callback function meaning

Question

What is the meaning of callback function in javascript.

Answer 1

It's just a name for a function that should be called back after something.

It's often used with XMLHttpRequest:

var x = new XMLHttpRequest();
x.onreadystatechange = function(){
    if(x.readyState == 4){
        callbackfunction(x.responseText);
    }
}
x.open('get', 'http://example.com/', true);
x.send(null);

callbackfunction is just a plain function, in this case:

function callbackfunction(text){
    alert("I received: " + text);
}