How to debug javascript in embedded V8?

Question

I'm trying to learn how to use the Debug object in V8 to debug javascript in an embedded-javascript c++ application.

I've called v8::Debug::SetDebugEventListener and set a callback. I then call v8::Debug::GetDebugContext to get a debug context, and then run something like: Debug.scripts()

If I print the results of that call from C++, I get:

 [{}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {}]

What I actually want to do is set a breakpoint for a given line number in a script, but can't get even the most basic things working.

I get callbacks to my debug event listener, but the callback input parameter (non-user-provided) data doesn't make any sense to me, either.

{script_: {context_: {data_: undefined, handle_: 0, type_: "context"}, handle_: 1, script_: {}, type_: "script"}, type_: 4}
{break_id: 2, selected_frame: 0}

Thank you.

One thing I didn't know when I posted this was that a bunch of information about a v8::Script is stored in v8::UnboundScript, which can be obtained from a script with v8::Script::GetUnboundScript(), even if you didn't explicitly make one with v8::ScriptCompiler first. — xaxxon
– xaxxon, Commented Oct 2, 2016 at 4:20
@Qwertman I posetd a self-answer with what I've learned. Debugging javascript in v8 seems to be VERY difficult and extremely poorly (even for v8) documented. — xaxxon
– xaxxon, Commented Oct 4, 2016 at 23:03

xaxxon · Accepted Answer · 2016-10-05 08:38:07Z

(Note: this may or may not be the right way to do things, but it's working for me in some simple cases)

v8::DebugEvent debug_event_type = event_details.GetEvent();

then switch on the results:

if (debug_event_type == v8::DebugEvent::Break) {

Here's the passed in data for a pretty trivial breakpoint being hit:

 /* GetEventData() when a breakpoint is hit returns:
     * {
     *      break_points_hit_: [{active_: true, actual_location: {column: 4, line: 13, script_id: 55}, condition_: null,
     *      script_break_point_: {
     *          active_: true,
     *          break_points_: [],
     *          column_: undefined,
     *          condition_: undefined,
     *          groupId_: undefined,
     *          line_: 13,
     *          number_: 1, <== breakpoint number - v8-assigned
     *          position_alignment_: 0,
     *          script_id_: 55, <== script id passed in from v8::ScriptOrigin when compiled
     *          type_: 0
 *          },
 *          source_position_: 175}], frame_: {break_id_: 8, details_: {break_id_: 8, details_: [392424216, {}, function a(){
    println("Beginning of a()");
    let some_var = 5;
    some_var += 5;
    b(some_var);
    println("End of a()");
}, {sourceColumnStart_: [undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, 4, undefined, undefined, undefined, undefined, undefined, undefined, undefined]}, 0, 1, 175, false, false, 0, "some_var", 5]}, index_: 0, type_: "frame"}}

shameless self promotion: You can see the progress I've made towards debugging v8 in my v8 integration simplification library v8toolkit here: https://github.com/xaxxon/v8toolkit/blob/master/src/debugger.cpp

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