JavaScript-based Debugging API

[RyanLamansky]

Although there are some expectations in the Tooling support document, I'd like to see formal support for JavaScript-based debugging as part of the WebAssembly object itself. With a handful of features (read-only access to the stack, break points), it becomes possible to build a self-hosting compiler and IDE within a web browser.

[lukewagner]

I think this sort of API isn't something we could expose to content by default (there's no distinguishing debugger vs. debuggee; it's all just content). However, #708 proposes a design that does separate out a content-debugger which could be given a special debugging API.

[RyanLamansky]

Although I wrote about this in a general sense, I'm particularly interested in iOS, which has a large user base and a severely locked down web browser. Cooperation from Apple on a debugger interface is far from assured. Having it built into the JavaScript spec allows it to work independently of Apple's vision of how their products are to be used.

[kevinbarabash]

Providing a debugging API that's accessible from JS allows debugging from web-based IDEs. This is particular important for web-based development environments that are targeted at novice developers.

[kevinbarabash]

there's no distinguishing debugger vs. debuggee; it's all just content

I think one way to address is to provide a for the JS code that initializes a WebAssembly module to specify a debugging "delegate" that can control things like setting break points, stepping, and whether to break on load (to debug the "start" segment if necessary).