javascript - Post Ajax function to return true or false

use callback

function validateLogin(uid, pass, i, callback) {
    var uvalue = uid.value;
    var pvalue = pass.value;
    var xmlhttp;
    if (window.XMLHttpRequest)
    {// code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp = new XMLHttpRequest();
    }
    else
    {// code for IE6, IE5
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.open("POST", "validateCheck.php", true);
    xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xmlhttp.onreadystatechange = function ()
    {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200)
        {
            if (xmlhttp.responseText == "Error")
                callback(false);
            else if (xmlhttp.responseText == "Yes")
                callback(a);
            else if (xmlhttp.responseText >= 1 && xmlhttp.responseText <= 5)
                callback(true);
            else
                callback(true);
        }
    };
    xmlhttp.send("uid=" + uvalue + "&pass=" + pvalue);
}

and call your function this way

function validateForm(i){
   var uid = document.myform1.username;
   var pass = document.myform1.password;
    validateLogin(uid, pass, i, function (resp) {
        if (resp) {
            alert(resp);
        }
    });
}

using Jquery Deffered Object

function validateLogin(uid, pass, i) {
   var result = jQuery.Deferred();
   ...
    if (xmlhttp.responseText == "Error")
       result.value = false;
    else if (xmlhttp.responseText == "Yes")
       result.value = a;
   ...
   xmlhttp.send("uid=" + uvalue + "&pass=" + pvalue);
   return result.value;
}