Implement debug object for CLI (#2772)

Currently some debugging stuff in JavaScript land is difficult to impossible, like triggering a GC collect, this is not impossible to do in JavaScript the way I triggered it was by creating a huge amount of object `for (let i = 0; i < 100000; ++i) { ({}) }` but this is cumbersome and not guaranteed to trigger a gc. This PR implements `--debug-object` flag that injects the `$boa` debug object in the context, the object is separated into modules currently `gc`, `function`, `object`. We can now do `$boa.gc.collect()`, which force triggers a GC collect. Or sometimes I wanted a trace (the current solution is great, you can trace stuff like `>>> 1 + 1` but that is also it's limitation), it traces everything, I sometimes have a scenario and just want to trace a single function in that scenario, that's why I added the `$boa.function.trace(func, this, ...args)` It only traces the function. ```js >> $boa.function.trace((a, b) => a + b, undefined, 1, 2) -------------------------Compiled Output: ''-------------------------- Location Count Opcode Operands 000000 0000 DefInitArg 0000: 'a' 000005 0001 DefInitArg 0001: 'b' 000010 0002 RestParameterPop 000011 0003 GetName 0000: 'a' 000016 0004 GetName 0001: 'b' 000021 0005 Add 000022 0006 Return 000023 0007 PushUndefined 000024 0008 Return ... (cut for brevity) ... ``` It also implements `$boa.function.flowgraph(func, options)`: ```js $boa.function.flowgraph(func, 'graphviz') $boa.function.flowgraph(func, { format: 'mermaid', direction: 'TopBottom' }) ``` Printing the object pointer: ```js $boa.object.id({}) // '0x566464F33' ``` It currently implements some functionality which we can grow it with our debugging needs since we are not restricted by a spec we can add whatever we want :) I was originally going to implement this in #2723 (but the PR is too big), for shapes having functions like: ```js $boa.shape.type({}) // Shared shape $boa.shape.id({}) // 0x8578FG355 (objects, shape pointer) $boa.shape.flowgraph({}) // printing the shape transition chain, like $boa.function.flowgraph ``` Shapes chains are very hard to debug once they are big... so having this type of debugging capability would make it much easier.
2 years ago · 570bc85ebe
14 changed files with 586 additions and 1 deletions
--- a/Cargo.lock
+++ b/Cargo.lock
@ -366,6 +366,8 @@ version = "0.16.0"
 dependencies = [
 "boa_ast",
 "boa_engine",
+ "boa_gc",
+ "boa_interner",
 "boa_parser",
 "clap 4.2.1",
 "colored",
--- a/boa_cli/Cargo.toml
+++ b/boa_cli/Cargo.toml
@ -15,6 +15,8 @@ rust-version.workspace = true
 boa_engine = { workspace = true, features = ["deser", "console", "flowgraph", "trace", "annex-b"] }
 boa_ast = { workspace = true, features = ["serde"] }
 boa_parser.workspace = true
+boa_gc.workspace = true
+boa_interner.workspace = true
 rustyline = { version = "11.0.0", features = ["derive"]}
 clap = { version = "4.2.1", features = ["derive"] }
 serde_json = "1.0.95"
--- a/boa_cli/src/debug/function.rs
+++ b/boa_cli/src/debug/function.rs
@ -0,0 +1,193 @@
+use boa_engine::{
+    builtins::function::Function,
+    object::ObjectInitializer,
+    vm::flowgraph::{Direction, Graph},
+    Context, JsArgs, JsNativeError, JsObject, JsResult, JsValue, NativeFunction,
+};
+use boa_interner::ToInternedString;
+
+use crate::FlowgraphFormat;
+
+fn flowgraph_parse_format_option(value: &JsValue) -> JsResult<FlowgraphFormat> {
+    if value.is_undefined() {
+        return Ok(FlowgraphFormat::Mermaid);
+    }
+
+    if let Some(string) = value.as_string() {
+        return match string.to_std_string_escaped().to_lowercase().as_str() {
+            "mermaid" => Ok(FlowgraphFormat::Mermaid),
+            "graphviz" => Ok(FlowgraphFormat::Graphviz),
+            format => Err(JsNativeError::typ()
+                .with_message(format!("Unknown format type '{format}'"))
+                .into()),
+        };
+    }
+
+    Err(JsNativeError::typ()
+        .with_message("format type must be a string")
+        .into())
+}
+
