Understanding JavaScript Callbacks

What is a Callback?

Simply put: A callback is a function that is to be executed after another function has finished executing — hence the name ‘call back’.

More complexly put: In JavaScript, functions are objects. Because of this, functions can take functions as arguments, and can be returned by other functions. Functions that do this are called higher-order functions. Any function that is passed as an argument is called a callback function.

^ That’s a lot of words. Lets look at some examples to break this down a little more.

Why do we need Callbacks?

For one very important reason — JavaScript is an event driven language. This means that instead of waiting for a response before moving on, JavaScript will keep executing while listening for other events. Lets look at a basic example:

function first(){
 console.log(1);
}
function second(){
 console.log(2);
}
first();
second();

As you would expect, the function first is executed first, and the function second is executed second — logging the following to the console:

// 1
// 2

All good so far.

But what if function first contains some sort of code that can’t be executed immediately? For example, an API request where we have to send the request then wait for a response? To simulate this action, were going to use setTimeout which is a JavaScript function that calls a function after a set amount of time. We’ll delay our function for 500 milliseconds to simulate an API request. Our new code will look like this:

function first(){
 // Simulate a code delay
  setTimeout( function(){
    console.log(1);
  }, 500 );
}
function second(){
 console.log(2);
}
first();
second();

It’s not important that you understand how setTimeout() works right now. All that matters is that you see we’ve moved our console.log(1); inside of our 500 millisecond delay. So what happens now when we invoke our functions?

first();
second();
// 2
// 1

Even though we invoked the first() function first, we logged out the result of that function after the second() function.

It’s not that JavaScript didn’t execute our functions in the order we wanted it to, it’s instead that JavaScript didn’t wait for a response from <strong>first()</strong> before moving on to execute <strong>second()</strong>.

So why show you this? Because you can’t just call one function after another and hope they execute in the right order. Callbacks are a way to make sure certain code doesn’t execute until other code has already finished execution.

Create a Callback

Alright, enough talk, lets create a callback!

First, open up your Chrome Developer Console (Windows: Ctrl + Shift + J)(Mac: Cmd + Option + J) and type the following function declaration into your console:

function doHomework(subject) {
  alert(`Starting my ${subject} homework.`);
}

Above, we’ve created the function doHomework . Our function takes one variable, the subject that we are working on. Call your function by typing the following into your console:

doHomework('math');
// Alerts: Starting my math homework.

Now lets add in our callback — as our last parameter in the doHomework() function we can pass in callback. The callback function is then defined in the second argument of our call to doHomework().

function doHomework(subject, callback) {
  alert(`Starting my ${subject} homework.`);
  callback();
}

doHomework('math', function() {
  alert('Finished my homework');
});

As you’ll see, if you type the above code into your console you will get two alerts back to back: Your ‘starting homework’ alert, followed by your ‘finished homework’ alert.

But callback functions don’t always have to be defined in our function call. They can be defined elsewhere in our code like this:

function doHomework(subject, callback) {
  alert(`Starting my ${subject} homework.`);
  callback();
}
function alertFinished(){
  alert('Finished my homework');
}
doHomework('math', alertFinished);

This result of this example is exactly the same as the previous example, but the setup is a little different. As you can see, we’ve passed the alertFinished function definition as an argument during our doHomework() function call!

#javascript

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Understanding JavaScript Callbacks

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As a JavaScript developer of any level, you need to understand its foundational concepts and some of the new ideas that help us developing code. In this article, we are going to review 16 basic concepts. So without further ado, let’s get to it.

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Jeromy  Lowe

Jeromy Lowe

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Intro to Callback in JavaScript

During my journey of learning JavaScript and a few days in with React, I had to come to terms with callback functions. It was impossible to avoid its existence and blindly using it without understanding. To understand callback function we have to know a bit about functions. Here I want to talk about function expression, arrow function, and callback function.


What is a Callback function?

According to MDN doc:

A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.

In JavaScript, functions are objects, which means, like any other objects it can be assigned to a variable and passed as an argument to another function. In a nutshell, a callback function is a function that is provided as a parameter for other methods such as forEach method or addEventListener method and gets invoked at a certain point in time.

Passing functions as arguments

So how do we do this? Let’s see with an example below:

document.addEventListener(‘click’,whoAmI);

//whoAmI Function Declaration(Function Statement)
function whoAmI(event){
  console.log(event.target.tagName)
}

We attached the ‘click’ event listener to document with whoAmI function as a parameter that logs the tag name of the clicked target. Whenever ‘click’ happens whoAmI function will be called with an _event_ argument. We call whoAmI a callback function.

When we call a function by its name followed by ( ), we are telling the function to execute its code. When we name a function or pass a function without the ( ), the function does not execute.** The callback function doesn’t execute right away. Instead, the addEventListener method executes the function when the event occurs.**

One more thing I want to mention is because we used function declaration, we were able to call thewhoAmI function before it was declared. It’s the magic of hoisting in JS. But with function expression, it doesn’t get hoisted. So the order of writing function expressions and using them as callback would be crucial.

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What is JavaScript - Stackfindover - Blog

Who invented JavaScript, how it works, as we have given information about Programming language in our previous article ( What is PHP ), but today we will talk about what is JavaScript, why JavaScript is used The Answers to all such questions and much other information about JavaScript, you are going to get here today. Hope this information will work for you.

Who invented JavaScript?

JavaScript language was invented by Brendan Eich in 1995. JavaScript is inspired by Java Programming Language. The first name of JavaScript was Mocha which was named by Marc Andreessen, Marc Andreessen is the founder of Netscape and in the same year Mocha was renamed LiveScript, and later in December 1995, it was renamed JavaScript which is still in trend.

What is JavaScript?

JavaScript is a client-side scripting language used with HTML (Hypertext Markup Language). JavaScript is an Interpreted / Oriented language called JS in programming language JavaScript code can be run on any normal web browser. To run the code of JavaScript, we have to enable JavaScript of Web Browser. But some web browsers already have JavaScript enabled.

Today almost all websites are using it as web technology, mind is that there is maximum scope in JavaScript in the coming time, so if you want to become a programmer, then you can be very beneficial to learn JavaScript.

JavaScript Hello World Program

In JavaScript, ‘document.write‘ is used to represent a string on a browser.

<script type="text/javascript">
	document.write("Hello World!");
</script>

How to comment JavaScript code?

  • For single line comment in JavaScript we have to use // (double slashes)
  • For multiple line comments we have to use / * – – * /
<script type="text/javascript">

//single line comment

/* document.write("Hello"); */

</script>

Advantages and Disadvantages of JavaScript

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