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

In this lesson, we will discuss common promise-related anti-patterns that should be avoided. Following is a list of anti-patterns we will discuss:

Unnecessary use of the Promise Constructor

One of the most common mistakes made by JavaScript developers, especially those who don't have much experience with promises, is creating promises unnecessarily using the 

 constructor function. Let's take a look at an example:

function fetchData(url) {
  return new Promise((resolve, reject) => {
    fetch(url)
      .then((res) => res.json(res))
      .then(resolve)
      .catch(reject);
  });
}

The above code will work if you pass a URL to the 

 function and then wait for the promise to resolve, but the use of the 

 constructor is unnecessary in the above code example. The 

 function already returns a promise, so instead of wrapping the 

 function call with the promise constructor, we can re-write the above function as shown below:

function fetchData(url) {
  return fetch(url).then((res) => res.json(res));
}

 function is concise, easy to read, free from the creation of any unnecessary promises, and allows the code that calls the 

 function to catch and handle any errors. The older version of the fetchData function also allowed the calling code to handle errors, but the revised version does it without using the catch method call.

Unnecessary use of the promise constructor can lead to another problem: if we forget to add the 

 method call to the promise chain inside the 

 constructor, then any error thrown during the HTTP request won't be caught. Forgetting to call the 

 function inside the executor function can hide the failure of the asynchronous operation inside the executor function.

When writing code that uses promises, one of the most important rules to keep in mind is to 

either catch and handle the error or return the promise to allow the calling code to catch and handle it

. This fundamental rule can help you avoid hidden bugs in the code that uses promises.

Let's take a look at an example of incorrect handling of errors that breaks the above rule:

function fetchData(url) {
  fetch(url).then((response) => response.json());
}

fetchData("https://jsonplaceholder.typicode.com/todos/1")
  .then((data) => console.log(data))
  .catch((error) => console.log(error));

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

The above code throws an error because the 

 function doesn't return the promise. It also doesn't allow the calling code to do any kind of error handling.

There are two ways to fix the above code:

---
title: Promise anti-patterns
description: Explanation of common anti-patterns to avoid when using promises in JavaScript
isPublicLesson: false
useMdx: true
code: https://gitlab.com/fullstackio/books/newline-course-apps/advanced-javascript-unleashed-app/-/tree/main/module_09/lesson_09.09
---

In this lesson, we will discuss common promise-related anti-patterns that should be avoided. Following is a list of anti-patterns we will discuss:

-   Unnecessary use of the Promise constructor
-   Incorrect error handling
-   Converting promise rejection into fulfillment
-   Async executor function

## Unnecessary use of the Promise Constructor

One of the most common mistakes made by JavaScript developers, especially those who don't have much experience with promises, is creating promises unnecessarily using the `Promise` constructor function. Let's take a look at an example:

```js
function fetchData(url) {
  return new Promise((resolve, reject) => {
    fetch(url)
      .then((res) => res.json(res))
      .then(resolve)
      .catch(reject);
  });
}
```

The above code will work if you pass a URL to the `fetchData` function and then wait for the promise to resolve, but the use of the `Promise` constructor is unnecessary in the above code example. The `fetch` function already returns a promise, so instead of wrapping the `fetch` function call with the promise constructor, we can re-write the above function as shown below:

```js
function fetchData(url) {
  return fetch(url).then((res) => res.json(res));
}
```

The revised version of the `fetchData` function is concise, easy to read, free from the creation of any unnecessary promises, and allows the code that calls the `fetchData` function to catch and handle any errors. The older version of the fetchData function also allowed the calling code to handle errors, but the revised version does it without using the catch method call.

Unnecessary use of the promise constructor can lead to another problem: if we forget to add the `catch` method call to the promise chain inside the `Promise` constructor, then any error thrown during the HTTP request won't be caught. Forgetting to call the `reject` function inside the executor function can hide the failure of the asynchronous operation inside the executor function.

## Incorrect Error Handling

When writing code that uses promises, one of the most important rules to keep in mind is to _either catch and handle the error or return the promise to allow the calling code to catch and handle it_. This fundamental rule can help you avoid hidden bugs in the code that uses promises.

Let's take a look at an example of incorrect handling of errors that breaks the above rule:

```js
function fetchData(url) {
  fetch(url).then((response) => response.json());
}

fetchData("https://jsonplaceholder.typicode.com/todos/1")
  .then((data) => console.log(data))
  .catch((error) => console.log(error));
```

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

<ReplitEmbed src="https://replit.com/@newlineauthors/promise-anti-patterns-example3" />

The above code throws an error because the `fetchData` function doesn't return the promise. It also doesn't allow the calling code to do any kind of error handling.

There are two ways to fix the above code:

-   Return the promise from the `fetchData` function by adding the `return` keyword before `fetch(...)`.

    ```js
    function fetchData(url) {
      return fetch(url).then((response) => response.json());
    }
    ```

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

    <ReplitEmbed src="https://replit.com/@newlineauthors/promise-anti-patterns-example4" />

    As the above function just makes the HTTP request and returns the response data after calling the `json()` method on the response object, the calling code is responsible for using the response data as well as handling any error.

    ```js
    fetchData(/* some url */)
      .then((data) => {
        /* do something with the data */
      })
      .catch((error) => {
        /* handle error */
      });
    ```
-   Handle the error inside the `fetchData` function by chaining the `catch` method to the `then` method.

    ```js
    function fetchData(url) {
      fetch(url)
        .then((response) => response.json())
        .then((data) => {
          /* do something with the data */
        })
        .catch((err) => {
          /* error handling code */
        });
    }
    ```