+fn flowgraph_parse_direction_option(value: &JsValue) -> JsResult<Direction> {
+    if value.is_undefined() {
+        return Ok(Direction::LeftToRight);
+    }
+
+    if let Some(string) = value.as_string() {
+        return match string.to_std_string_escaped().to_lowercase().as_str() {
+            "leftright" | "lr" => Ok(Direction::LeftToRight),
+            "rightleft" | "rl" => Ok(Direction::RightToLeft),
+            "topbottom" | "tb" => Ok(Direction::TopToBottom),
+            "bottomtop" | "bt " => Ok(Direction::BottomToTop),
+            direction => Err(JsNativeError::typ()
+                .with_message(format!("Unknown direction type '{direction}'"))
+                .into()),
+        };
+    }
+
+    Err(JsNativeError::typ()
+        .with_message("direction type must be a string")
+        .into())
+}
+
+/// Get functions instruction flowgraph
+fn flowgraph(_this: &JsValue, args: &[JsValue], context: &mut Context<'_>) -> JsResult<JsValue> {
+    let Some(value) = args.get(0) else {
+        return Err(JsNativeError::typ()
+        .with_message("expected function argument")
+        .into());
+    };
+
+    let Some(object) = value.as_object() else {
+        return Err(JsNativeError::typ()
+        .with_message(format!("expected object, got {}", value.type_of()))
+        .into());
+    };
+
+    let mut format = FlowgraphFormat::Mermaid;
+    let mut direction = Direction::LeftToRight;
+    if let Some(arguments) = args.get(1) {
+        if let Some(arguments) = arguments.as_object() {
+            format = flowgraph_parse_format_option(&arguments.get("format", context)?)?;
+            direction = flowgraph_parse_direction_option(&arguments.get("direction", context)?)?;
+        } else if value.is_string() {
+            format = flowgraph_parse_format_option(value)?;
+        } else {
+            return Err(JsNativeError::typ()
+                .with_message("options argument must be a string or object")
+                .into());
+        }
+    }
+
+    let object = object.borrow();
+
+    let Some(function) = object.as_function() else {
+        return Err(JsNativeError::typ()
+        .with_message("expected function object")
+        .into());
+    };
+
+    let code = match function {
+        Function::Ordinary { code, .. }
+        | Function::Async { code, .. }
+        | Function::Generator { code, .. }
+        | Function::AsyncGenerator { code, .. } => code,
+        Function::Native { .. } => {
+            return Err(JsNativeError::typ()
+                .with_message("native functions do not have bytecode")
+                .into())
+        }
+    };
+
+    let mut graph = Graph::new(direction);
+    code.to_graph(context.interner(), graph.subgraph(String::default()));
+    let result = match format {
+        FlowgraphFormat::Graphviz => graph.to_graphviz_format(),
+        FlowgraphFormat::Mermaid => graph.to_mermaid_format(),
+    };
+
+    Ok(JsValue::new(result))
+}
+
+fn bytecode(_: &JsValue, args: &[JsValue], context: &mut Context<'_>) -> JsResult<JsValue> {
+    let Some(value) = args.get(0) else {
+        return Err(JsNativeError::typ()
+        .with_message("expected function argument")
+        .into());
+    };
+
+    let Some(object) = value.as_object() else {
+        return Err(JsNativeError::typ()
+        .with_message(format!("expected object, got {}", value.type_of()))
+        .into());
+    };
+    let object = object.borrow();
+    let Some(function) = object.as_function() else {
+        return Err(JsNativeError::typ()
+        .with_message("expected function object")
+        .into());
+    };
+    let code = match function {
+        Function::Ordinary { code, .. }
+        | Function::Async { code, .. }
+        | Function::Generator { code, .. }
+        | Function::AsyncGenerator { code, .. } => code,
+        Function::Native { .. } => {
+            return Err(JsNativeError::typ()
+                .with_message("native functions do not have bytecode")
+                .into())
