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Caffeinated Simpleton » Blog Archive » An Introduction to JavaScript's "this" - 2 views

shared by Javier Neira on 02 Oct 09 - Cached

That’s what this is expected to be, anyway. It’s expected to be a reference to the current instance of whatever object it’s defined within.
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It’ll give an error saying that this doesn’t have a member called condiments, even though it clearly does. What happened?!
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This is because there is no binding of functions to instances in JavaScript.
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...3 more annotations...
The setTimeout function, however, just has a reference to that function. When it calls it, it’s not aware of myHotDog, so JavaScript sets this to window
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function HotDog() { var my = this; // my references the current this, which is correct. my.condiments = "mustard, ketchup"; my.getCondiments = function() { return my.condiments; //my is guaranteed to be a reference to the original "this" } }
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In constructors, this is always your instance. So we created a new variable, my, that references the HotDog instance. This allows you to always refer to the HotDog instance, no matter how the getCondiments function is called.
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Perfection kills » Understanding delete - 3 views

perfectionkills.com/understanding-delete javascript delete tutorial

shared by Javier Neira on 11 Jan 10 - Cached

yc c liked it

All because it’s not possible to delete variables in Javascript. At least not when declared in such way.
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It’s almost as if Firebug follows some other rules of deletion. It is Firebug that has led Stoyan astray! So what is really going on here?
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we need to understand how delete operator works in Javascript: what exactly can and cannot be deleted and why.
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...35 more annotations...
var o = { x: 1 }; delete o.x; // true o.x; // undefined
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var x = 1; delete x; // false x; // 1
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function x(){} delete x; // false typeof x; // "function"
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Note that delete only returns false when a property can not be deleted.
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variable instantiation and property attributes
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Global code, Function code and Eval code.
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When a source text is treated as a Program, it is executed in a global scope, and is considered a Global code.
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Anything that’s executed directly within a function is, quite obviously, considered a Function code. In browsers, content of event attributes (e.g. <p onclick="...">) is usually parsed and treated as a Function code.
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text that’s supplied to a built-in eval function is parsed as Eval code. We will soon see why this type is special.
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And now that we know the difference between property assignment and variable declaration — latter one sets DontDelete, whereas former one doesn’t — it should be clear why undeclared assignment creates a deletable property:
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As you can see, execution contexts can logically form a stack. First there might be Global code with its own execution context; that code might call a function, with its own execution context; that function could call another function, and so on and so forth. Even if function is calling itself recursively, a new execition context is being entered with every invocation.
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Every execution context has a so-called Variable Object associated with it. Similarly to execution context, Variable object is an abstract entity, a mechanism to describe variable instantiation. Now, the interesing part is that variables and functions declared in a source text are actually added as properties of this Variable object.
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When control enters execution context for Global code, a Global object is used as a Variable object. This is precisely why variables or functions declared globally become properties of a Global object:
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The behavior is actually very similar: they become properties of Variable object. The only difference is that when in Function code, a Variable object is not a Global object, but a so-called Activation object. Activation object is created every time execution context for Function code is entered.
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and a special Arguments object (under arguments name). Note that Activation object is an internal mechanism and is never really accessible by program code.
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within Eval code are created as properties of calling context’s Variable object. Eval code simply uses Variable object of the execution context that it’s being called within:
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Now that it’s clear what happens with variables (they become properties), the only remaining concept to understand is property attributes. Every property can have zero or more attributes from the following set — ReadOnly, DontEnum, DontDelete and Internal. These attributes serve as sort of flags — an attribute can either exist on a property or not. For the purposes of today’s discussion, we are only interested in DontDelete.
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When declared variables and functions become properties of a Variable object — either Activation object (for Function code), or Global object (for Global code), these properties are created with DontDelete attribute. However, any explicit (or implicit) property assignment creates property without DontDelete attribute. And this is essentialy why we can delete some properties, but not others:
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Special arguments variable (or, as we know now, a property of Activation object) has DontDelete. length property of any function instance has DontDelete as well:
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As you might remember, undeclared assignment creates a property on a global object.
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Execution context When ECMAScript code executes, it always happens within certain execution context.
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Variables declared within Eval code are actually created as properties without DontDelete:
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This interesting eval behavior, coupled with another aspect of ECMAScript can technically allow us to delete non-deletable properties. The thing about function declarations is that they can overwrite same-named variables in the same execution context:
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Note how function declaration takes precedence and overwrites same-named variable (or, in other words, same property of Variable object). This is because function declarations are instantiated after variable declarations, and are allowed to overwrite them
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If we declare function via eval, that function should also replace that property’s attributes with its own. And since variables declared from within eval create properties without DontDelete, instantiating this new function should essentially remove existing DontDelete attribute from the property in question, making that property deletable (and of course changing its value to reference newly created function).
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Unfortunately, this kind of spoofing doesn’t work in any implementation I tried. I might be missing something here, or this behavior might simply be too obscure for implementors to pay attention to
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this.x = 1; delete x; // TypeError: Object doesn't support this action
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var x = 1; delete this.x; // TypeError: Cannot delete 'this.x'
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It’s as if variable declarations in Global code do not create properties on Global object in IE.
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Not only is there an error, but created property appears to have DontDelete set on it, which of course it shouldn’t have:
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“The global variable object is implemented as a JScript object, and the global object is implemented by the host.
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Note how this and window seem to reference same object (if we can believe === operator), but Variable object (the one on which function is declared) is different from whatever this references.
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delete doesn’t differentiate between variables and properties (in fact, for delete, those are all References) and really only cares about DontDelete attribute (and property existence).
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The moral of the story is to never trust host objects.
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Few restrictions are being introduced. SyntaxError is now thrown when expression in delete operator is a direct reference to a variable, function argument or function identifier. In addition, if property has internal [[Configurable]] == false, a TypeError is thrown:
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Adobe Creative Suite 3 Web Premium All-in-One Desk Reference For Dummies|free ebooks do... - 0 views

