About $.ajax callback function cannot return boolean value solution

function tjm1(){
            var tjm = $("#tjzh");
            var tjmid = $("#tjzhId");
            if(tjm.val() != ""){
                $.ajax({
                    type : "get",
                    async : false,
                    url:"${pageContext.request.contextPath}/pcUser/checkUserName",
                    data :
                    {
                        "userName" : tjm.val()
                    },
                    dateType:'html',
                    success : function(data)
                    {
                        if (data == "0")
                        {
                            tjmid.className="error_prompt";
                            tjmid.html("Incorrect input! This user does not exist.");
                            return false;//Can't get
                        }
                        tjmid.className="ok_prompt";
                        tjmid.html("Correct input! This user exists.");
                        return true;//Can't get
                    }
                });
            }
        }

 -----------------------------******************correct answer** ***************** ---------------------------

function tjm2(){
            var tjm = $("#tjzh");
            var tjmid = $("#tjzhId");
            var flag = false;//declare a variable
            if(tjm.val() != ""){
                $.ajax({
                    type : "get",
                    async : false,
                    url:"${pageContext.request.contextPath}/pcUser/checkUserName",
                    data :
                    {
                        "userName" : tjm.val()
                    },
                    dateType:'html',
                    success : function(data)
                    {
                        if (data == "0")
                        {
                            tjmid.className="error_prompt";
                            tjmid.html("Incorrect input! This user does not exist.");
                        }
                        tjmid.className="ok_prompt";
                        tjmid.html("Correct input! This user exists.");
                        flag = true;//You can get
                    }
                });
            }
            return flag;
        }

    

function checkForm()
        {
            var flagUserName=userNameBlur();
            var flagPwd = pwdBlur ();
            var flagRepwd = repwdBlur ();
            var flagTel = telBlur ();
            var flagEmail=emailBlur();
            var flagVerificationCode = verificationCodeBlur ();
            var flagVerificationCheckBox = verificationCheckBox ();
            var flagtjm1 = tjm1();//Unable to return the boolean value of the function
            var flagtjm = tjm2();//The boolean value returned by the function can be obtained
            if(flagVerificationCheckBox==true){
                if(flagPwd==true &&flagRepwd==true&&flagTel==true
                        &&flagEmail==true &&flagVerificationCode==true&&flagtjm==true)
                {
                    return true;
                }
                else
                {
                    alert("Please fill in according to the prompt information!");
                    return false;
                }
            }else{
                alert("Please read <<Registration Terms and Privacy Protection>>, if you have read it, please tick!");
                return false;
            }
        }

 

 The search is always varied, and I always know the results after my own experiments. I still don’t know why this happens, and I can’t get to the bottom of it.