+        }
+    };
+
+    Ok(code.to_interned_string(context.interner()).into())
+}
+
+fn set_trace_flag_in_function_object(object: &JsObject, value: bool) -> JsResult<()> {
+    let object = object.borrow();
+    let Some(function) = object.as_function() else {
+        return Err(JsNativeError::typ()
+        .with_message("expected function object")
+        .into());
+    };
+    let code = match function {
+        Function::Ordinary { code, .. }
+        | Function::Async { code, .. }
+        | Function::Generator { code, .. }
+        | Function::AsyncGenerator { code, .. } => code,
+        Function::Native { .. } => {
+            return Err(JsNativeError::typ()
+                .with_message("native functions do not have bytecode")
+                .into())
+        }
+    };
+    code.set_trace(value);
+    Ok(())
+}
+
+/// Trace function.
+fn trace(_: &JsValue, args: &[JsValue], context: &mut Context<'_>) -> JsResult<JsValue> {
+    let value = args.get_or_undefined(0);
+    let this = args.get_or_undefined(1);
+
+    let Some(callable) = value.as_callable() else {
+        return Err(JsNativeError::typ()
+        .with_message("expected callable object")
+        .into());
+    };
+
+    let arguments = args.get(2..).unwrap_or(&[]);
+
+    set_trace_flag_in_function_object(callable, true)?;
+    let result = callable.call(this, arguments, context);
+    set_trace_flag_in_function_object(callable, false)?;
+
+    result
+}
+
+pub(super) fn create_object(context: &mut Context<'_>) -> JsObject {
+    ObjectInitializer::new(context)
+        .function(NativeFunction::from_fn_ptr(flowgraph), "flowgraph", 1)
+        .function(NativeFunction::from_fn_ptr(bytecode), "bytecode", 1)
+        .function(NativeFunction::from_fn_ptr(trace), "trace", 1)
+        .build()
+}
--- a/boa_cli/src/debug/gc.rs
+++ b/boa_cli/src/debug/gc.rs
@ -0,0 +1,13 @@
+use boa_engine::{object::ObjectInitializer, Context, JsObject, JsResult, JsValue, NativeFunction};
+
+/// Trigger garbage collection.
+fn collect(_: &JsValue, _: &[JsValue], _: &mut Context<'_>) -> JsResult<JsValue> {
+    boa_gc::force_collect();
+    Ok(JsValue::undefined())
+}
+
+pub(super) fn create_object(context: &mut Context<'_>) -> JsObject {
+    ObjectInitializer::new(context)
+        .function(NativeFunction::from_fn_ptr(collect), "collect", 0)
+        .build()
+}
--- a/boa_cli/src/debug/mod.rs
+++ b/boa_cli/src/debug/mod.rs
@ -0,0 +1,48 @@
+// Allow lint so it, doesn't warn about `JsResult<>` unneeded return on functions.
+#![allow(clippy::unnecessary_wraps)]
+
+use boa_engine::{object::ObjectInitializer, property::Attribute, Context, JsObject};
+
+mod function;
+mod gc;
+mod object;
+mod optimizer;
+
+fn create_boa_object(context: &mut Context<'_>) -> JsObject {
+    let function_module = function::create_object(context);
+    let object_module = object::create_object(context);
+    let optimizer_module = optimizer::create_object(context);
+    let gc_module = gc::create_object(context);
+
+    ObjectInitializer::new(context)
+        .property(
+            "function",
+            function_module,
+            Attribute::WRITABLE | Attribute::NON_ENUMERABLE | Attribute::CONFIGURABLE,
+        )
+        .property(
+            "object",
+            object_module,
+            Attribute::WRITABLE | Attribute::NON_ENUMERABLE | Attribute::CONFIGURABLE,
+        )
+        .property(
+            "optimizer",
+            optimizer_module,
+            Attribute::WRITABLE | Attribute::NON_ENUMERABLE | Attribute::CONFIGURABLE,
+        )
+        .property(
+            "gc",
+            gc_module,
+            Attribute::WRITABLE | Attribute::NON_ENUMERABLE | Attribute::CONFIGURABLE,
+        )
+        .build()
+}
+
+pub(crate) fn init_boa_debug_object(context: &mut Context<'_>) {
+    let boa_object = create_boa_object(context);
+    context.register_global_property(
+        "$boa",
+        boa_object,
+        Attribute::WRITABLE | Attribute::NON_ENUMERABLE | Attribute::CONFIGURABLE,
