Class 04 Node Architecture, Asynchronous, Events

Questions and Feedback

• file system command are relative to cwd (pwd)
  • not the same as require which is relative to the file in which it appears
• mocha -w hangs on some errors
  • nodemon
• ?

Learning Objectives

• Have a working model of asynchronous programming in NodeJS
• Utilize events and event emitters

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

Event Emitter pattern

• Subscribe and use an event emitter
• Streams
  • as event emitters:
    • on: data and close
  • write
• Files as streams
  • Demo fs.createReadStream()

Managing asynchronicity

• 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 (returns a promise)!
• Wrapping with promises
  • require('util').promisify for node callbacks
  • new Promise((resolve, reject) => { /*...*/ })
• Demo:
  • Evolve CoolStringifier
    • "static" methods
  • ~~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)
  • return a promise!

Use nodemon to run mocha in watch mode

• Demo: package.json

Developer Focus

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