require([
    "dijit/registry",
    "dojo/request",
    "dojo/Deferred"
], function(registry, request, Deferred) {

// NOTE: This is written as if it is not a module, but your main JS file that gets executed on page load...
//       checkPassword would NOT be a global method so you would be calling it from some sibling code inside this file...

function checkPassword(password) {
    var dfd = new Deferred();   // Create a deferred object that we will resolve at a later time
    console.log("c1: inside checkPassword()");
    request.post("checkPwd.php", {  // Make XHR Post request to checkPwd.php
        handleAs: "json",
        data: {
            'user': registry.byId("username").get("value"), // accessing .value isnt the best practice either, should almost always use the getter/setter
            'pwd': password
        }
    }).then(function(data) {        // Our XHR request completed... lets check the status and see if its a success or a failure
        console.log("c2: checkPassword() response", data);
        if (data.status == "true") {// Success!
            console.log ("true");
            dfd.resolve(data);      // Resolve our deferred object with the data from the XHR response incase we want to expose the results from the promise
        } else {                    // Failure!
            console.log ("false");
            dfd.reject(data);       // Reject our deferred object again with the data from the XHR response so that you could tailor an error message or something from it
        }
    });

    return dfd.promise;     // Return the deferred's promise which is then'able.
}

// For demo purposes I have added a console message in each stage of the Async process and I have numbered them in the order they are written in code just to help illustrate the round about way Async happens c# are generated by code in checkPassword and m# are from the code surrounding the checkPassword call

console.log("m1: before checkPassword()");
// NOTE: To use this method you would then do something like this... and since we used resolve and reject you could do this
checkPassword(yourPass).then(logThemIn, kickThemOut);   // What this means is if checkPassword success, pass its data to LogThemIn() else pass the failure data to kickThemOut()
console.log("m2: after checkPassword()");
});