+    );
+}
--- a/boa_cli/src/debug/object.rs
+++ b/boa_cli/src/debug/object.rs
@ -0,0 +1,27 @@
+use boa_engine::{
+    object::ObjectInitializer, Context, JsNativeError, JsObject, JsResult, JsValue, NativeFunction,
+};
+
+/// Returns objects pointer in memory.
+fn id(_: &JsValue, args: &[JsValue], _: &mut Context<'_>) -> JsResult<JsValue> {
+    let Some(value) = args.get(0) else {
+        return Err(JsNativeError::typ()
+        .with_message("expected object argument")
+        .into());
+    };
+
+    let Some(object) = value.as_object() else {
+        return Err(JsNativeError::typ()
+        .with_message(format!("expected object, got {}", value.type_of()))
+        .into());
+    };
+
+    let ptr: *const _ = object.as_ref();
+    Ok(format!("0x{:X}", ptr as usize).into())
+}
+
+pub(super) fn create_object(context: &mut Context<'_>) -> JsObject {
+    ObjectInitializer::new(context)
+        .function(NativeFunction::from_fn_ptr(id), "id", 1)
+        .build()
+}
--- a/boa_cli/src/debug/optimizer.rs
+++ b/boa_cli/src/debug/optimizer.rs
@ -0,0 +1,82 @@
+use boa_engine::{
+    object::{FunctionObjectBuilder, ObjectInitializer},
+    optimizer::OptimizerOptions,
+    property::Attribute,
+    Context, JsArgs, JsObject, JsResult, JsValue, NativeFunction,
+};
+
+fn get_constant_folding(
+    _: &JsValue,
+    _: &[JsValue],
+    context: &mut Context<'_>,
+) -> JsResult<JsValue> {
+    Ok(context
+        .optimizer_options()
+        .contains(OptimizerOptions::CONSTANT_FOLDING)
+        .into())
+}
+
+fn set_constant_folding(
+    _: &JsValue,
+    args: &[JsValue],
+    context: &mut Context<'_>,
+) -> JsResult<JsValue> {
+    let value = args.get_or_undefined(0).to_boolean();
+    let mut options = context.optimizer_options();
+    options.set(OptimizerOptions::CONSTANT_FOLDING, value);
+    context.set_optimizer_options(options);
+    Ok(JsValue::undefined())
+}
+
+fn get_statistics(_: &JsValue, _: &[JsValue], context: &mut Context<'_>) -> JsResult<JsValue> {
+    Ok(context
+        .optimizer_options()
+        .contains(OptimizerOptions::STATISTICS)
+        .into())
+}
+
+fn set_statistics(_: &JsValue, args: &[JsValue], context: &mut Context<'_>) -> JsResult<JsValue> {
+    let value = args.get_or_undefined(0).to_boolean();
+    let mut options = context.optimizer_options();
+    options.set(OptimizerOptions::STATISTICS, value);
+    context.set_optimizer_options(options);
+    Ok(JsValue::undefined())
+}
+
+pub(super) fn create_object(context: &mut Context<'_>) -> JsObject {
+    let get_constant_folding =
+        FunctionObjectBuilder::new(context, NativeFunction::from_fn_ptr(get_constant_folding))
+            .name("get constantFolding")
+            .length(0)
+            .build();
+    let set_constant_folding =
+        FunctionObjectBuilder::new(context, NativeFunction::from_fn_ptr(set_constant_folding))
+            .name("set constantFolding")
+            .length(1)
+            .build();
+
+    let get_statistics =
+        FunctionObjectBuilder::new(context, NativeFunction::from_fn_ptr(get_statistics))
+            .name("get statistics")
+            .length(0)
+            .build();
+    let set_statistics =
+        FunctionObjectBuilder::new(context, NativeFunction::from_fn_ptr(set_statistics))
+            .name("set statistics")
+            .length(1)
+            .build();
+    ObjectInitializer::new(context)
+        .accessor(
+            "constantFolding",
+            Some(get_constant_folding),
+            Some(set_constant_folding),
+            Attribute::WRITABLE | Attribute::CONFIGURABLE | Attribute::NON_ENUMERABLE,
+        )
+        .accessor(
+            "statistics",
+            Some(get_statistics),
+            Some(set_statistics),
+            Attribute::WRITABLE | Attribute::CONFIGURABLE | Attribute::NON_ENUMERABLE,
+        )
+        .build()
+}
--- a/boa_cli/src/main.rs
+++ b/boa_cli/src/main.rs
@ -58,6 +58,7 @@
 )]
 #![allow(clippy::option_if_let_else, clippy::redundant_pub_crate)]

