ajax cannot get php return value

The solution to the problem that ajax cannot get the php return value:

1. First check whether ajax is It is asynchronous. Async cannot directly return the result.

/*常见错误示例  直接在 ajax 里面return 结果*/

 function demo(){
     $.ajax({
         url : 'test.do',
         type : "post",
         data : {},
         async : false,
         success : function(data) {             return 2;
         }
     });
 }/* 结果  无返回 */

2. Then ensure that ajax does not return data in the nested function. The function that calls ajax should return the data.

/**
 * （1）同步调用 （2）且在ajax对全局变量进行设值 (3)ajax函数外将变量return
 * 结果：返回 2。成功获取返回值
 * 成功原因：先执行result = 2;再往下执行return result;
 */
function demo1(){
    var result = 1;
    $.ajax({
        url : 'test.do',
        type : "post",
        data : {},
        async : false,
        success : function(data) {
            result = 2;
        }
    });
    return result;  //2
}

3. It can run normally. However, changing ajax to a synchronous request will cause blocking; ajax requires an asynchronous request.

/**
  * 添加async:true.即修改为异步
 * 结果以callback的形式回调
  */
 function demo1(params,callback){
    var result = 1;
     $.ajax({
        url : 'test.do',
         type : "post",
         data : {"params ":params },
         async : true,
         success : function(data) {
             result = 2;
             callback(result);
         }
     });
 }    

demo1("Value",function(rs){
   //do someting
})

Note: The php requested by ajax requires echo or print data, otherwise ajax will not be able to obtain the data.

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