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13.1 JS Callbacks

"I will call back later!"
A callback is a function passed as an argument to another function
This technique allows a function to call another function
A callback function can run after another function has finished

Function Sequence

JavaScript functions are executed in the sequence they are called. Not in the sequence they are defined.
This example will end up displaying "Goodbye":

Example 1: JS Functions - Function Sequence part 1

function myFirst() {
  myDisplayer("Hello");
}

function mySecond() {
  myDisplayer("Goodbye");
}

myFirst();
mySecond();

This example will end up displaying "Hello":

Example 2: JS Functions - Function Sequence part 2

function myFirst() {
  myDisplayer("Hello");
}

function mySecond() {
  myDisplayer("Goodbye");
}

mySecond();
myFirst();

Sequence Control

Sometimes you would like to have better control over when to execute a function.
Suppose you want to do a calculation, and then display the result.
You could call a calculator function (myCalculator), save the result, and then call another function (myDisplayer) to display the result:

Example 3: JS Functions - Function Sequence part 3

function myDisplayer(some) {
  document.getElementById("demo").innerHTML = some;
}

function myCalculator(num1, num2) {
  let sum = num1 + num2;
  return sum;
}

let result = myCalculator(5, 5);
myDisplayer(result);

Or, you could call a calculator function (myCalculator), and let the calculator function call the display function (myDisplayer):

Example 4: JS Functions - Function Sequence part 4

function myDisplayer(some) {
  document.getElementById("demo").innerHTML = some;
}

function myCalculator(num1, num2) {
  let sum = num1 + num2;
  myDisplayer(sum);
}

myCalculator(5, 5);

JavaScript Callbacks

A callback is a function passed as an argument to another function.
Using a callback, you could call the calculator function (myCalculator) with a callback, and let the calculator function run the callback after the calculation is finished:

Example 5: JS Functions - Callback Functions

function myDisplayer(some) {
  document.getElementById("demo").innerHTML = some;
}

function myCalculator(num1, num2, myCallback) {
  let sum = num1 + num2;
  myCallback(sum);
}

myCalculator(5, 5, myDisplayer);

In the example above, myDisplayer is the name of a function.
It is passed to myCalculator() as an argument.
When you pass a function as an argument, remember not to use parenthesis.
Right: myCalculator(5, 5, myDisplayer);
Wrong: myCalculator(5, 5, myDisplayer());

When to Use a Callback?

The examples above are not very exciting.
They are simplified to teach you the callback syntax.
Where callbacks really shine are in asynchronous functions, where one function has to wait for another function (like waiting for a file to load).
Asynchronous functions are covered in the next chapter.

JavaScript Fundamentals course home
Module 13. JS Async

Navigate this module

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Example files created in this module:
JS Functions - Function Sequence part 1 JS Functions - Function Sequence part 2 JS Functions - Function Sequence part 3 JS Functions - Function Sequence part 4 JS Functions - Callback Functions JS Functions - setTimeout() with a Callback part 1 JS Functions - setTimeout() with a Callback part 2 JS Functions - setInterval() with a Callback JS Promise Object - The then() Method part 1 JS Promise Object - The then() Method part 2 JS Callbacks - Waiting for a file part 1 JS Callbacks - Waiting for a file part 2 JS Promise Object - Waiting for a file part 1 JS Promise Object - Waiting for a file part 2 JS async / await part 1 JS async / await part 2