Debugger

The Debug object provides functions for debugging code running in a separate compartment. You can provide functions for SpiderMonkey to call when events like steps, calls, and breakpoint hits occur in the debuggee, examine the debuggee's stack frames, and inspect and manipulate the debuggee's objects.

Debug object event hook functions run in the same thread as the debuggee, on the same stack: when the event occurs, the debuggee pauses while your hook functions run, and resumes (unless you say otherwise) when your functions return.

The debugger and debuggee must be in separate compartments. Your hook functions run in the debugger's compartment. SpiderMonkey mediates their access to the debuggee's objects, and prevents the debuggee from accessing the debugger's objects at all.

The Debug object provides objects representing the debuggee's stack frames, scripts, and other internal interpreter structures, for your hook functions to examine and manipulate.

Debugger access to debuggee values

The Debug object follows certain conventions to help debuggers safely inspect and modify the debuggee's objects and values. Primitive values are passed freely between debugger and debuggee; copying or wrapping is handled transparently, as appropriate. Objects (including host objects like DOM nodes) received from the debuggee are fronted in the debugger by Debug.Object instances (described in detail below), which provide reflection-oriented methods for inspecting the referent object's properties and other characteristics. Of the debugger's objects, only Debug.Object instances may be passed to the debuggee: when this occurs, the debuggee receives the Debug.Object's referent, not the Debug.Object instance itself.

In the descriptions below, the term "debuggee value" means either a primitive value or a Debug.Object instance; it is a value that might be received from the debuggee, or that could be passed to the debuggee.

Beginning to Debug

To begin debugging another compartment's code, you create a Debug object for the debuggee compartment, and install your hook functions.

Debug(object)

Create a debugger object debugging object's compartment. Object is typically a global object, but can be any JavaScript object from the debuggee's compartment. The object must be in a different compartment than the calling code, and debugger/debuggee compartments may not form a cycle. Object's compartment must not be in use by another thread while this call runs.

D.setHooks(hooks)

Use the functions in hooks to handle events occurring in D's debuggee. Hooks should be an object; each property should be named after a debugging event, and its value should be a function SpiderMonkey should call when the named event occurs. See below for descriptions of specific debugging hooks. This removes all previously registered hooks; after the call, only the hooks mentioned in hooks are in force. Thus, a call like D.setHooks({}) removes all debugging hooks. Hook function calls are cross-compartment, same-thread calls. Hook functions run in the thread in which the event occurred, not in the thread that registered the hooks. (It is your responsibility to ensure that two threads don't try to run in the same compartment). Hook functions run in the compartment to which they belong, not in the debuggee's compartment.

D.getHooks()

Return an object holding all the event hooks currently in force. The returned object is suitable for use with D.setHooks.

Debugging hooks

For each debugging hook, we give the name of the hook and the arguments passed to its handler function, and describe the circumstances under which SpiderMonkey calls it.

interrupt(frame)

A bytecode instruction is about to execute in the stack frame represented by frame, a Debug.Frame instance. Naturally, frame is the youngest debuggee frame. This hook function's return value determines how execution should continue:

• If it returns true, execution continues normally.
• If it returns an object of the form { throw: value }, then value is thrown as an exception from the current bytecode instruction. value must be a debuggee value.
• If it returns an object of the form { return: value }, then value is immediately returned as the current value of the function. value must be a debuggee value.
• If the hook throws an exception, ... well, we're in trouble. That's an error in the debugger which should be reported somehow, but certainly not handled by the debuggee.

newScript(script, [function])

New code, represented by the Debug.Script instance script, has been loaded into the debuggee's compartment. If the new code is part of a function, function is a Debug.Object reference to the function object. Note that script may be a temporary script, created by eval and destroyed when its execution is complete.

destroyScript(script)

SpiderMonkey has determined that script will no longer be needed, and is about to throw it away. The garbage collector may have found that the script is no longer in use, or perhaps eval has finished executing the script, and is about to destroy it. In any case, operations on script after this hook function returns will throw an error.

debuggerHandler(frame)

The debuggee has executed a debugger statement in frame. This hook function's return value determines how execution proceeds, as for the interrupt hook function.

sourceHandler(ASuffusionOfYellow)

This hook function is never called. If it is ever called, a contradiction has been proven, and the debugger is free to assume that everything is true.

enterFrame(frame, call)

The stack frame frame is about to begin executing code. (Naturally, frame is currently the youngest debuggee frame.) If call is true, it is a function call; if call is false, it is global or eval code. If this hook function returns a function F, SpiderMonkey will call F when execution of frame completes, passing one argument indicating how it completed. If the argument is of the form { return: value }, then the code completed normally, yielding value. If the argument is of the form { throw: value }, then the code threw value as an exception. In either case, value is a debuggee value.

throw(frame, value)

The code running in frame is about to throw value as an exception. The value this hook function returns determines how execution proceeds, as for interrupt.

error(frame, report)

SpiderMonkey is about to report an error in frame. Report is an object describing the error, with the following properties:

message

The fully formatted error message.

file

If present, the source file name, URL, etc. (If this property is present, the line property will be too, and vice versa.)

line

If present, the source line number at which the error occurred.

lineText

If present, this is the source code of the offending line.

offset

The index of the character within lineText at which the error occurred.

warning

Present and true if this is a warning; absent otherwise.

strict

Present and true if this error or warning is due to the strict option (not to be confused with ES strict mode)

exception

Present and true if an exception will be thrown; absent otherwise.

arguments

An array of strings, representing the arguments substituted into the error message.

Debug.Script

weak reference

Debug.Object

strong reference