I am trying to understand callback functions in javascript.
There is a function something like
function load() {
var func = function(data){
///
};
}
Can anyone explain me from where the parameter "data" will be returned, as I dont see any variable declared in the file.
In JavaScript, functions are first class objects. You could store them in objects (variables) and pass them around as arguments to functions. Every function is actually a Function object.
You don't have a callback function in that example. You'd have one when you pass a function as an argument to another function.
This is a function that invokes a callback function when it is ready:
function load (ready_callback) {
// do some stuff
ready_callback();
}
Which can be invoked as follows:
function callback () {
alert('Ready');
}
load(callback);
Or:
var callback = function () {
alert('Ready');
}
load(callback);
Or:
load(function () {
alert('Ready');
});
The above three example are pretty much equivalent.
Callback functions are typically used for event handling and asynchronous methods. One example is the setTimeout() method, which invokes a callback function when a timeout expires:
var timerCallback = function () {
alert('Timeout Expired!');
}
setTimeout(timerCallback, 5000); // Set the timeout to 5 seconds