Understanding Multi-Threading in JavaScript

In this Hasty Treat, Scott and Wes define some JavaScript Buzzwords and talk about what they mean!

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03:24 - Synchronous / Async

05:23 - Multi-threaded

• JavaScript has a single “main thread”
• But you can have more threads with web workers

08:12 - Blocking

• JavaScript can stop other things on the page from running
• A script tag can block HTML from being parsed
• Most stuff in JavaScript is non-blocking
• Node.js write to filesystem can be blocking

10:27 - Concurrent + Parallel

• JavaScript start/stop are concurrent
• The API runs on a different thread
• Doesn’t REALLY matter
• https://joearms.github.io/published/2013-04-05-concurrent-and-parallel-programming.html

13:22 - Consecutive / Waterfall

• One after another

13:48 - Callback

• A function to run when this thing happens or is done
• Click event callback
• Websocket on data callback
  • Like a tweet stream
• Data fetch callback
  • Almost entirely replaced with async + await
• http://callbackhell.com/
• https://caolan.github.io/async/v3/

17:56 - Script Tag Async + Defer

• Doesn’t block other content
• Runs when ready - doesn’t care about DOMcontentLoaded
• Wait until the page is loaded before running
• If the script tag is above content, don’t wait for it
• Good for things that aren’t called on page load

21:54 - Lazy

• Load it in later - maybe when it’s scrolled into view, or as needed
• Not mission-critical

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