+mod debug;
 mod helper;

 use boa_ast::StatementList;
@ -70,6 +71,7 @@ use boa_engine::{
 };
 use clap::{Parser, ValueEnum, ValueHint};
 use colored::{Color, Colorize};
+use debug::init_boa_debug_object;
 use rustyline::{config::Config, error::ReadlineError, EditMode, Editor};
 use std::{cell::RefCell, collections::VecDeque, fs::read, fs::OpenOptions, io, path::PathBuf};

@ -146,6 +148,10 @@ struct Opt {
        requires = "graph"
    )]
    flowgraph_direction: Option<FlowgraphDirection>,
+
+    /// Inject debugging object `$boa`.
+    #[arg(long)]
+    debug_object: bool,
 }

 impl Opt {
@ -308,6 +314,10 @@ fn main() -> Result<(), io::Error> {
    // Trace Output
    context.set_trace(args.trace);

+    if args.debug_object {
+        init_boa_debug_object(&mut context);
+    }
+
    // Configure optimizer options
    let mut optimizer_options = OptimizerOptions::empty();
    optimizer_options.set(OptimizerOptions::STATISTICS, args.optimizer_statistics);
--- a/boa_engine/src/bytecompiler/mod.rs
+++ b/boa_engine/src/bytecompiler/mod.rs
@ -1327,6 +1327,8 @@ impl<'b, 'host> ByteCompiler<'b, 'host> {
            is_class_constructor: self.is_class_constructor,
            class_field_initializer_name: self.class_field_initializer_name,
            function_environment_push_location: self.function_environment_push_location,
+            #[cfg(feature = "trace")]
+            trace: std::cell::Cell::new(false),
        }
    }

--- a/boa_engine/src/object/mod.rs
+++ b/boa_engine/src/object/mod.rs
@ -2124,6 +2124,33 @@ impl<'ctx, 'host> ObjectInitializer<'ctx, 'host> {
        self
    }

