Class 02 Node Architecture and Asynchronous Programming

Questions and Feedback

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Learning Objectives

• Have a working model of asynchronous programming in NodeJS
• Write and utilize asynchronous callback functions effectively:
  • When calling node asynchronous APIs
  • When exposing a function
• Utilize the filesystem
• Use a class to hold onto state

Agenda

Node.JS Architecture

• What is it?
• Node Event Loop
  • Basic node architecture 101
  • v8 + event-loop + os-lib
  • Thread - actual "thread of execution"
  • Event loop explained
  • JavaScript single threaded event model
• Demo: event-loop.js

Passing functions in javascript

• How do we
  • return values?
  • Propagate errors?
• "Old School" Node callback pattern: callback(err, result)
• Promises FTW!
• ~~Promises~~ async/await FTW!
• Wrapping with promises
  • require('util').promisify for node callbacks
  • new Promise((resolve, reject) => { /*...*/ })
• Some hard rules:
  1. You can't create asynchronicity with just js
  2. If your building functionality on top of asynchronous APIs, then your library must be asynchronous!
  3. If a function or method uses asynchronous activity to complete its work, then that function must have an asynchronous interface (takes a callback and/or returns a promise)!

Demo: Copy Dir

• ~~Mocha done~~
  • test parameter
  • Tests function.length
  • if > 0, test is async
  • calling done with any non-null argument is failure (matches node callback signature)
• and node fs module
• Mocha before and friends
• Orchestration?
  • Sequential
  • Parallel
    • Async order is "completion" based

Demo: Test drive "copy dir"

Demo CoolStringifier

• ES6 Classes
• Hold onto state