Getting a better understanding of callback functions in JavaScript

Getting a better understanding of callback functions in JavaScript

I understand passing in a function to another function as a callback and having it execute, but I’m not understanding the best implementation to do that. I’m looking for a very basic example, like this:
var myCallBackExample = {
myFirstFunction : function( param1, param2, callback ) {
// Do something with param1 and param2.
if ( arguments.length == 3 ) {
// Execute callback function.
// What is the “best” way to do this?
mySecondFunction : function() {
myFirstFunction( false, true, function() {
// When this anonymous function is called, execute it.

In myFirstFunction, if I do return new callback(), then it works and executes the anonymous function, but that doesn’t seem like the correct approach to me.


Solution 1:

You can just say


Alternately you can use the call method if you want to adjust the value of this within the callback. newValueForThis);

Inside the function this would be whatever newValueForThis is.

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Solution 2:

You should check if the callback exists, and is an executable function:

if (callback && typeof(callback) === "function") {
    // execute the callback, passing parameters as necessary

A lot of libraries (jQuery, dojo, etc.) use a similar pattern for their asynchronous functions, as well as node.js for all async functions (nodejs usually passes error and data to the callback). Looking into their source code would help!

Solution 3:

There are 3 main possibilities to execute a function:

var callback = function(x, y) {
    // "this" may be different depending how you call the function
  1. callback(argument_1, argument_2);
  2., argument_1, argument_2);
  3. callback.apply(some_object, [argument_1, argument_2]);

The method you choose depends whether:

  1. You have the arguments stored in an Array or as distinct variables.
  2. You want to call that function in the context of some object. In this case, using the “this” keyword in that callback would reference the object passed as argument in call() or apply(). If you don’t want to pass the object context, use null or undefined. In the latter case the global object would be used for “this”.
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Docs for,

Solution 4:

Callbacks are about signals and “new” is about creating object instances.

In this case it would be even more appropriate to execute just “callback();” than “return new callback()” because you aren’t doing anything with a return value anyway.

(And the arguments.length==3 test is really clunky, fwiw, better to check that callback param exists and is a function.)

Solution 5:

the proper implementation would be:

if( callback ) callback();

this makes the callback parameter optional..

Solution 6:

You can use:

if (callback && typeof(callback) === "function") {

The below example is little more comprehensive:

function mySandwich(param1, param2, callback) {
  alert('Started eating my sandwich.\n\nIt has: ' + param1 + ', ' + param2);
  var sandwich = {
      toppings: [param1, param2]
    madeCorrectly = (typeof(param1) === "string" && typeof(param2) === "string") ? true : false;
  if (callback && typeof(callback) === "function") {
    callback.apply(sandwich, [madeCorrectly]);

mySandwich('ham', 'cheese', function(correct) {
  if (correct) {
    alert("Finished eating my " + this.toppings[0] + " and " + this.toppings[1] + " sandwich.");
  } else {
    alert("Gross!  Why would I eat a " + this.toppings[0] + " and " + this.toppings[1] + " sandwich?");