www.ebook-x.com/esk-reference-for-dummies.html computer design download ebook ebooks free programming web2.0 webdesign

shared by Adildi ldinlio on 18 Oct 08 - Cached

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gotAPI.com :: Reference Lookup Service - 0 views

www.gotapi.com/index.html css development html imported delicious javascript language perl programming reference sql web

shared by Julian Knight on 29 Aug 08 - Cached

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Regexp Syntax Summary - 0 views

www.greenend.org.uk/...regexp.html cheatsheet compare reference regex

shared by yc c on 13 Jun 08 - Cached

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HtmlUnit - Welcome to HtmlUnit - 2 views

htmlunit.sourceforge.net java testing html unit api main reference

shared by Javier Neira on 16 Nov 09 - Cached

Javier Neira on 16 Nov 09

HtmlUnit is a "GUI-Less browser for Java programs". It models HTML documents and provides an API that allows you to invoke pages, fill out forms, click links, etc... just like you do in your "normal" browser. It has fairly good JavaScript support (which is constantly improving) and is able to work even with quite complex AJAX libraries, simulating either Firefox or Internet Explorer depending on the configuration you want to use. It is typically used for testing purposes or to retrieve information from web sites. HtmlUnit is not a generic unit testing framework. It is specifically a way to simulate a browser for testing purposes and is intended to be used within another testing framework such as JUnit or TestNG. Refer to the document "Getting Started with HtmlUnit" for an introduction. HtmlUnit is used as the underlying "browser" by different Open Source tools like Canoo WebTest, JWebUnit, WebDriver, JSFUnit, Celerity, ... HtmlUnit was originally written by Mike Bowler of Gargoyle Software and is released under the Apache 2 license. Since then, it has received many contributions from other developers, and would not be where it is today without their assistance.

<div class="cArrow"> </div><div class="cContentInner"> HtmlUnit is a "GUI-Less browser for Java programs". It models HTML documents and provides an API that allows you to invoke pages, fill out forms, click links, etc... just like you do in your "normal" browser. It has fairly good JavaScript support (which is constantly improving) and is able to work even with quite complex AJAX libraries, simulating either Firefox or Internet Explorer depending on the configuration you want to use. It is typically used for testing purposes or to retrieve information from web sites. HtmlUnit is not a generic unit testing framework. It is specifically a way to simulate a browser for testing purposes and is intended to be used within another testing framework such as JUnit or TestNG. Refer to the document "Getting Started with HtmlUnit" for an introduction. HtmlUnit is used as the underlying "browser" by different Open Source tools like Canoo WebTest, JWebUnit, WebDriver, JSFUnit, Celerity, ... HtmlUnit was originally written by Mike Bowler of Gargoyle Software and is released under the Apache 2 license. Since then, it has received many contributions from other developers, and would not be where it is today without their assistance. </div>

