What is a callback function?

A callback function gets called later, at an unknown time. It's old terminology which harkens back to the first GUI's. So your program fires and all sorts of things are created and initialized.    On the screen is a Save button. You would assign a callback function to the Save button, to be executed when it's pressed. Callbacks may be invoked by user-events like a click, timers, or lots of other mechanisms. Another example is you may do something which takes some time, like an Ajax call to a web server. You don't want to wait around being unresponsive while the call completes, so you fire it asynchronously with a callback. Meaning, your JS continues immediately after submitting the request.    Later, when your Ajax call has the data you want, your callback is invoked. Callbacks are a way to handle events which occur at unknown times.

There is a simple example in JS: function mySandwich(param1, param2, callback) { alert("There we start to eat sandwich. Parameters: " + param1 + " , " + param2); callback(); } mySandwich("ham", "cheese", function() { alert("Finish eating our sandwich"); }); There we have function mySandwich which takes three parameters. The third one is callback function. While main function is executing it generates message with output parameters. And then callback function executes. The code of the callback function is defined in the third argument of the main function. The callback function just displays a message that it's been executed. Sorry for my English if I made mistakes.

The logic of callback functions remains the same irrespective of the language. I will explain it using C. A callback function is one which is passed as an argument to another  function and is invoked after the completion of the parent function. In other words callback is a piece of executable code that is passed as an argument to other code, which is expected to call back (execute) the argument at some convenient time. The invocation may be immediate as in a synchronous callback or it might happen at later time, as in an asynchronous callback. #include <stdio.h> int a(int (*callback)(void)) { printf("inside parent function"); callback(); } int test() { printf("inside callback function"); } int main (void) { a(&test); printf("main gotta end"); return 0; } In this example what we want to do is to call our callback function test  when the function a is finished with its execution. What we do is that  we pass the address of our callback function test to the function a .  After we are done with the code of function a we call the callback  function by calling callback(), internally this function will call our  required callback function test as we have passed it as an argument. Hope it helps to clear your understanding regarding callback functions. This concept is very useful for application as well as system developers.

The answer is explained with the help of JavaScript code. Whenever we set up a function to be called at a later time, whether by the browser or other code, we’re setting up what is termed a callback. The term stems from the fact that we establish a function that some other code will later “call back” into at an appropriate point of execution. function useless(callback) { // few lines of code... return callback(); } function myCallback(){ console.log('Thats the way love goes ..'); } useless(myCallback); In this case myCallback is a function which is passed as a parameter to the useless function. At the end of useless function we have invoked the callback passed as parameter. This is taken from the book : by .

An 8-year old explanation could go like this : A callback function is a function that says “I am just a simple function. Please call me when a complex function completes its work. Just put my name in the complex function’s argument”.Lets say your complex function is named computeSummary(). This function’s job is to create a summary report of your company’s 1 billion users. In the real world, the processing of this complex function can take several minutes/hours. But immediately after your complex function’s computation is done, lets say you need to send(email) the results to your company’s CEO. For this you can write a simple sendSummary() function. This simple sendSummary function is an example of a callback function.Its like saying : Execute the complex function. Immediately after that, execute the simple function.In asynchronous programming, the setup (“workflow”) could look like the below : Run your complex computeSummary function (create event1) Run another complex computeSummary function (create event2) Event 2 is done. So run the simple sendSummary function (create event3) Event 1 is done. So run the simple sendSummary function (create event4) As you can see above, event2 could get completed before event1 (asynchronous events). So the simple “callback” function sendSummary gets executed (event3) immediately after complex event2.Typical (newbie) example of callback functions are : print or display something on screen commit or save something to a database send email to users log errors to a file

The mechanism of calling a function from another function by passing its memory address is known as callback function. Callbacks are achieved in java with the help of inerface references.

I have tried to explain it here. https://knownodejs.quora.com/Callback-functions