feature(display): Implement pretty printing of arguments inside of console.log (closes #1437)

[gluzandii]

This pull request closes: #1437.

Firstly, it restructures the core::boa_engine::value::display module into different files. Initially the display module was a single file with everything in it, this made it hard to read and maintain, I have split it up into different modules inside of display, each pertaining to a certain data type for formatting (array, map, set etc).

Added compact formatting of certain Js types, so that it can be used in console logging the arguments object:

• array.rs
• map.rs
• set.rs
• object.rs

Along with the compact formatting methods that are used for logging arguments, there are also methods for regular logging (not compact) for the same data types as well, present in the same file.

This PR also added ability to log:

• Regex -> In the form "/{regex pattern}/{flags}"
• Date -> In the ISO 8601 format

The above 2 changes have been added for formatting in general (not only restricted to how arguments object is displayed)

Added method in value.rs called log_value_compact which is used for compact formatting of objects. This is what is internally used when the console.log(arguments) is called.

Compact formatting is the same as regular formatting but smartly compacts deeply nested types (like objects, arrays to just mentioning what type they are). It does this by keeping track of the depth. The current max depth is 2.

This can be changed by modifying the constant COMPACT_DEPTH_LIMIT constant inside of value.rs.

[Copilot]

Pull request overview

This PR addresses #1437 by improving how console.log formats and displays certain JavaScript values—especially arguments objects—while also refactoring the core::boa_engine::value::display module into smaller, type-focused files.

Changes:

• Refactors core/engine/src/value/display from a single large file into multiple modules (array/map/set/object/value/etc.).
• Adds specialized formatting for Arguments, RegExp (/pattern/flags), and Date (ISO 8601 / Invalid Date).
• Introduces “compact” formatting used when printing nested values within Arguments, plus new console tests covering the new output.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
core/runtime/src/console/tests.rs	Adds regression tests for logging arguments, RegExp, and Date.
core/engine/src/value/mod.rs	Removes now-unneeded HashSet import after display refactor.
core/engine/src/value/display/mod.rs	New display module entry point wiring submodules and ValueDisplay.
core/engine/src/value/display/value.rs	Main dispatch for value formatting; adds compact formatting + Date/RegExp handling.
core/engine/src/value/display/arguments.rs	Implements [Arguments] { ... } formatting and compact formatting of individual entries.
core/engine/src/value/display/array.rs	Array formatting + compact array formatting for nested contexts.
core/engine/src/value/display/map.rs	Map formatting + compact map formatting.
core/engine/src/value/display/set.rs	Set formatting + compact set formatting.
core/engine/src/value/display/object.rs	Object formatting helpers + compact plain-object formatting.
core/engine/src/value/display/typed_array.rs	Adds TypedArray formatting.
core/engine/src/value/display/primitives.rs	Extracts float formatting helper (-0 handling).
core/engine/src/value/display.rs	Removes the old monolithic display implementation (replaced by the new module layout).
core/engine/src/builtins/regexp/mod.rs	Exposes original source/flags for display formatting.
core/engine/src/builtins/date/mod.rs	Adds ISO 8601 display helper for Date.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

In log_array_compact, elements backed by accessor descriptors are currently treated as missing because it only reads descriptor.value(). That makes accessor elements display as <empty> even when present. Consider matching on DescriptorKind (like log_array_to) and printing a [Getter]/[Setter] marker (or similar) for accessors, reserving <empty> only for actual holes (no property at that index).

Suggested change

	if let Some(value) = x
	.borrow()
	.properties()
	.get(&i.into())
	.and_then(|x| x.value().cloned())
	{
	super::value::log_value_compact(f, &value, depth + 1, print_internals, encounters)?;
	if let Some(desc) = x.borrow().properties().get(&i.into()) {
	match desc.kind() {
	DescriptorKind::Data { value, .. } => {
	if let Some(value) = value {
	super::value::log_value_compact(
	f,
	value,
	depth + 1,
	print_internals,
	encounters,
	)?;
	} else {
	// Data descriptor with no value is treated as `undefined`, matching `log_array_to`.
	super::value::log_value_compact(
	f,
	&JsValue::undefined(),
	depth + 1,
	print_internals,
	encounters,
	)?;
	}
	}
	DescriptorKind::Accessor { get, set } => {
	let display = match (get.is_some(), set.is_some()) {
	(true, true) => "[Getter/Setter]",
	(true, false) => "[Getter]",
	(false, true) => "[Setter]",
	_ => "<empty>",
	};
	f.write_str(display)?;
	}
	DescriptorKind::Generic => {
	unreachable!("found generic descriptor in array")
	}
	}

[Copilot]

Array length is read as a JS number and cast to i32. This can overflow/turn negative for large arrays (length can be up to 2^32-1), leading to incorrect output and skipping elements because the 0..len loop becomes empty. Prefer parsing the length as u32/usize (or clamping) to keep iteration and Array({len}) accurate for large lengths.

[Copilot]

Arguments length is cast to i32 and then iterated with for i in 0..len. While typical argument counts are small, length is conceptually a non-negative integer; casting from a JS number to i32 can overflow/turn negative and make the loop skip printing. Prefer using u32/usize (or clamping) for len to ensure correct behavior for large lengths.

