when onEnterFrame fires, we have not yet run the script's prologue, which runs JSOP_FUNCTIONTHIS and initializes the |this| binding, but onEnterFrame passes a Debugger.Frame object to the script, which will seemingly always see |undefined|.

Shu and I did some digging here. It seems to me like we need to initalize that local or CallObject binding directly, during function setup, as we used to for the frame |this|. Jan, how feasible is that? ni? Jan as he wrote all the |this| binding code.

(In reply to Eric Faust [:efaust] from comment #1) > Shu and I did some digging here. > > It seems to me like we need to initalize that local or CallObject binding > directly, during function setup, as we used to for the frame |this|. Jan, > how feasible is that? Agreed - the args to a function stepped into are populated, why not `this`? https://bugzilla.mozilla.org/show_bug.cgi?id=1250785#c4

I've a patch, will finish it (and write more tests) tomorrow.

I had some time today so here's the approach we discussed on IRC.

Comment on attachment 8723208 [details] [diff] [review] Patch Review of attachment 8723208 [details] [diff] [review]: ----------------------------------------------------------------- Nice tests! ::: js/src/vm/ScopeObject.cpp @@ +3188,5 @@ > + // Figure out if we already executed JSOP_FUNCTIONTHIS. > + bool executedInitThisOp = false; > + if (script->functionHasThisBinding()) { > + jsbytecode* initThisPc = script->code(); > + while (*initThisPc != JSOP_FUNCTIONTHIS) Maybe also guard on initThisPc < main() to guard against possible bad bytecode generation. @@ +3200,5 @@ > + // in strict mode), and assign it to the this-argument slot so > + // JSOP_FUNCTIONTHIS will use it and not box a second time. > + if (!GetFunctionThis(cx, frame, res)) > + return false; > + frame.thisArgument() = res; Cool.

Hm I just realized DebugScopeProxy needs a similar fix (and tests).

Comment on attachment 8724656 [details] [diff] [review] Part 2 - Fix DebugScopeProxy Review of attachment 8724656 [details] [diff] [review]: ----------------------------------------------------------------- Again, beautiful tests. Sorry for the delay.