1:What is ajax? What is the function of ajax?

异步的javascript和xml  AJAX 是一种用于创建快速动态网页的技术。  ajax用来与后台交互

[Topic recommendation]:2020 ajax interview questions and answers (latest)

2: There are several steps for native js ajax requests ? What are they?

//创建 XMLHttpRequest 对象
var ajax = new XMLHttpRequest();
//规定请求的类型、URL 以及是否异步处理请求。
ajax.open('GET',url,true);
//发送信息至服务器时内容编码类型
ajax.setRequestHeader("Content-type", "application/x-www-form-urlencoded"); 
//发送请求
ajax.send(null);  
//接受服务器响应数据
ajax.onreadystatechange = function () {    if (obj.readyState == 4 && (obj.status == 200 || obj.status == 304)) { 
    }
};

3: json string conversion set json object, json object conversion json string

//字符串转对象
JSON.parse(json)eval('(' + jsonstr + ')')   
// 对象转字符串
JSON.stringify(json)

4: What are the several request methods of ajax? Their pros and cons?

Commonly used post, get, delete. Copy, head, link, etc. are not commonly used.

###代码上的区别
 1:get通过url传递参数
 2:post设置请求头  规定请求数据类型###使用上的区别
 1:post比get安全
 (因为post参数在请求体中。get参数在url上面)
 2:get传输速度比post快 根据传参决定的。
 (post通过请求体传参，后台通过数据流接收。速度稍微慢一些。而get通过url传参可以直接获取)
 3:post传输文件大理论没有限制  get传输文件小大概7-8k ie4k左右
 4:get获取数据	post上传数据
 (上传的数据比较多  而且上传数据都是重要数据。所以不论在安全性还是数据量级 post是最好的选择)

5: What situation causes cross-domain?

Same-origin policy restrictions Different sources will cause cross-domain. If any of the following situations are different, they are all from different sources.

http://www.baidu.com/8080/index.html

• Principle: Create a script tag dynamically. Using the src attribute of the script tag is not restricted by the same origin policy. Because all src attributes and href attributes are not restricted by the same origin policy. Can request third-party server data content.

• Steps:

To create a script tag
The src attribute of script sets the interface address
Interface parameters must bring a custom function name, otherwise the backend cannot return data.
Receive background return data by defining a function name

//去创建一个script标签
var  script = document.createElement("script");
//script的src属性设置接口地址 并带一个callback回调函数名称
script.src = "http://127.0.0.1:8888/index.php?callback=jsonpCallback";
//插入到页面
document.head.appendChild(script);
//通过定义函数名去接收后台返回数据function jsonpCallback(data){
    //注意  jsonp返回的数据是json对象可以直接使用
    //ajax  取得数据是json字符串需要转换成json对象才可以使用。
}

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• Principle: Server settings Access After the -Control-Allow-OriginHTTP response header, the browser will allow cross-domain requests

• Restrictions: The browser needs to support HTML5 and can support POST, PUT and other methods. It is compatible with ie9 and above

需要后台设置
Access-Control-Allow-Origin: *              //允许所有域名访问，或者
Access-Control-Allow-Origin: http://a.com   //只允许所有域名访问

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• Principle: For pages under the same main domain name but different subdomain names, you can set document.domain to make them the same domain

• Restrictions: Documents in the same domain provide interoperability between pages, and the iframe page needs to be loaded

// URL http://a.com/foo
var ifr = document.createElement('iframe');
ifr.src = 'http://b.a.com/bar'; 
ifr.onload = function(){
    var ifrdoc = ifr.contentDocument || ifr.contentWindow.document;
    ifrdoc.getElementsById("foo").innerHTML);
};

ifr.style.display = 'none';
document.body.appendChild(ifr);

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2xx (success) indicates a status code that successfully processed the request

200 (成功) 服务器已成功处理了请求。 通常。

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3xx (redirect) indicates that further action is required to complete the request. Typically, these status codes are used for redirects.

304 (未修改) 自从上次请求后，请求的网页未修改过。 服务器返回此响应时，不会返回网页内容。

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4xx (request error) These status codes indicate that the request may be in error, hindering the server's processing

1:400 (错误请求) 服务器不理解请求的语法。
 
2:403 (禁止) 服务器拒绝请求。

3:404 (未找到) 服务器找不到请求的网页。

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5xx (Server Error) These status codes indicate that an internal error occurred while the server was trying to process the request. These errors may be errors on the server itself, rather than request errors

500 (服务器内部错误) 服务器遇到错误，无法完成请求。

501 (尚未实施) 服务器不具备完成请求的功能。 例如，服务器无法识别请求方法时可能会返回此代码。

502 (错误网关) 服务器作为网关或代理，从上游服务器收到无效响应。

503 (服务不可用) 服务器目前无法使用(由于超载或停机维护)。 通常，这只是暂时状态。

504 (网关超时) 服务器作为网关或代理，但是没有及时从上游服务器收到请求。

505 (HTTP 版本不受支持) 服务器不支持请求中所用的 HTTP 协议版本。

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