Course Overview

Overview of history of the JavaScript language

History of JavaScript

Overview of the relationship between JavaScript and Java

JavaScript vs Java

Overview of how JavaScript is evolved as a programming language

How is it evolved?

Overview of how we can track upcoming features in the JavaScript language

How to track upcoming features?

Overview of how the ECMAScript is versioned

How is ECMAScript versioned?

Explanation of whether JavaScript is interpreted or a compiled language

Is it interpreted or compiled?

Explanation of how JavaScript is executed

How is it executed?

Execution Context

Explanation of what is a callstack and how it is used in JavaScript

Callstack

Overview of memory management in JavaScript

Memory

What is JavaScript

What is hoisting?

Explanation of common misconception about hoisting in JavaScript

Misconception about hoisting

Hoisting

What is scope?

Explanation of global scope in JavaScript

Global scope

Explanation of function scope in JavaScript

Function scope

Block scope

Explanation of module scope in JavaScript

Module scope

Scope chain

Scope

What is coercion?

Diving into inner details of  coercion in JavaScript

Deeper look at coercion

Explanation of what abstract operations are and common types of abstract operations in JavaScript

Abstract operations

Explanation of how abstract or loose equality works in JavaScript

Abstract equality operator

Explanation of how the addition operator works in JavaScript

Addition operator

Explanation of how relational operators work in JavaScript

Relational operators

Coercion exercise for testing and practicing the knowledge learned in this module

Coercion exercise

Coercion

What is a closure?

Explanation of a common misconception about closures in JavaScript

Misconception about closures

Explanation of the problem of closures inside loops and how to fix it

Closures inside loops

Explanation of closures can be used to hide data

Data hiding using closures

Closures

What is prototypal inheritance?

Prototype chain

__proto__ property

Explanation of how we can create objects with custom prototypes in JavaScript

Custom prototypes

Explanation of how prototype chain is set with ES6 classes

ES6 classes and prototypes

Prototypes

Explanation of this keyword in JavaScript

What is this?

Explanation of how "this" works inside arrow functions

Arrow functions and "this"

Explanation of how value of "this" can be set explicitly

Explicitly binding "this"

globalThis

Summarizing "this"

'this' keyword

What is a symbol?

Explanation of why symbols were added in JavaScript

Why symbols?

Symbols and privacy

Explanation of global symbols in JavaScript

Global symbols

Explanation of what well-known symbols are in JavaScript

Well-known symbols

Symbol

Overview of asynchronous behavior of JavaScript

Overview

Overview of how callbacks help us write asynchronous code in JavaScript

Callbacks

Event loop

Promise

Explanation of promise chaining in JavaScript

Promise chaining

Explanation of some static methods of promises and their use cases

Use cases for static Promise methods

Explanation of async-await syntax in JavaScript

async await

Explanation of microtasks, how they are processed, and how they are linked to promises

Microtasks

Explanation of common anti-patterns to avoid when using promises in JavaScript

Promise anti-patterns

Asynchronous JavaScript

Explanation of what is an iterator in JavaScript

Iterators

Explanation of what is a generator in JavaScript

Generators

Explanation of how we can create asynchronous iterators in JavaScript

Asynchronous iterators

Explanation of how we can create asynchronous generators in JavaScript

Asynchronous generators

Iterators and Generators

Overview of what is debugging and different ways to debug JavaScript

Debugging

Overview of using the "debugger" statement to debug JavaScript

Debugger statement

Overview of using the breakpoints in browser developers' tools to debug JavaScript

Breakpoints in browser

Overview of using the VS Code debugger to debug JavaScript

VS Code debugger

Debugging JavaScript

Final thoughts and advice regarding how to continue the exciting journey of learning JavaScript

Conclusion

Wrap up

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Become a confident JavaScript pro by unraveling the most challenging aspects of the language. With easy-to-understand explanations and code examples, you’ll gain a deeper and solid understanding of JavaScript's most complex features.

Advanced JavaScript Unleashed is for beginner and intermediate developers alike, who want to elevate their JavaScript skills and become senior engineers by gaining a deeper understanding of the core topics of the language.

Advanced JavaScript Unleashed

We learned in one of the earlier lessons in this module that executing DOM event listeners and 

 callbacks require scheduling a "task". Tasks are enqueued in a task queue until the event loop processes them. What about the promise fulfillment or rejection handlers? Does their execution also require scheduling a task? Not exactly a task, but a "microtask".

Microtasks, which the ECMAScript specification calls 

, are scheduled for things that have higher priority than "tasks". Microtasks are processed after:

Whereas "tasks" are executed in the order they are enqueued in the task queue, only one task is executed per one turn or tick of the event loop.

