I'm working on a jQuery widget that attaches events to the widget's parent, but I'm unable to tell if it has a parent.
For example;
var x = $('<div>');
x.mywidget();
........... in mywidget
_create : function () {
var y = this.element.parent() === undefined ? this.element : this.element.parent();
y.bind(....);
}
I need to check if the widget has been added to the DOM before I do the bind statement. If it has not been added to the DOM, then I'll just bind this.element.bind(....) instead.
The problem is that $('<div>').parent() returns a jQuery object! I was expecting that it would return undefined.
So I'm wondering what parent could it be returning when it shouldn't have a parent?
You may use myDiv.parent().length to know if the jQuery set is empty or not.
But this will yield false positives if the object wasn't removed from the DOM directly but it parent was.
If you want a reliable detection, then, you should use jQuery.contains(document.documentElement, myDiv).
It will always return an object. If you want to see whether anythings in the object, you can check for .length == 0, so $("<div>").parent().length == 0 would be your check.
Check the length of the jQuery object returned. If your div has no parent, the jQuery object returned by .parent will wrap zero elements.
All jQuery DOM searching and manipulation methods return a jQuery collection with 0 or more elements. $("<div>").parent() returns a collection with no elements (an empty collection). You can still call any jQuery method on it, but without being tied to a DOM element what you can do is very limited. It will have .length of zero, and the callback will not be reached when iterating over with .each.
I would check the length of the jQuery object since jQuery will always return an object.
it will be always a parent, can be the body for example
use jquery data for example to set your init marker
var wasInit = ( $(this).data("mypluginwasinit") !== undefined );
if(wasInit) return;
$(this).data("mypluginwasinit","yes");
Related
I understand how the pushstack function works and I also understand how to use it for my plugins (I guess that's what its most used for , just for internal use and for end() to function properly and other similar methods) .
now below is the jquery source of pushstack , have a look :
pushStack: function( elems ) {
// Build a new jQuery matched element set
var ret = jQuery.merge( this.constructor(), elems );
// Add the old object onto the stack (as a reference)
ret.prevObject = this;
ret.context = this.context;
// Return the newly-formed element set
return ret;
},
theres a lot going on in that function really and I kind of get most part of it , but I have a small problem , understanding the below lines of code :-
ret.prevObject = this;
ret.context = this.context;
what is prevObject and context ? can somebody give me a clue , it definitely does't seem to be a javascript thing ?
Basically prevObject is used to allow flexibility for the jquery selectors chaining syntax.
Most of jQuery's DOM traversal methods operate on a jQuery object instance and produce a new one, matching a different set of DOM elements. When this happens, it is as if the new set of elements is pushed onto a stack that is maintained inside the object. Each successive filtering method pushes a new element set onto the stack
Everytime you make a new filter a new jquery instance is constructed matching your selector and the previous one is stored in it to allow the .end() and the addBack() functions to work (most of the DOM transversal functions use pushStack internally). If this property were not used every selector will forget about the previous one and it will not behave like a stack. Think parent instead of prevObject and it will make it easier to understand.
The context property is deprecated in jQuery 1.10 and only used for supporting live() method but it must be added because the returning object is constructed merging an empty jQuery constructor as first parameter and a set of elements as the second.
var ret = jQuery.merge( this.constructor(), elems );
Because .merge returns the first array modified it's context property might not have the correct value so is overwritten with the correct one this.context
Check this jsfiddle and open the console. You will see that the first value of prevObject is the document as we are making a new selector and then is the ul element since we filter using find. Moreover you can go to the last filter and lookup the whole chain of selectors up to the document again.
I recommend you to use the JQuery API instead of this property as reference in production but this will allow you for example to know the result of all the selectors that have been applied to obtain a given set of DOM elements.
I have to set a specific value to an element if the element is exists.
var a = jQuery("#abc");
if(a) {
a.val("something");
}
For this, I've to check a.length to check the element is exits.
What happen if I directly set the value without checking the element is present or not?
Because, If I do the following
jQuery("#abc").val("dfd");
I don't get any error in chrome when the element is not present. So, can I continue to use like this?
or
any workaround?
Help appreciated!
What happen if I directly set the value without checking the element is present or not?
Nothing. Calling jQuery methods on an empty jQuery object (set) doesn't cause a problem, it just does nothing. This is one of the great things about the set-based concept used in jQuery. The equivalent DOM code (document.getElementById("abc").value = "something";) would throw an error, but the jQuery version doesn't.
Specifically, if the jQuery set is empty:
Calling setter methods (like your val call) becomes a no-op.
Calling getter methods — for instance, var x = $("#foo").val(); — returns the value undefined.
Calling traversal methods — for instance, var divs = $("#foo).find("div"); — gives you a new empty set.
You only need to check (using if (a.length) as you said, or if (a[0])) if you actually care.
jQuery("#abc").val("dfd");
I don't get any error in chrome when the element is not present. So, can I continue to use like this?
