This series is a guided tour of what it is like to work on the SpiderMonkey compiler and improve conformance with ECMA-262, the JavaScript Specification. In this series, we go step by step through open issues, understand spec text, and implement fixes. Viewers will be invited to ask questions, make suggestions, and, if they fix a bug themselves, have their solutions discussed on the stream.


All sessions start at 15:00 UTC (8am PST, 17:00 CEST) for 90 minutes (60 min talk, 30 min dedicated Q&A in chat) and are streamed via the Twitch channel codehag. Sessions are recorded and videos linked from this page.

Upcoming sessions:

  • July 3 - The JIT, and Implementation in JIT.
  • July 10

Past episodes:

  • June 26
    • Last week we generalized our fix to element access. However there are two problems: error messages are less informative, and there is a lot of extra bytecode. How can we fix this? With MOAR BYTECODE.
  • June 12
    • Continuing on from the stream two weeks ago, we will look at the finishing work for the pre-increment bug, before moving on to our next (related) issue.
    • Video
  • June 9
    • Curious about how JavaScript gets standardized? Join us for a fireside chat about the most recent TC39 meeting. Questions will be answered and a little bit about the process explained.
    • Video
  • May 29
    • Introducing Compiler Compiler.
    • Video

Extra resources for viewers:


Watch the stream




Learn more about Spidermonkey on MDN

Yulia Startsev on Hacks