Well, let's break this down line by line: console.log("first") is on the stack first, so it gets printed. But once we break down the pieces of the definition, understanding what it means may not be as arduous as it seems. It is far better to understand the reason for these rules; the single-threaded nature of JavaScript, and how it loads the page and manages the event loop. This means that by design, JavaScript engines — originally browsers — have one main thread of execution, and, to put it simply, process or function B cannot be executed until process or function A is finished. And a heap space where the object references are stored and garbage collected. These shady deals come in the form of asynchronous calls. Exactly, multi-threading or parallel computing is the key when it comes to a lot of tasks. Once the callback finally does hits we get console.log("second") printed. There are no window, document and parent objects inside the worker. JavaScript is an interpreted language, not a compiled one. Developing JavaScript for web pages can be perplexing. We could imagine it as the following piece of code: The next example shows us that JavaScript code is executed as soon as possible after the inline element or file reference is found, and so it doesn’t wait for the creation of the DOM tree to be completed (JSFiddle link):Inline JavaScript code is executed in the same manner, but without the file loading phase. It could be handling AJAX request, an onload handler within the DOM or maybe a web worker response. Modern JavaScript engines implement and heavily optimize the described semantics. One more thing worth mentioning is that page render has higher priority than any handler waiting in the Event handler queue. Javascript is the single-threaded asynchronous programming language. Javascript uses low level programming languages like C++ to perform these behind the scenes. I highly recommend reading it: In most operating systems, the wait or sleep functions are lower bounds - wait(n) will wait for at least n time-units, but once the time is up, you will never know when your tasked is scheduled next - sometimes that can be really long. Outside of work he likes to travel and read Sci-Fi books. The former must be handled properly before the latter can be executed. Here's what you'd learn in this lesson: Will introduces what will be covered in the next few sections, then goes on to review how JavaScript executes code through a single thread. Would you please help me understand how does this happen. devquora. So if you are completely new to this question, in computer science, thread of execution is the sequence or amount of programmed instructions that can be processed independently at a given time. It maintains a single stack where the instructions are pushed to control the order of execution and popped to get executed. We strive for transparency and don't collect excess data. JavaScript is single-threaded, that is a well publicized fact. Taking into account this new information our, abstractly-coded Event loop is extended with a render-related check: On desktops, it is hard to notice the render part of the event loop, but it could be noticeable on mobile devices where resources are more limited. Templates let you quickly answer FAQs or store snippets for re-use. When the call stack is cleared, the browser checks whether it is time to render the page: only then does it check the Event handler queue. Next, the engine notices setTimeout, which isn't handled by Javascript and pushes it off to the WebAPI to be done asynchronously. Here's what you'd learn in this lesson: In the context of programming, Parallelism is the utilization of multiple threads in an operating system. But why? It has limitation that if any item of array contained number in billion or millions it will keep in waiting unless the time finish. JavaScript is single-threaded therefore it can only do one task at a time. Copyright 1999 - 2020 Red Gate Software Ltd. Runtime. Because of that we have two options to achieve parallelism in JavaScript. See, JavaScript knows it’s supposed to run one piece of code at a time, but it has no problem making shady deals under the table. In this case, the user would have seen all the steps of page building till the final result but we don’t want such behavior. This determines that it can only be single-threaded, otherwise it can … Open your dev tools, follow the steps with… JavaScript is a single-threaded synchronous language. :P. I still haven't learned about those yet, but I definitely will write something up once I do. 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