+    /// Add new accessor property to the object.
+    ///
+    /// # Panics
+    ///
+    /// If both getter or setter are [`None`].
+    pub fn accessor<K>(
+        &mut self,
+        key: K,
+        get: Option<JsFunction>,
+        set: Option<JsFunction>,
+        attribute: Attribute,
+    ) -> &mut Self
+    where
+        K: Into<PropertyKey>,
+    {
+        // Accessors should have at least one function.
+        assert!(set.is_some() || get.is_some());
+
+        let property = PropertyDescriptor::builder()
+            .maybe_get(get)
+            .maybe_set(set)
+            .enumerable(attribute.enumerable())
+            .configurable(attribute.configurable());
+        self.object.borrow_mut().insert(key, property);
+        self
+    }
+
    /// Build the object.
    #[inline]
    pub fn build(&mut self) -> JsObject {
--- a/boa_engine/src/vm/code_block.rs
+++ b/boa_engine/src/vm/code_block.rs
@ -128,6 +128,11 @@ pub struct CodeBlock {
    /// We execute the parameter expressions in the function code and push the function environment afterward.
    /// When the execution of the parameter expressions throws an error, we do not need to pop the function environment.
    pub(crate) function_environment_push_location: u32,
+
+    #[cfg(feature = "trace")]
+    /// Trace instruction execution to `stdout`.
+    #[unsafe_ignore_trace]
+    pub(crate) trace: std::cell::Cell<bool>,
 }

 impl CodeBlock {
@ -153,9 +158,18 @@ impl CodeBlock {
            is_class_constructor: false,
            class_field_initializer_name: None,
            function_environment_push_location: 0,
+            #[cfg(feature = "trace")]
+            trace: std::cell::Cell::new(false),
        }
    }

+    /// Enable or disable instruction tracing to `stdout`.
+    #[cfg(feature = "trace")]
+    #[inline]
+    pub fn set_trace(&self, value: bool) {
+        self.trace.set(value);
+    }
+
    /// Read type T from code.
    ///
    /// # Safety
--- a/boa_engine/src/vm/mod.rs
+++ b/boa_engine/src/vm/mod.rs
@ -216,7 +216,7 @@ impl Context<'_> {

