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JavaScript code
/ /Get the number of select items
jQuery.fn.size = function()
{
return jQuery(this).get(0).options.length;
}
// Get the index of the selected item
jQuery.fn.getSelectedIndex = function()
{
return jQuery(this).get(0).selectedIndex;
}
//Get the currently selected item The text of
jQuery.fn.getSelectedText = function()
{
if(this.size() == 0)
{
return "No options in the drop-down box";
}
else
{
var index = this.getSelectedIndex();
return jQuery(this).get(0).options[index].text;
}
}
//Get the value of the currently selected item
jQuery.fn.getSelectedValue = function()
{
if(this.size() == 0)
{
return "There is no selected value in the drop-down box";
}
else
{
return jQuery(this).val();
}
}
//Setting select The item with value value is selected
jQuery.fn.setSelectedValue = function(value)
{
jQuery(this).get(0).value = value;
}
// Set the first item with text in select to be selected
jQuery.fn.setSelectedText = function(text)
{
var isExist = false;
var count = this.size();
for(var i=0;i
if(jQuery(this).get(0).options[i].text == text)
{
jQuery(this).get(0).options[i].selected = true;
isExist = true;
break;
}
}
if(!isExist)
{
alert("This item does not exist in the drop-down box");
}
}
//Set the selected index item
jQuery.fn.setSelectedIndex = function(index)
{
var count = this.size();
if(index >= count || index {
alert("The selected item index is out of range");
}
else
{
jQuery(this).get(0).selectedIndex = index;
}
}
//Judge whether there is a value in the select item. Item of value
jQuery.fn.isExistItem = function(value)
{
var isExist = false;
var count = this.size();
for(var i=0; i
if(jQuery(this).get(0).options[i].value == value)
{
isExist = true;
break ;
}
}
return isExist;
}
//Add an item to the select, the display content is text and the value is value. If the item value already exists, it will prompt
jQuery.fn.addOption = function(text,value)
{
if(this.isExistItem(value))
{
alert("The value of the item to be added already exists");
}
else
{
jQuery(this).get(0).options.add(new Option(text,value));
}
}
/ /Delete the item with value in the select. If the item does not exist, prompt
jQuery.fn.removeItem = function(value)
{
if(this.isExistItem(value))
{
var count = this.size();
for(var i=0;i
if(jQuery(this).get(0).options[ i].value == value)
{
jQuery(this).get(0).remove(i);
break;
}
}
}
else
{
alert("The item to be deleted does not exist!");
}
}
//Delete the item with the specified index in select
jQuery.fn.removeIndex = function(index)
{
var count = this.size();
if(index >= count || index {
alert("Item to be deleted Index out of range");
}
else
{
jQuery(this).get(0).remove(index);
}
}
//Delete Selected items in select
jQuery.fn.removeSelected = function()
{
var index = this.getSelectedIndex();
this.removeIndex(index);
}
//Clear all items in the select
jQuery.fn.clearAll = function()
{
jQuery(this).get(0).options.length = 0;
}
When using it, first import the jquery.js file, then import the jquery.liu.select.js file, and then call the plug-in method. For example, if I want to clear all the items in the drop-down box with the ID selEmail, then I can do this: $("#selEmail").clearAll();
Note: The method in this plug-in is available in ie7 and firefox The verification has been passed. If there are any errors or areas that need improvement, I hope you will criticize and correct them.

Yes, the engine core of JavaScript is written in C. 1) The C language provides efficient performance and underlying control, which is suitable for the development of JavaScript engine. 2) Taking the V8 engine as an example, its core is written in C, combining the efficiency and object-oriented characteristics of C. 3) The working principle of the JavaScript engine includes parsing, compiling and execution, and the C language plays a key role in these processes.

JavaScript is at the heart of modern websites because it enhances the interactivity and dynamicity of web pages. 1) It allows to change content without refreshing the page, 2) manipulate web pages through DOMAPI, 3) support complex interactive effects such as animation and drag-and-drop, 4) optimize performance and best practices to improve user experience.

C and JavaScript achieve interoperability through WebAssembly. 1) C code is compiled into WebAssembly module and introduced into JavaScript environment to enhance computing power. 2) In game development, C handles physics engines and graphics rendering, and JavaScript is responsible for game logic and user interface.

JavaScript is widely used in websites, mobile applications, desktop applications and server-side programming. 1) In website development, JavaScript operates DOM together with HTML and CSS to achieve dynamic effects and supports frameworks such as jQuery and React. 2) Through ReactNative and Ionic, JavaScript is used to develop cross-platform mobile applications. 3) The Electron framework enables JavaScript to build desktop applications. 4) Node.js allows JavaScript to run on the server side and supports high concurrent requests.

Python is more suitable for data science and automation, while JavaScript is more suitable for front-end and full-stack development. 1. Python performs well in data science and machine learning, using libraries such as NumPy and Pandas for data processing and modeling. 2. Python is concise and efficient in automation and scripting. 3. JavaScript is indispensable in front-end development and is used to build dynamic web pages and single-page applications. 4. JavaScript plays a role in back-end development through Node.js and supports full-stack development.

C and C play a vital role in the JavaScript engine, mainly used to implement interpreters and JIT compilers. 1) C is used to parse JavaScript source code and generate an abstract syntax tree. 2) C is responsible for generating and executing bytecode. 3) C implements the JIT compiler, optimizes and compiles hot-spot code at runtime, and significantly improves the execution efficiency of JavaScript.

JavaScript's application in the real world includes front-end and back-end development. 1) Display front-end applications by building a TODO list application, involving DOM operations and event processing. 2) Build RESTfulAPI through Node.js and Express to demonstrate back-end applications.

The main uses of JavaScript in web development include client interaction, form verification and asynchronous communication. 1) Dynamic content update and user interaction through DOM operations; 2) Client verification is carried out before the user submits data to improve the user experience; 3) Refreshless communication with the server is achieved through AJAX technology.


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