Questions in: JAVA

Using jQuery length property, we can ensure whether element exists or not.
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$(document).ready(function(){
if ($('#element').length > 0){
//Element exists
}
});

jQuery .size() method returns number of element in the object. But it is not preferred to use the size() method as jQuery provide .length property and which does the same thing. But the .length property is preferred because it does not have the overhead of a function call.

There are 2 ways to check if element is empty or not. We can check using ":empty" selector.
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$(document).ready(function(){
if ($('#element').is(':empty')){
//Element is empty
}
});
And the second way is using the "$.trim()" method.
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$(document).ready(function(){
if($.trim($('#element').html())=='') {
//Element is empty
}
});

class selectors are the slow compare to ID and element.

Your last selectors is always executed first. For example, in below jQuery code, jQuery will first find all the elements with class ".myCssClass" and after that it will reject all the other elements which are not in "p#elmID".
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$("p#elmID .myCssClass");

Native JavaScipt is always fast. jQuery method to select txtName "$('#txtName')" will internally makes a call to document.getElementByID('txtName'). As jQuery is written on top of JavaScript and it internally uses JavaScript only So JavaScript is always fast.

this and $(this) refers to the same element. The only difference is the way they are used. 'this' is used in traditional sense, when 'this' is wrapped in $() then it becomes a jQuery object and you are able to use the power of jQuery.
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$(document).ready(function(){
$('#spnValue').mouseover(function(){
alert($(this).text());
});
});
In below example, this is an object but since it is not wrapped in $(), we can't use jQuery method and use the native JavaScript to get the value of span element.
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$(document).ready(function(){
$('#spnValue').mouseover(function(){
alert(this.innerText);
});
});

This will select all the div elements on page.

$('.selected'). This selector is known as class selector. We need to prefix the class name with "." (dot).

All the div element with parent class.

To select element use ID selector. We need to prefix the id with "#" (hash symbol). For example, to select element with ID "txtName", then syntax would be,
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$('#txtName')

To work with an element on the web page, first we need to find them. To find the html element in jQuery we use selectors. There are many types of selectors but basic selectors are:
Name: Selects all elements which match with the given element Name.
#ID: Selects a single element which matches with the given ID
.Class: Selects all elements which match with the given Class.
Universal (*): Selects all elements available in a DOM.
Multiple Elements E, F, G: Selects the combined results of all the specified selectors E, F or G.
Attribute Selector: Select elements based on its attribute value.

There are 3 popular jQuery CDNs.
1. Google.
2. Microsoft
3. jQuery.
Advantage of using CDN.
It reduces the load from your server.
It saves bandwidth. jQuery framework will load faster from these CDN.
The most important benefit is it will be cached, if the user has visited any site which is using jQuery framework from any of these CDN

jQuery library comes in 2 different versions Development and Production/Deployment. The deployment version is also known as minified version. So .min.js is basically the minified version of jQuery library file. Both the files are same as far as functionality is concerned. but .min.js is quite small in size so it loads quickly and saves bandwidth.

jQuery library comes in 2 different versions.
Development
Production/Deployment
The development version is quite useful at development time as jQuery is open source and if you want to change something then you can make those changes in development version. But the deployment version is minified version or compressed version so it is impossible to make changes in it. Because it is compressed, so its size is very less than the production version which affects the page load time.

To start with jQuery, one need to make reference of it's library. The latest version of jQuery can be downloaded from jQuery.com.

The starting point of jQuery code execution is $(document).ready() function which is executed when DOM is loaded.