            // 1. Run the next instruction.
            #[cfg(feature = "trace")]
-            let result = if self.vm.trace {
+            let result = if self.vm.trace || self.vm.frame().code_block.trace.get() {
                let mut pc = self.vm.frame().pc;
                let opcode: Opcode = self
                    .vm
--- a/docs/boa_object.md
+++ b/docs/boa_object.md
@ -0,0 +1,150 @@
+# Boa Debug Object
+
+The `$boa` object contains useful utilities that can be used to debug JavaScript in JavaScript.
+
+It's injected into the context as global variable with the `--debug-object` command-line flag,
+the object is separated into modules.
+
+## Module `$boa.gc`
+
+This module contains functions that are related the garbage collector. It currently has the `.collect()` method.
+
+```JavaScript
+$boa.gc.collect()
+```
+
+This force triggers the GC to scan the heap and collect garbage.
+
+## Module `$boa.function`
+
+In this module are untility functions related to execution and debugging function.
+
+### Function `$boa.function.bytecode(func)`
+
+This function returns the compiled bytecode of a function as a string,
+
+```JavaScript
+>> function add(x, y) {
+  return x + y
+}
+>> $boa.function.bytecode(add)
+"
+------------------------Compiled Output: 'add'------------------------
+Location  Count   Opcode                     Operands
+
+000000    0000    DefInitArg                 0000: 'a'
+000005    0001    DefInitArg                 0001: 'b'
+000010    0002    RestParameterPop
+000011    0003    GetName                    0000: 'a'
+000016    0004    GetName                    0001: 'b'
+000021    0005    Add
+000022    0006    Return
+000023    0007    PushUndefined
+000024    0008    Return
+
+Literals:
+    <empty>
+
+Bindings:
+    0000: a
+    0001: b
+
+Functions:
+    <empty>
+"
+>>
+```
+
+### Function `$boa.function.trace(func, this, ...args)`
+
+It only traces the specified function. If the specified function calls other functions,
+their instructions aren't traced.
+
+```JavaScript
+>> const add = (a, b) => a + b
+>> $boa.function.trace(add, undefined, 1, 2)
+5μs           DefInitArg                 0000: 'a'                  2
+4μs           DefInitArg                 0001: 'b'                  <empty>
+0μs           RestParameterPop                                      <empty>
+3μs           GetName                    0000: 'a'                  1
+1μs           GetName                    0001: 'b'                  2
+2μs           Add                                                   3
+1μs           Return                                                3
+3
+>>
+```
+
+The `this` value can be changed as well as the arguments that are passed to the function.
+
+## Function `$boa.function.flowgraph(func, options)`
+
+It can be used to get the instruction flowgraph, like the command-line flag.
+This works on the function level, allows getting the flow graph without
+quiting the boa shell and adding the specified flags.
+
+Besides the function it also takes an argument that, can be a string or an object.
+If it is a string it represets the flowgraph format, otherwire if it's an object:
+
+```JavaScript
+// These are the defaults, if not specified.
+{
+    format: 'mermaid'
+    direction: 'LeftRight' // or 'LR' shorthand.
+}
+```
+
+Example:
+
+```JavaScript
+$boa.function.flowgraph(func, 'graphviz')
+$boa.function.flowgraph(func, { format: 'mermaid', direction: 'TopBottom' })
+```
+
+## Module `$boa.object`
+
+Contains utility functions for getting internal information about an object.
+
+## Function `$boa.object.id(object)`
+
+This function returns memory address of the given object, as a string.
+
+Example:
+
+```JavaScript
+let o = { x: 10, y: 20 }
+$boa.object.id(o)    // '0x7F5B3251B718'
+
+// Geting the address of the $boa object in memory
+$boa.object.id($boa) // '0x7F5B3251B5D8'
+```
+
+## Module `$boa.optimizer`
+
+This modules contains getters and setters for enabling and disabling optimizations.
+
+### Getter & Setter `$boa.optimizer.constantFolding`
+
+This is and accessor property on the module, its getter returns `true` if enabled or `false` otherwise.
+Its setter can be used to enable/disable the constant folding optimization.
+
+```JavaScript
+$boa.optimizer.constantFolding = true
+$boa.optimizer.constantFolding // true
+```
+
+### Getter & Setter `$boa.optimizer.statistics`
+
+This is and accessor property on the module, its getter returns `true` if enabled or `false` otherwise.
+Its setter can be used to enable/disable optimization statistics, which are printed to `stdout`.
+
+```JavaScript
+>> $boa.optimizer.constantFolding = true
+>> $boa.optimizer.statistics = true
+>> 1 + 1
+Optimizer {
+    constant folding: 1 run(s), 2 pass(es) (1 mutating, 1 checking)
+}
+
+2
+>>
+```
--- a/docs/debugging.md
+++ b/docs/debugging.md
@ -82,6 +82,21 @@ for example `--flowgraph-direction=left-to-right`, the default is `top-to-bottom
 [gihub-mermaid]: https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams
 [graphviz]: https://graphviz.org/

+## Debugging through the debug object $boa
+
+Certain debugging actions in JavaScript land are difficult to impossible, like triggering a GC collect.
+
+For such puroposes we have the `$boa` object that contains useful utilities that can be used to debug JavaScript in JavaScript.
+The debug object becomes available with the `--debug-object` cli flag, It injects the `$boa` debug object in the context as global variable,
+the object is separated into modules `gc`, `function`, `object`, etc.
+
+We can now do `$boa.gc.collect()`, which force triggers a GC collect.
+
+If you want to trace only a particular function (without being flodded by the `--trace` flag, that traces everything),
+for that we have the `$boa.function.trace(func, this, ...args)`.
+
+The full documentation of the `$boa` object's modules and functionalities can be found [`here`](./boa_object.md).
+
 ## Compiler panics

 In the case of a compiler panic, to get a full backtrace you will need to set