Question

I have an AJAX post function I am using for my communication between my server & the users webpage. But I have a problem with dealing with the response from the server in my resuable function.

If you see the following example you will see that the function outputEmailDetailsResult() is called & run on the line “post( “forgotPass.py”, emailParam, outputEmailDetailsResult() );”, ie, when the function is passed as a param. But it should only run inside the post function in reation to the server response:

var xmlhttp;
    function post( dest, params, callbackFunction )
{
    if (window.XMLHttpRequest)
    {// code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
    }
    else
    {// code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }

    xmlhttp.onreadystatechange = function()
    {
        if (xmlhttp.readyState==4 && xmlhttp.status==200)
        {
            callbackFunction;
        }
    }

    xmlhttp.open("POST",dest,true);
    xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
    xmlhttp.send( params );  // "fname=Henry&lname=Ford");
}


    function emailLoginDetails()
{
    var emailParam = "email=sam"; // + document.getElementById( "forgotEmail" ).innerText;
    alert( emailParam );
    post( "forgotPass.py", emailParam, outputEmailDetailsResult() );
}

function outputEmailDetailsResult()
{
    try
    {
        document.getElementById( "statusOutput" ).value = xmlhttp.responseText;
    }
    catch ( ex )
    {
        document.getElementById( "statusOutput" ).value = "Failed to get response from server";
    }
}

I am new to passing functions as a parameters(I think they’re called callbacks?) & I am unsure I am doing it correctly?

Answer 1

Move the parentheses from the callback parameter to the function. Like this.

if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
  callbackFunction();
}

...

post( "forgotPass.py", emailParam, outputEmailDetailsResult);

What you did was calling outputEmailDetailsResult() before post, so you were actually passing the result of the outputEmailDetailsResult() function as the parameter rather than the function itself as the parameter.