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Asynchronous JavaScript: Introductory Guide with Examples

25th Apr, 2024
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    Asynchronous JavaScript: Introductory Guide with Examples

    Synchronous JavaScript

    Synchronous JavaScript is a concept that means anything to be “in a sequence.” This phrase implies that every line of code gets executed one after the other. A particular statement waits for some time while the overhead lines get executed.

    Synchronous JavaScript is used to execute the code in an enhanced way.

    The working of the synchronous JavaScript, the async await JavaScript, depends on the two main attributes- execution and stack.

    Execution Context- Execution context is where the JavaScript code is evaluated at execution. The execution of the JavaScript code takes place within the execution context. Different functions have different execution contexts where the code runs simultaneously.

    Call Stack- Call stack is a LIFO structure that stores all the contexts captured during the execution of a code. Since JavaScript is a single-threaded programming language, it has a single call stack. Any element can easily be added or removed from the top of the stack.

    Working process- Various functions are added to the stack during the execution of a code. Later, the functions are executed one by one. The function that appears at the top of the stack is executed before the others. So, push and pop are the two primary operations performed on the stack.

    Synchronous JavaScript is of great use for the developers, where they can track each and every activity or the executed functions.

    To get personal assistance, you can even refer to the best place to learn Node.js course to receive personalized training and expertise.

    What is Asynchronous

    Asynchronous JavaScript executes the code immediately after being compiled. These are the areas where the synchronous JavaScript blocks the execution. Asynchronous JavaScript makes it easy for the users to get the desired output of the program.

    Let us look closely at how the asynchronous JavaScript code executes.

    The async await js is of great use to the company as it loads the application and screen faster to provide the best user experience.

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    Blocking in JavaScript is a technique used to wait till the execution of a non-JavaScript operation takes place. This occurs because the event loop cannot continue the processing of the JavaScript code till the blocking operation occurs. The JavaScript async feature helps the users to hold the execution of a code for the time being. Using processes such as image processing and the network requests in a program consumes a lot of time.

    The Blocking technique makes it difficult for the users to execute the code. This is because it delays the execution time making it an impossible task to overcome the processing issues. 

    Blocking operations occurs in node.js because JavaScript is a single-threaded language. Nodejs runs all the operations in a single process instead of creating a new thread for every incoming request.

    Here’s a code snippet for the same.

    To know more about the working of this code in JavaScript, you need to learn more about execution and stack. It is also known as the execution stack. 


    The callback is just an event handler used to execute the requests stored in the memory. The callback is a function passed as an argument to another function. Callback functions are executed automatically with respect to the pending requests. Callbacks are the main source through which the asynchronous functions implement.

    The async-await JavaScript callback function provides an ease to waiting for the results of a previous function while executing another function call. Let us discuss this with an example. Here we will use a setTimeout() method to execute the program.

    Knowing the fact that the setTimeout method executes only after the given amount of time, the other method executes by then. It has two arguments- callback and the time(in milliseconds).

    Now, we will take an example of a callback function.

    In the above example, the second function is executed before waiting for the first function. However, waiting for a function is a concept used by the callback function.

    The output for this function is given below.

    The code was executed synchronously. Here, sayName() function is used as an argument in the greet() function. 

    Also, check ut the blog – Node.js Process 

    Handling Events

    Event handlers are a form of asynchronous programming language that helps you to call a specific function when an event occurs. The users are then notified about the calling function.

    The XMLHttpRequest API is used to create http requests to a server. This process takes up a lot of time and hence is known as an asynchronous API, that notifies the user about the progress of a request. This progress is shared using the event listeners that are attached together with the object of the XMLHttpRequests.

    Let us look at it with an example. We will use a click to start the request handler. Here, we will first create an XMLHttpRequest and then we will capture the event.

    Once, the code is executed, you will get a message saying “Finished!”. This is where the execution ends.

    Once we add the event listener, the user gets a request.

    There are 3 common ways to assign event handlers to a specific request. We will discuss each of them briefly. 

    1. HTML Event Handler Attributes

    The most common event handler names begin with on. The event handler for a click event is known as a click. To attach the event handler along with an HTML element, you can use the event handler attribute. Here’s how you can use this event handler.

    While assigning the event handler, you need to avoid using special characters such as &, “ “, <, etc.

    The script function in the handler can be written as follows- 

    2. DOM Level O Event handlers

    Every element has an event handler attribute named onclick. You just need to assign the values to a specific function. Here’s how you can do this. 

    In the above code, the function acts as a button element. You can even access the properties within the event handling function. The Dom Level 0 event handler is most widely used because of its browser support and its simplicity level.

    3. DOM Level 2 Event handlers

    The DOM level 2 Event handler is the other form of event handling technique. It provides two methods to deal with the event listeners. They are- addEvenetListener and removeEventListener. The former is used to register a specific event. While the latter is used to remove that particular event.

    Let us discuss both of them in detail.

    A) addEvenetListener()

    This method takes 3 arguments- event handler, event name, and a boolean value. This value is used to call the event handler when it has a true value. You can add more than 1 event handler to a single event. 

    B) removeEventListener()

    As the name suggests, the removeEventListener removes an event that was added using the addEventListener method.

    The event handling loop finds the status of the stack used within the heap memory. When the stack gets empty, it calls out for the remaining requests. While we handle the events, the queue contains only a few pending requests. Hence, the callback shifts at the top of the stack and executes the remaining queries.


    After going through the above sections, you might now be familiar with the java script await and the async js. We have discussed all the essential aspects that will help you gain advanced knowledge about NodeJS async JavaScript. Besides this, we have a fantastic course for you.

    The ﷟HYPERLINK ""KnowledgeHut’s best place to learn Node.js, where you will find all the basics and the fundamentals regarding the async await in JavaScript. This article combines all the main points such as callbacks, blocking, synchronous and asynchronous JavaScript. All these concepts together build up the entire JavaScript runtime environment.

    Frequently Asked Questions (FAQs)

    1Is JavaScript asynchronous or synchronous?

    JavaScript is an asynchronous programming language that offers various features and flexibility to users. It is a single-threaded and non-blocking language with functions such as async and await in JavaScript. 

    2What is Asynchronous JavaScript?

    Asynchronous JavaScript is a form of language that helps the user make various ajax calls. These calls are used to stop and start the execution of a code simultaneously.

    3What is the difference between asynchronous and synchronous JS?

     In synchronous js, one code is executed at a time. The other codes are kept on hold for a while. The next one is unblocked as soon as the previous execution takes place.

    In asynchronous js, you can execute more than 1 code at a time without waiting for the results. 

    4How does Asynchronous JS work?

    Asynchronous JS is a concept that allows the users to run their time-taking codes and still respond to other tasks as well. The users no longer need to wait for the execution of extended programs. They can compiler other codes while waiting for a response.

    5Why do we need asynchronous JS?

    Async js is used to perform long network requests simultaneously. It does not block the main thread execution. Instead, it runs them parallelly.


    Sachin Bhatnagar

    Blog Author

    Sachin Bhatnagar is an experienced education professional with 20+ years of expertise in Media & Entertainment and Web Technologies. Currently, as the Program Director - Full-Stack at KnowledgeHut, he excels in curriculum development, hands-on training, and strategic deployment of industry-centric educational programs. His online training programs on have attracted over 25,000 learners since 2014. He actively contributes to the development of full-stack training products, leveraging KnowledgeHut's advanced learning platform. Collaborating with organizational leaders, he ensures the success of these programs and has created several technology programs prominently featured in KnowledgeHut's course offerings.

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