Event Loop with JavaScript
Engineering 5 min read

Event Loop in JavaScript

An event loop in JavaScript is a single thread that handles all the asynchronous callbacks, such as user inputs, network requests, and other events.

It helps to manage code execution better and provide faster response times than if everything was being executed on one thread at a time.  

In addition, by allowing asynchronous operations, developers can create more efficient applications with less overhead.

An essential aspect of an event loop is its ability to pause execution when needed so resources can be freed up for other tasks. It is instrumental when dealing with high-latency services such as networks or databases.  

Event loops can also provide better scalability and performance for applications as they are more efficiently managed. As a result, they are a crucial part of modern web development frameworks and libraries.

Understanding JavaScript terminologies with an example

Here is an example of how heap, stack, browser API, call stack, callback queue, and event queue work together in JavaScript.  

Suppose you have a web page that displays a button. When the user clicks the button, an event is generated and added to the event queue.   

The event loop continuously checks the event queue for new events. For instance, when it finds the button click event, it adds the event handler function for the button click to the call stack to be executed.  

The event handler function calls a browser API function that creates an animation on the page. Finally, the browser API function is added to the call stack and executed.   

When the animation is complete, the browser API function returns and is popped off the stack.  

Meanwhile, the event handler function continues to execute and calls to another function that processes some data.   

This function is added to the call stack and executed. When the data processing function is finished, it returns and is popped off the stack.  

Now, the event handler function is the only function left on the call stack. It calls to a function that updates the page with the processed data.   

This function is added to the call stack and executed. When it is finished, it returns and is popped off the stack.  

The call stack is empty at this point, and the event loop checks the callback queue for any functions waiting to be called.   

It finds the function used to update the page with the processed data. Then, this function is added to the call stack and executed.  

In this example,   

  • The heap stores objects such as the animation and the processed data. 
  • The stack keeps track of function calls and the current function execution. 
  • The browser API provides access to browser features such as creating an animation. 
  • The call stack holds the current function execution.
  • The callback queue holds functions waiting to be called. 
  • The event queue holds events waiting to be processed by the event loop.

How does it work?

In JavaScript, an event loop is a mechanism that allows the execution of code to be scheduled in a non-blocking way.  

That means the code can continue to run and handle new events as they occur without being stopped or blocked by long-running operations.

The event loop works by continuously checking a queue of pending events and executing each event turn by turn. It allows JavaScript code to handle multiple events concurrently without waiting for one event to complete before handling the next.

Examples of Event loop

To use the event loop in your code, you can use the setTimeout function to schedule the execution of a piece of code later.

It allows you to write non-blocking code that can handle multiple events concurrently without having to stop and wait for any one event to complete before moving on to the next.

Here is an example of an event loop in JavaScript:

code snippet 1

In this example, the event loop runs in an infinite loop, checking for new events and dispatching them to the appropriate handlers. When there are no more events, the loop exits.

In practice, JavaScript event loops are typically implemented using the setTimeout or setInterval functions, which allow the event loop to run asynchronously and in the background.

In addition, it will enable the program to continue executing while the event loop waits for new events.

Here is an example of an event loop implemented using setTimeout:

code snippet 2

In this example, the runEventLoop function is scheduled to run again using setTimeout, which allows the event loop to run asynchronously and in the background. In addition, it will enable the program to continue executing while the event loop waits for new events.

Here is an example of how you might use the setTimeout() function to schedule the execution of a piece of code at a later time:

code snippet 3

In this example, the greet function is passed to setTimeout, along with a delay time of 1000 milliseconds (1 second). It causes setTimeout to schedule the greet function to be executed after a delay of 1 second.

The setTimeout function returns a timeout ID, which can be used to cancel the scheduled execution of the greet function using the clearTimeout function.

For example:

code snippet 4

In this example, the timeoutID variable is used to cancel the scheduled execution of the greet function using the clearTimeout function. It ensures that the greet function will not be executed if it is no longer needed.

Closing lines

In JavaScript, the event loop is essential to how the language works.  

It is a programming construct that waits for and dispatches events or messages in a program, allowing it to respond to external events. JavaScript is an event-driven language, meaning the program’s execution is determined by external events such as user input or incoming data.  

The event loop is the central mechanism that drives the program’s behaviour and allows it to respond to these events. By understanding how an event loop works and its benefits, developers can create better applications with improved performance and scalability.

Niyati Madhvani

A flamboyant, hazel-eyed lady, Niyati loves learning new dynamics around marketing and sales. She specializes in building relationships with people through her conversational and writing skills. When she is not thinking about the next content campaign, you'll find her traveling and dwelling in books of any genre!

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