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JQuery Selectors - 0 views

codylindley.com/jqueryselectors jquery javascript reference development

shared by Julian Knight on 30 Nov 09 - Cached

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How to Use JavaScript to Create Simple Animation - - 1 views

hostforlife.eu/...ipt-to-create-simple-animation asp net tutorial how to javascript

shared by Hostforlife Hosting on 29 Jun 15 - No Cached

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HTML Reference V.2 @ yasininat.com - 0 views

www.yasininat.com/ref css html javascript php programming reference scripting

shared by Julian Knight on 29 Aug 08 - No Cached

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FreeComputerBooks.com - Free Computer Books, Tutorials &amp; Lecture Notes - JavaScript - 0 views

freecomputerbooks.com/webJavaScriptBooksIndex.html development imported delicious javascript programming reference

shared by Julian Knight on 29 Aug 08 - Cached

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Bookmarklets, Favelets and Snippets | Smashing Magazine - 0 views

www.smashingmagazine.com/...marklets-favelets-and-snippets bookmarklet firefox imported delicious javascript reference

shared by Julian Knight on 29 Aug 08 - Cached

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Closure Compiler - Google Code - 1 views

code.google.com/...compiler javascript compiler

shared by yc c on 02 Feb 10 - Cached

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jQuery Desktop - 5 views

desktop.sonspring.com jquery desktop html5

shared by yc c on 22 Mar 10 - Cached

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3 reasons why you should let Google host jQuery for you | Encosia - 3 views

encosia.com/...let-google-host-jquery-for-you development javascript jquery

shared by Julian Knight on 09 Oct 09 - Cached

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JavaScript setTimeout Function - JavaScript Timing Events - 0 views

www.bloggingdeveloper.com/...-JavaScript-Timing-Events.aspx javascript function callback time

shared by Javier Neira on 14 Nov 09 - Cached

Javier Neira on 14 Nov 09

JavaScript setTimeout Function - JavaScript Timing Events November 16, 2007 by Blogging Developer JavaScript features a couple of methods that lets you run a piece of JavaScript code (javascript function) at some point in the future. These methods are: * setTimeout() * setInterval() In this tutorial, I'll explain how setTimetout() method works, and give a real world example. You may find the details of setInterval() method in JavaScript setInterval Function - JavaScript Timing Events setTimeout() window.setTimeout() method allows you to specify a piece of JavaScript code (expression) will be run after specified number of miliseconds from when the setTimeout() method is called. Syntax var t = setTimeout ( expression, timeout ); The setTimeout() method returns a numeric timeout ID which can be used to refer the timeout to use with clearTimeout method. The first parameter (expression) of setTimeout() is a string containing a javascript statement. The statement could be a call to a JavaScript function like "delayedAlert();" or a statement like "alert('This alert is delayed.');". The second parameter (timeout), indicates the number of miliseconds to pass before executing the expression. Example An alert box will be shown 5 seconds later when you clicked the button. clearTimeout() Sometimes it's useful to be able to cancel a timer before it goes off. The clearTimeout() method lets us do exactly that. Its syntax is: clearTimeout ( timeoutId ); where timeoutId is the ID of the timeout as returned from the setTimeout() method call.

<div class="cArrow"> </div><div class="cContentInner">JavaScript setTimeout Function - JavaScript Timing Events November 16, 2007 by Blogging Developer JavaScript features a couple of methods that lets you run a piece of JavaScript code (javascript function) at some point in the future. These methods are: * setTimeout() * setInterval() In this tutorial, I'll explain how setTimetout() method works, and give a real world example. You may find the details of setInterval() method in JavaScript setInterval Function - JavaScript Timing Events setTimeout() window.setTimeout() method allows you to specify a piece of JavaScript code (expression) will be run after specified number of miliseconds from when the setTimeout() method is called. Syntax var t = setTimeout ( expression, timeout ); The setTimeout() method returns a numeric timeout ID which can be used to refer the timeout to use with clearTimeout method. The first parameter (expression) of setTimeout() is a string containing a javascript statement. The statement could be a call to a JavaScript function like "delayedAlert();" or a statement like "alert('This alert is delayed.');". The second parameter (timeout), indicates the number of miliseconds to pass before executing the expression. Example An alert box will be shown 5 seconds later when you clicked the button. clearTimeout() Sometimes it's useful to be able to cancel a timer before it goes off. The clearTimeout() method lets us do exactly that. Its syntax is: clearTimeout ( timeoutId ); where timeoutId is the ID of the timeout as returned from the setTimeout() method call. </div>

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