals pretty well. But as your code looks right now, you can wrap the entire thing in (function(){ ... all your current code in here ... }()) and leave nothing behind in global scope.
b. For "use strict" see this question
c. typeof questions['c'] === "undefined"...
d. Currently, your js is too tied to the markup (html) and vice-versa. If you add or remove something in the form, you'll have to edit your js. If you want to use the validation for another form, you'll have to start the whole thing over again.
Validators are prime candidates for "unobtrusive javascript"/progressive enhancement (here's some material: A List Apart & Wikipedia). Basically, your markup (the html), your styling (the css), and your "behaviors" (the javascript) should all be separate, so they can be changed independently. Here's something you can try:
Add a class name to your form(s) (e.g. "validate")
Set up your js to look for form.validate when the page has loaded
For each form it finds, add a submit event handler
When the handler fires, you search the given form for inputs with various other class names you specify (e.g. "required" or "phone-number" or whatever). The class names tell your code, what kinds of validations should be done. For instance, <input type="text" class="required zip-code"> would mean that a) it's a zip-code, and b) it's a required field.
If there are any validation errors, cancel the submit and alert the user somehow. Otherwise, let the submit proceed.
That's obviously a very rough outline, but I'm not gonna write your code for you :)
The point is, that if you ever need more forms or different forms or something like that you have a generic validation script, and all you need to do is "mark" the inputs with the appropriate class names.
All that being said: There are tons of jQuery plugins out there that do form validation. It's definitely still a good exercise to write one yourself, but it's also a good idea to look at what's already there, and how it works. Don't look at the code right away, but read up on how those validators are used (e.g. do they use class names or something else?) and try to figure out how they work.
You know what I liked best about obtrusive javascript? You always knew what it was going to do when you triggered an event.
<a onclick="thisHappens()" />
Now that everybody's drinking the unobtrusive kool-aid it's not so obvious. Calls to bind events can happen on any line of any number of javascript file that get included on your page. This might not be a problem if you're the only developer, or if your team has some kind of convention for binding eventhandlers like always using a certain format of CSS class. In the real world though, it makes it hard to understand your code.
DOM browsers like Firebug seem like they could help, but it's still time consuming to browse all of an element's event handler properties just to find one that executes the code you're looking for. Even then it usually just tells you it's an anonymous function() with no line number.
The technique I've found for discovering what JS code gets executed when events are triggered is to use Safari's Profiling tool which can tell you what JS gets executed in a certain period of time, but that can sometimes be a lot of JS to hunt through.
There's got to be a faster way to find out what's happening when I click an element. Can someone please enlighten me?
Check out Visual Event... it's a bookmarklet you can use to expose events on a page.
If you're using jQuery you can take advantage of its advanced event system and inspect the function bodies of event handlers attached:
$('body').click(function(){ alert('test' )})
var foo = $('body').data('events');
// you can query $.data( object, 'events' ) and get an object back, then see what events are attached to it.
$.each( foo.click, function(i,o) {
alert(i) // guid of the event
alert(o) // the function definition of the event handler
});
Or you could implement your own event system.
To answer your question, try using the Firebug command line. This will let you use JavaScript to quickly grab an element by an ID, and then iterate through its listeners. Often, if used with console.log, you'll even be able to get the function definitions.
Now, to defend the unobtrusive:
The benefit I find in unobtrusive JavaScript is that it is a lot easier for me to see the DOM for what it is. That said, I feel that it is generally bad practice to create anonymous functions (with only few exceptions). (The biggest fault I find with JQuery is actually in their documentation. Anonymous functions can exist in a nether-world where failure does not lead to useful output, yet JQuery has made them standard.) I generally have the policy of only using anonymous functions if I need to use something like bindAsListener from Prototype.
Further, if the JS files are properly divided, they will only be addressing one sub-set of the DOM at a time. I have an "ordered checkbox" library, it is in only one JS file which then gets re-used in other projects. I'll also generally have all of the methods of a given sub-library as member methods of either a JSON object or a class and I have one object/class per js file (just as if I were doing everything in a more formalized language). If I have a question about my "form validation code", I will look at the formValidation object in formvalidation.js.
At the same time, I'll agree that sometimes things can become obtuse this way, especially when dealing with others. But disorganized code is disorganized code, and it is impossible to avoid unless you are working by yourself and are a good programmer.
In the end, though, I would rather deal with using /* */ to comment out most of two or three js files to find misbehaving code, then go through the HTML and remove the onclick attributes.
Calling it "kool-aid" seems unfair. DOM Level 2 events solve specific problems with inline event handling, like the conflicts that always result. I don't look back to writing code to use window.onload that has to check whether someone else has assigned it before, and sometimes having it overriden by accident or out of sloppiness. It also ensures a better separation of the structure (HTML) and behaviour (JS) layers. All in all, it's a good thing.
Regarding debugging, I don't think there's any way to solve the event handlers being anonymous functions, other than nagging the authors to use named functions where possible. If you can, tell them that it produces more meaningful call stacks and makes the code more maintainable.
One thing: you shouldn't be able to see what will happen in JavaScript by looking at the HTML code. What nuisance is that? HTML is for structure.
If you want to check what events are bound to certain elements, there's a bookmarklet called visual event for now, and firebug 1.6 (IIRC) will have some sort of event inspector.