Javascript - Callback


This page is about callbacks in Javascript.

It is the primary method of enabling asynchrony.


Without argument

callback = function () {
   console.log("Hello Callback");

function highOrder(callbackFunctionToCall){
  // Call the callback function


With argument

With argument, you are not passing the function but the return value of the function. You need then to wrap it up in a anonymous function.


  • The callback function with an argument that returns an Hello
callback = function (who) {
   return "Hello "+who;
  • The High order function that expects a function as an argument
function highOrder(callbackFunctionToCall){
  // Call the callback function
  • The call of the highOrder needs then to wrap the callback function in a anonymous function because callback(“Nico”) is not a function but the returned value (ie Hello Nico)
   function() {
      return callback("Nico");

Pyramid of doom

doSomething(function(result) {
  doSomethingElse(result, function(newResult) {
    doThirdThing(newResult, function(finalResult) {
      console.log('Got the final result: ' + finalResult);
    }, failureCallback);
  }, failureCallback);
}, failureCallback);

getData( a => {
     getMoreData(a, b => {
         getMoreData(b, c => {
             getMoreData(c, d => {
                 getMoreData(d, e => {

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