In the simple term, Event Loop in javascript is a constantly running process that checks if the call stack is empty. In our example, secondFunction has now completed its execution and main.js has now completed its execution as well. And when the event does happen, the event table will simply move the function over to the Event Queue. The browser may even show a “the script takes too long” warning. What does above line meaning is whenever in javascript, a function is call is pushed to the top of the stack and when another function is called then that will also put on the top of the stack and continue.

Browser JavaScript execution flow, as well as in Node.js, is based on an event loop. And I’m hopeful the explanation above will provide you with some clarity into what’s happening under the hood that’ll suffice for the majority of our work as web developers. Learn the basic concepts about the JavaScript Event Loop. If call stack is empty then it looks into the event queue. The first example is very straight forward, now take another example which has setTimeout in the code. They simply printed in the console in the same order as they are written. We can avoid problems by splitting the big task into pieces. In this chapter we first cover theoretical details about how things work, and then see practical applications of that knowledge. Here’s the demo, the changes to i won’t show up until the function finishes, so we’ll see only the last value: …But we also may want to show something during the task, e.g. While debugging the javascript code, you always see a long list of the function in the console in the same order in which they are called like below image. onclick event) appears while the engine is busy executing part 1, it gets queued and then executes when part 1 finished, before the next part. They are usually created by promises: an execution of .then/catch/finally handler becomes a microtask. Imagine that you’re on a web site and click on a button … and then it just hangs there. Highlight first 100 lines, then schedule setTimeout (with zero-delay) for the next 100 lines, and so on. For long heavy calculations that shouldn’t block the event loop, we can use Web Workers. Once firstFunction has completed its execution, we return to a state where there’s nothing in the call stack, the event table doesn’t have any events to listen for, and the event queue is empty. It may even cause the browser to “hiccup” or even “hang” for a bit, which is unacceptable. Understanding how event loop works is important for optimizations, and sometimes for the right architecture. So one may want to queueMicrotask to execute a function asynchronously, but within the environment state. But JavaScript is deep — it’s got a complex interior that takes a while to really understand. Let’s understand these terms in details then we will have a clear idea about event loop in javascript. The JavaScript engine does nothing most of the time, it only runs if a script/handler/event activates. JavaScript event-loop call stack event queue multi-threaded. Javascript single thread property has its own benefits and cause. A Stack is a data structure which works in Last In First Out(LIFO) manners. So how does the JavaScript engine know how to process a single line of JavaScript at a time? f3 first console f3 then calling the function f2 with setTimeout and then doing a promise request. Browsers have defined APIs which developers can be used to make the complex process which knows as web API.

Another benefit of splitting heavy tasks for browser scripts is that we can show progress indication. Any JavaScript code that takes too long to return back control to the event loop will block the execution of any JavaScript code in the page, even block the UI thread, and the user cannot click around, scroll the page, and so on. It’s the engine behind Chrome and is also used in Node. Despite these cause then why Javascript is one of the most used programming languages over the past few years. The notable thing is that both variants – with and without splitting the job by setTimeout – are comparable in speed. September 19, 2020. Now, let’s see how the event loop works in javascript with plenty of examples. Luckily for us, JavaScript provides a mechanism and it’s via asynchronous callback functions. It may happen that a task comes while the engine is busy, then it’s enqueued. Once the string is logged out though, there are no more lines to execute in secondFunction and secondFunction is removed from the call stack: Finally, since there are no more lines of code to execute in main.js, main.js is removed from the call stack as well: Now that we get how the call stack works in the JavaScript engine, let’s get back to the idea of blocking code. In the above code, we are using three functions f1, f2 and f3. No worries — an asynchronous callback function is just like any other function you’re used to writing in JavaScript, with the added caveat that it doesn’t get executed till later. Use case 3: doing something after the event. Along with macrotasks, described in this chapter, there are microtasks, mentioned in the chapter Microtasks.

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