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Table of Contents

• ↓ Motivation
• ↓ What is a promise?
• ↓ Constructing a promise
• ↓ Awaiting a promise
• ↓ Transformation / Chaining
• ↓ Implementations / Polyfills
• ↓ Top Alternatives to promisejs.org

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Content Snippets from promisejs.org

"Consider the following synchronous JavaScript function to read a file and parse it as JSON. It is simple and easy to read, but you wouldn't want to use it in most applications as it is blocking. This means that while you are reading the file from disk (a slow operation) nothing else can happen."

"To make our application performant and responsive, we need to make all the operations that involve IO be asynchronous. The simplest way to do this would be to use a callback. However, a naive implementation will probably go wrong:"

"We need to handle errors thrown by JSON.parse but we also need to be careful not to handle errors thrown by the callback function. By the time we've done all of this our code is a mess of error handling:"

"Despite all this mess of error handling code, we are still left with the problem of the extra callback parameter hanging around. Promises help you naturally handle errors, and write cleaner code by not having callback parameters, and without modifying the underlying architecture (i.e. you can implement them in pure JavaScript and use them to wrap existing asynchronous operations)."

Key Topics & Focus Areas for promisejs.org:

Motivation

What is a promise?

Constructing a promise

Awaiting a promise

Transformation / Chaining

Implementations / Polyfills