19 classic interview questions for ajax

1. What is AJAX and why should we use Ajax (Please talk about your understanding of Ajax) What is ajax: AJAX is the abbreviation of "Asynchronous JavaScript and XML". It refers to a web development technology for creating interactive web applications. Ajax includes the following technologies: Based on web standards (standards-basedpresentation) XHTML CSS representation; Use DOM (Document Object Model) for dynamic display and interaction; Use XML and XSLT for data exchange and Related operations; Use XMLHttpRequest for asynchronous data query and retrieval; Use JavaScript to bind everything together.

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2. Why use ajax:The advantages of Ajax applications are: 1. Improve the user experience through asynchronous mode2. Optimize the transmission between the browser and the server, reduce unnecessary data round-trips, and reduce bandwidth usage3. The Ajax engine runs on the client and takes on part of the work originally performed by the server, thus reducing the server load under large user volumes. 2. What is the biggest feature of AJAX. Ajax can achieve dynamic non-refresh (partial refresh) That is, it can maintain data without updating the entire page. This allows web applications to respond more quickly to user actions and avoids sending unchanged information over the network. 3. Please introduce the XMLhttprequest object. The core of Ajax is the JavaScript object XmlHttpRequest. This object was first introduced in Internet Explorer 5 and is a technology that supports asynchronous requests. In short, XmlHttpRequest allows you to use JavaScript to make requests to the server and handle the responses without blocking the user. Through the XMLHttpRequest object, web developers can perform partial updates to the page after the page is loaded. 4. What are the components of the AJAX technology system. HTML, css, dom, xml, xmlHttpRequest, javascript5. What is the difference between AJAX applications and traditional Web applications. In traditional Javascript programming, if you want to get information from a server-side database or file, or send client information to the server, you need to create an HTML form and then GET or POST the data to the server. Users need to click the "Submit" button to send or receive data information, and then wait for the server to respond to the request and the page to reload. Because the server returns a new page every time, traditional web applications may be slow and user-unfriendly. Using AJAX technology, Javascript can interact directly with the server through the XMLHttpRequest object. Through HTTP Request, a web page can send a request to the web server and accept the information returned by the web server (without reloading the page). What is displayed to the user is still the same page. The user feels that the page is refreshed and cannot see it. Javascript sends requests and receives responses in the background. 6. How many types of CALLBACK are there in AJAX requests? Ajax requests have a total of eight CallbacksonSuccess onFailure onUninitialized onLoading onLoaded onInteractive onComplete onException7. The difference between Ajax and javascript. Javascript is a scripting language that is executed on the browser side. Ajax is a development technology for creating interactive web applications. It uses a series of related technologies, including javascript. Javascript is a scripting language developed by Netscape. It has nothing to do with Sun's Java language. Their similar names are just a marketing strategy. In general web development, javascript is executed on the browser side. We can use javascript to control the behavior and content of the browser. How information is transferred between the browser and the server in Ajax applications Through XML data or strings8. How to get the server-side response XML on the browser side data. The responseXMl attribute of the XMLHttpRequest object 9. Is there any difference in how XMLHttpRequest objects are created in IE and Firefox. Yes, it is obtained through new ActiveXObject() in IE, and it is obtained through newXMLHttpRequest() in Firefox10. Introduce the common methods and properties of XMLHttpRequest object. open("method","URL") establishes a call to the server. The first parameter is an HTTP request. The method can be GET, POST or any method you want to call supported by the server. The second parameter is the URL of the requested page. send() method, send a specific request abort() method, stop the current request readyState attribute The status of the request has 5 possible values 0=not initialized, 1=loading 2= Loading, 3=interaction, 4=complete responseText attribute The server's response, expressed as a string responseXML attribute The server's response, expressed as XML status The server's HTTP status code, 200 corresponds to ok 400 Corresponding to not found11. What is XML XML is an extended markup language that can describe data with a series of simple tags12. XML The parsing methods are commonly used to use dom parsing and sax parsing. DOM parsing is to read the XML file at one time and construct it into a DOM object for use by the program. The advantage is that it is easy to operate, but it consumes more memory. Sax is parsed in an event-driven manner, which takes up less memory, but is complicated to program. 13. What framework (package) are you using? This question is a must-ask , is usually asked at the very beginning. The more popular ones in java are dojo, Prototype, JQuery, Dwr, extjs, etc. 14. If you are familiar with a certain ajax framework, he may ask how to use it in the program. Kind of frameworkDWR framework introduction DWR (DirectWeb Remoting) is a WEB remote calling framework. Using this framework can make AJAX development very simple. Using DWR, you can use JavaScript on the client to directly call the server Java methods and return values to JavaScript are just like direct local client calls (DWR dynamically generates JavaScript code based on Java classes). The implementation principle of DWR is to translate java into javascript through reflection, and then use the callback mechanism, thus Implemented javascript calling Java code15、介绍一下Prototype的 $() 函数， "> () 函数， "> () 函数$

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$A() function can convert the single parameter it receives into an Array object.

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The XMLHttpRequest object is supported in IE5.0, Safari 1.2, Mozilla1.0/Firefox, Opera 8 and NetScapt7.

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AJAX technology is based on Javascript and HTTP Request.

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AJAX applications use web browsers that support the above technologies as the running platform. These browsers currently include: Mozilla, Firefox, Internet Explorer, Opera, Konqueror and Safari. But Opera does not support XSL format objects, nor does it support XSLT.

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