A User-land JavaScript Stepping Debugger

• Name : James Long
• Twitter : @jlongster
• Github : jlongster
• Website : jlongster.com
• Job : Mozilla, Apps Engineering

Abstract

Programming today can still feel like a black box, even with all the new developer tools for debugging code. I think this is because we don't have the ability to truly customize the debugging experience for our application. What if we could have a user-land JavaScript debugger that you could shape to your needs?

It turns out that you can build a real stepping debugger by abusing yield from ES6 generators. By instrumenting every expression with yield, we gain control of the stack and execution while allowing the native JavaScript VM to still execute everything else. This special code is run inside our own VM in user-land, and allows you to pause and step through code.

An example of the instrumented code is here, and you can step through the code here. If we flesh this out, I think it could be a powerful tool.

Speaker Bio

I work for Mozilla and help developers write web apps. In my heart I'm an idealist but I've learned to mix practicality with idealism when approaching software problems. I have a heavy functional programming background with Scheme, and used to write DirectX games in high-school in C++. In fact, I wrote a text-based RPG on my TI-82 calculator during math class in 7th grade. It didn't take off like I thought it would.