Yup.
jQuery's val() method simply sets (or gets) the value of each matching element. If there are no matching element, there will be no value to set (or get). You don't need to check if the element exists first.
From jQuery's val() documentation:
Description: Set the value of each element in the set of matched elements.
If there are no matched elements, nothing will happen.
Try with -
jQuery("#abc").length > 0
Yes, you can safely continue. JQuery just executes a function on all elements found by the selector - if there are none, it does nothing. There's no error.
I'm writing a general purpose helper function in JavaScript. It accesses the className property of the element.
But since I'm using jQuery, it turns out that sometimes the element passed as an argument is actually an array (a jQuery set).
What is the simplest way to use an argument if it's an element or get the first element from the set if the argument is a set? Does jQuery have a tool for this?
You can test if the object is a jQuery object by using instanceof:
if (myObject instanceof jQuery) {
// use $.each to loop over myObject and get class names of all
// or just use myObject[0].className to return only the first
} else {
// it's not a jQuery object
}
You always want a native element? Each jQuery object has a property jquery (containing the jQuery version) so you can use it to check if the element you got is likely to be a jQuery object:
obj = obj.jquery ? obj[0] : obj;
If you are not into ducktyping:
obj = (obj instanceof jQuery) ? obj[0] : obj;
In case you want to pass a single element, an array of elements or a jQuery object to the same method, do the following:
function stuff(element) {
element = [].concat(element)[0];
}
If you have to deal with both jQuery objects and non-jQuery elements, this is a bit tougher. Keep in mind that the jQuery collection can possibly have different classNames for each element, i.e. the first and others may not be the same.
It's slightly inefficient, but this will work:
function (obj) {
var className = $(obj).get(0).className;
}
Wrapping a jQuery object in jQuery() does nothing, but wrapping the element allows you to access it again via .get. It's inefficient because you may have an extra $() call.
If that's no good, you could check obj.hasOwnProperty('className') since a jQuery object usually should not have it, but I think that is riskier.
Is a jQuery Object an Element or an Array of Elements?
Strictly, neither.
A jQuery object is a Javascript object that has some of the Array capabilities. In most aspects you can use it as an array of elements.
If the jQuery object may contain several elements, and you only want the first, you can use the first method to get that:
elements = elements.first();
If the parameter can be either an element, an array of elements, or a jQuery object, you can wrap it in a jQuery object to handle all the cases:
elements = $(elements).first();
Given the following code, why does the selector property work in the first instance but not the second? Aren't they both jQuery objects?
<span class='tst'>span</span>
var tst = $('.tst');
console.log(tst.selector);
// prints '.tst'
$('.tst').each(function() { console.log(this.selector);});
// prints undefined
this, in the context of the .each() loop, is not a jQuery object, so the selector property is undefined.
You need to make it a jQuery object first: $(this).selector
However, it should be noted that the selector property will return an empty string while inside the .each() loop.
Edit
If you absolutely need the selector property within the .each(), one option would be to cache your selector:
var cached = $('.tst');
cached.each(function() {
console.log(cached.selector); // Will display ".tst"
});
this != $(this)
In your first case tst is a reference to the jQuery object, but in the second this is simply the corresponding DOM element.
Within an .each() loop the .selector property is not available, however. To access '.tst' you can do $(this).attr("class") (when you use a class selector) -- though if you already use it in the each you can just cache it in a variable before hand.
Note that this will return all of the classes for that elements, so you can parse it later if it has more than one.
The best workaround based on your exact description is this:
var $tst = $(".tst");
$tst.each(function() {
console.log($tst.selector); //prints .tst
});
However I can't see any reason why you really need to do this.
Working Demo http://jsfiddle.net/wA6Yv/ or http://jsfiddle.net/a3CYR/2/
this != $(this)
If you keen: jQuery: What's the difference between '$(this)' and 'this'?
code
var tst = $('.tst');
console.log(tst.selector);
// prints '.tst'
$('.tst').each(function() {
alert($(this).text());
});
// prints undefined
I have a function that gets passed the document object like:
toggle( $('username') );
function Toggle(id)
{
/// ??
}
How can I get the actual name of the object passed i.e. username?
If I understand you correctly, can't you:
$(id).attr("id");
or
$(id).attr("name");
Or am I mistaken?
Either what Jonathon said or with
$(id).val();
The object that gets passed in is an instance of the jQuery object, which contains a set of elements (in this case with only one element in it). The documentation is on the jQuery documentation site.
The jQuery object that $() returns, wraps one or more DOM elements.
Using the accessors $().attr(), $().val(), $().text() and $().html() will act on the first of those wrapped elements.
If you want to drop out of jQuery mode so that you can work with the native DOM element (sometimes useful) use $().get( index ).
eg
var el = $('#mytextbox').get(0);
el.value = 'a new value';
alert(el.id);
etc...