Question

I’m learning Ajax and I’m going through this example. What does this do? I don’t understand the syntax variable = function(){ how is a function being assigned to a variable?

xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
    }
  }

Answer 1

I know everyone is saying it’s a callback specifically but the question you asked could better be answered by comparing what you’re puzzled over with some more familiar code.

function myFunction()
{
    ...
}

So we know that calling myFunction() will run that code.

In Javascript, you can declare a function in a number of ways. This means that this:

var myFunction = function()
{
    ...
}

Does exactly the same as the first example above. It creates a function that you can call using myFunction().

Add the callback into the mix in your question and we can see that

xmlhttp.onreadystatechange = function()
{
    ...
}

is nothing more than assigning a function and containing code to the onreadystatechange property of object xmlhttp. Meaning that your code within the function will be called every time there is a state change in the xmlhttp object.