Another important thing to note about microtasks is that while only one task is processed per tick of the event loop, microtasks are processed until the microtask queue is empty. If a task schedules another task, it won't be processed until the next turn or tick of the event loop, but in the case of microtasks, if any microtask is queued by a microtask, the queued microtask will also be processed. This means that the event loop can get stuck in an infinite loop if each microtask keeps queuing another microtask.

console.log("start");

setTimeout(() => {
  console.log("setTimeout callback with 500ms delay");
}, 500);

Promise.resolve()
  .then(() => {
    console.log("first 'then' callback");
  })
  .then(() => {
    console.log("second 'then' callback");
  })
  .then(() => {
    console.log("third 'then' callback");
  });

setTimeout(() => {
  console.log("setTimeout callback with 0ms delay");
}, 0);

console.log("end");

/*
start
end
first 'then' callback
second 'then' callback
third 'then' callback
setTimeout callback with 0ms delay
setTimeout callback with 500ms delay
*/

---
title: Microtasks
description: Explanation of microtasks, how they are processed, and how they are linked to promises
isPublicLesson: false
useMdx: true
code: https://gitlab.com/fullstackio/books/newline-course-apps/advanced-javascript-unleashed-app/-/tree/main/module_09/lesson_09.08/microtasks-example1
---

We learned in one of the earlier lessons in this module that executing DOM event listeners and `setTimeout` or `setInterval` callbacks require scheduling a "task". Tasks are enqueued in a task queue until the event loop processes them. What about the promise fulfillment or rejection handlers? Does their execution also require scheduling a task? Not exactly a task, but a "microtask".

Microtasks, which the ECMAScript specification calls [jobs](https://tc39.es/ecma262/#sec-promise-jobs), are scheduled for things that have higher priority than "tasks". Microtasks are processed after:

-   each callback, provided that the callstack is empty.
-   each task

Whereas "tasks" are executed in the order they are enqueued in the task queue, only one task is executed per one turn or tick of the event loop.

Another important thing to note about microtasks is that while only one task is processed per tick of the event loop, microtasks are processed until the microtask queue is empty. If a task schedules another task, it won't be processed until the next turn or tick of the event loop, but in the case of microtasks, if any microtask is queued by a microtask, the queued microtask will also be processed. This means that the event loop can get stuck in an infinite loop if each microtask keeps queuing another microtask.

Consider the following code example:

```js
console.log("start");

setTimeout(() => {
  console.log("setTimeout callback with 500ms delay");
}, 500);

Promise.resolve()
  .then(() => {
    console.log("first 'then' callback");
  })
  .then(() => {
    console.log("second 'then' callback");
  })
  .then(() => {
    console.log("third 'then' callback");
  });

setTimeout(() => {
  console.log("setTimeout callback with 0ms delay");
}, 0);

console.log("end");

/*
start
end
first 'then' callback
second 'then' callback
third 'then' callback
setTimeout callback with 0ms delay
setTimeout callback with 500ms delay
*/
```

Here's a Replit of the above code in action:

<ReplitEmbed src="https://replit.com/@newlineauthors/microtasks-example1" />

Executing the above code requires scheduling tasks and microtasks. The following steps explain how different tasks and microtasks are scheduled to execute the code above:

1.  A task is created to execute the script, starting the synchronous execution of the code.
2.  The first `console.log` statement is executed, logging "start" on the console.

        output:
        -------
        start
3.  Next, we have the `setTimeout` call with a 500ms delay. This starts a timer in the background, and its expiration will result in a task to execute the `setTimeout` callback getting queued in the task queue.

        task queue:
        -----------
        [task(execute setTimeout callback)]

        output:
        -------
        start
4.  Moving on with the synchronous execution of the code, [Promise.resolve](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/resolve) is called, which creates a resolved promise. To execute its fulfillment handler, a microtask or job is queued in the microtask queue.

        task queue:
        -----------
        [task(execute setTimeout callback)]

        microtask queue:
        ----------------
        [job(execute fulfillment callback)]

        output:
        -------
        start
5.  Next, we have a `setTimeout` call with a 0ms delay. This also schedules a task to execute its callback.

        task queue:
        -----------
        [
            task(execute setTimeout callback),
            task(execute setTimeout callback)
        ]

        microtask queue:
        ----------------
        [job(execute fulfillment callback)]

        output:
        -------
        start
6.  Finally, the synchronous execution reaches its end with the final `console.log` statement, logging "end" on the console. At this point, the callstack is empty, and the event loop can start processing the scheduled tasks and microtasks.

        task queue:
        -----------
        [
            task(execute setTimeout callback),
            task(execute setTimeout callback)
        ]