Suggested change

	.map(|n| n as i32)
	.map(|n| n.max(0.0).min(i32::MAX as f64) as i32)

[Copilot]

RegExp display uses original_source() verbatim inside /.../flags. For regexes created via new RegExp("a/b"), original_source will contain an unescaped /, so the output becomes /a/b/ (ambiguous / not a valid literal form). Consider escaping / (and other characters required by RegExp.prototype.toString, e.g. line terminators) when formatting so the printed form matches JS /.*/ syntax reliably.

Suggested change

	write!(
	f,
	"/{}/{}",
	regexp.original_source().to_std_string_escaped(),
	let source = regexp
	.original_source()
	.to_std_string_escaped()
	.replace('/', r"\/");
	write!(
	f,
	"/{}/{}",
	source,

[Copilot]

When formatting accessor properties, the key gets printed twice. print_obj_value_props already wrote the indented {key}: prefix, but the accessor branch writes {key}: {display} again, producing output like foo: foo: Getter. Write only the accessor marker after the prefix (e.g. just {display}) to avoid duplication.

Suggested change

	write!(f, "{key}: {display}")?;
	write!(f, "{display}")?;

[Copilot]

log_arguments_to reads indexed values directly from the object’s stored property descriptors (properties().get(...).value()), which can diverge from the actual value for MappedArguments (values are sourced from the environment via the parameter map). This also treats accessor descriptors as <empty> even when the property exists. Prefer detecting MappedArguments and using its get(index) for mapped entries (falling back to the stored value when unmapped), and for non-data descriptors print a [Getter]/[Setter] marker similar to array/object formatting instead of <empty>.

Suggested change

	// FIXME: handle accessor descriptors
	let val = x
	.borrow()
	.properties()
	.get(&i.into())
	.and_then(|d| d.value().cloned());
	match val {
	Some(v) => {
	write!(
	f,
	" {i}: {}",
	CompactValue {
	value: &v,
	print_internals
	}
	)?;
	}
	None => {
	// Retrieve the property descriptor once so we can distinguish between
	// data and accessor descriptors and, for mapped arguments, prefer the
	// live mapped value over the stored descriptor value.
	let desc = x
	.borrow()
	.properties()
	.get(&i.into())
	.cloned();
	match desc {
	Some(desc) => {
	if !desc.is_data_descriptor() {
	// Accessor descriptor: print a marker instead of `<empty>`.
	let has_get = desc.get().is_some();
	let has_set = desc.set().is_some();
	let marker = match (has_get, has_set) {
	(true, true) => "[Getter/Setter]",
	(true, false) => "[Getter]",
	(false, true) => "[Setter]",
	// An accessor descriptor with neither getter nor setter
	// is effectively empty.
	(false, false) => "<empty>",
	};
	write!(f, " {i}: {marker}")?;
	} else {
	// Data descriptor: start from the stored value.
	// For mapped arguments, prefer the live mapped value when present.
	let mut value = desc.value().cloned();
	// If this is a `MappedArguments` object, consult its mapping for `i`.
	if let Some(mapped) = x.borrow().as_mapped_arguments() {
	if let Some(mapped_value) = mapped.get(i as u32) {
	value = Some(mapped_value);
	}
	}
	match value {
	Some(v) => {
	write!(
	f,
	" {i}: {}",
	CompactValue {
	value: &v,
	print_internals
	}
	)?;
	}
	None => {
	write!(f, " {i}: <empty>")?;
	}
	}
	}
	}
	None => {
	// No property descriptor at all for this index.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,687	49,687	0
Ignored	2,262	2,262	0
Failed	1,014	1,014	0
Panics	0	0	0
Conformance	93.81%	93.81%	0.00%

Tested main commit: 34ca7f1f404c2712d6897c3e39e5f5e981ce9ed6
Tested PR commit: e7d82273a2600713ae1db74dce49e541afb26a82
Compare commits: 34ca7f1...e7d8227

[codecov]

Codecov Report

❌ Patch coverage is 41.12554% with 272 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.05%. Comparing base (6ddc2b4) to head (c10f628).
⚠️ Report is 742 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/value/display/value.rs	30.55%	100 Missing ⚠️
core/engine/src/value/display/object.rs	37.27%	69 Missing ⚠️
core/engine/src/value/display/typed_array.rs	0.00%	33 Missing ⚠️
core/engine/src/value/display/array.rs	40.81%	29 Missing ⚠️
core/engine/src/value/display/map.rs	47.05%	18 Missing ⚠️
core/engine/src/value/display/set.rs	50.00%	15 Missing ⚠️
core/engine/src/value/display/arguments.rs	89.65%	3 Missing ⚠️
core/engine/src/value/display/mod.rs	40.00%	3 Missing ⚠️
core/engine/src/builtins/date/mod.rs	89.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4853      +/-   ##
==========================================
+ Coverage   47.24%   57.05%   +9.81%     
==========================================
  Files         476      562      +86     
  Lines       46892    60721   +13829     
==========================================
+ Hits        22154    34645   +12491     
- Misses      24738    26076    +1338     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[gluzandii]

I force pushed since my branch was very behind the main branch, hence updated its base commit to the latest.