A good rule of thumb is to avoid touching host objects as much as possible. Trying to base architecture on something that—by definition—can behave so sporadically is hardly a good idea. Chance of collisions api based on dom element extensions is hard to scale. It's hard to scale for developers of the library—when adding new or changing core api methods, and for library users—when adding domain-specific extensions. The root of the issue is a likely chance of collisions. Dom implementations in popular browsers usually all have properietary api's. What's worse is that these api's are not static, but constantly change as new browser versions come out.
Aloysius Institute of Technology
In Internet Explorer, we can easily observe exactly this—an example of host object Get throwing error: eateElement. OffsetParent; / "Unspecified error." new Activexobject nd; / "Object doesn't support this essay property or method" Extending dom objects is kind of like walking in a minefield. By definition, you are working with something that's allowed to behave in unpredictable and completely erratic way. And not only things can blow up; there's also a possibility of silent failures, which is even worse scenario. An example of erratic behavior is applet, object and embed elements, which in certain cases throw errors on assignment of properties. Similar disaster happens with xml nodes: var xmlDoc new o 'bar / "Object doesn't support this property or method" There are other cases of failures in ie, such as yleSheets99999 throwing "Invalid procedure call or argument" or lters throwing "Member not found." exceptions. But not only mshtml dom is the problem. Trying to overwrite "target" property of event object in mozilla throws TypeError, complaining that property has only a getter (meaning that it's readonly and can not be set). Doing same thing in WebKit, results in silent failure, where "target" continues to refer to original object after assignment. When creating api for working with event objects, there's now a need to consider all of these readonly properties, instead of focusing on concise and descriptive names.
Host objects have no rules Next problem with dom extension is that dom objects are host objects, and host objects are the worst friendship bunch. By specification (ecma-262 3rd. Ed host objects are allowed to do things, no other objects can even dream. To" relevant section.6.2 : Host objects may implement these internal methods with any implementation-dependent behaviour, or it may be that a host object implements only some internal methods and not others. The internal methods specification talks about are get, put, delete, etc. Note how it says that internal methods behavior is implementation-dependent. What this means is that it's absolutely normal for host object to throw error on invocation of, say, get method. And unfortunatey, this isn't just a theory.
Js has ever done. So what are these problems? Lack of specification As I have already mentioned, surgery exposure of "prototype objects" is not part of any specification. Dom level 2 merely defines interfaces and their inheritance relations. In order for implementation to conform to dom level 2 fully, there's no need to expose those global Node, element, htmlelement, etc. Neither is there a requirement to expose them in any other way. Given that there's always a possibility to extend dom objects manually, this doesn't seem like a big issue. But the truth is that manual extension is a rather slow and inconvenient process (as we will see shortly). And the fact that fast, "prototype object" -based extension is merely somewhat of a de-facto standard among few browsers, makes this practice unreliable when it comes to future adoption or portability across non-convential platforms (e.g.
Will, dom value property assignment invoke toString?
This is because anchor elements prototype chain never includes object refered to by ototype, but instead includes that referred to by ototype. To fix this, we can assign to property of object positioned further in the prototype chain, such as that referred to by ototype, ototype or ototype. Similarly, creating a property on ototype would not make it available on all promotional nodes, but only on nodes of element type. If we wanted to have property on all nodes (e.g. Text nodes, comment nodes, etc. we would need to assign to property of ototype instead.
And speaking of text and comment nodes, this is how interface inheritance usually looks for them: eateTextNode foo now, it's important to understand that exposure of these dom object prototypes is not guaranteed. Dom level 2 specification merely defines interfaces, and inheritance between those interfaces. It does not state that there should exist global Element property, referencing object that's a prototype of all objects implementing Element interface. Neither does it state that there should exist global Node property, referencing object that's a prototype of all objects implementing Node interface. Internet Explorer 7 (and below) is an example of such environment; it does not expose global Node, element, htmlelement, htmlparagraphElement, or other properties. Another such browser is Safari.x (and most seeing likely safari.x). So what can we do in environments that don't expose these global "prototype" objects?
It is then invoked directly on an element, and elements display style is set to none. The reason this works is because object referred to by ototype is actually one of the objects in prototype chain of p element. When hide property is resolved on it, its searched throughout the prototype chain until found on this ototype object. In fact, if we were to examine prototype chain of p element in some of the modern browsers, it would usually look like this: / " denotes connection between objects in prototype chain eateElement p ototype ototype ototype ototype ototype null. Note how the nearest ancestor in the prototype chain of p element is object referred to by ototype.
This is an object specific to type of an element. For p element, its ototype; for div element, its ototype; for a element, its ototype, and. But why such strange names, you might ask? These names actually correspond to interfaces defined in dom level 2 html specification. That same specification also defines inheritance between those interfaces. It says, for example, that htmlparagraphElement interface have all properties and functions of the htmlelement interface ( source ) and that htmlelement interface have all properties and functions of the Element interface ( source and. Quite obviously, if we were to create a property on prototype object of paragraph element, that property would not be available on, say, anchor element: ototype. Hide; / "undefined" typeof eateElement.
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Id like to explain why extending dom is generally a bad idea, by showing some of the problems associated with. Well also look at possible alternatives to this harmful exercise. But first of all, what exactly is dom extension? And how does it all work? How dom extension works, dom extension is simply the process of adding custom methods/properties to dom objects. Custom properties are those that dont exist in simple a particular implementation. And what are the dom objects?
Declarative behavior assignment by css
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