docs/checklist/javascript.md - devtools/devtools-frontend - Git at Google


            
            # Chromium DevTools support checklist for JavaScript language features


            [goo.gle/v8-checklist](https://goo.gle/v8-checklist)


            Implementation for new language features (NLF) are often intrusive and affect many parts of

            
            [V8](https://v8.dev). This sometimes causes the debugger to not work seamlessly with the NLF

            
            out of the box. We generally make the distinction between _Basic functionality_ and _Extended

            
            functionality_ when talking about debugger support:


            - Basic functionality is required for every NLF in order to launch. The debugger must not crash

            
              or act in a confusing way when interacting with the NLF. For example, stepping into a `Proxy`

            
              trap handler should be possible.

            
            - Extended functionality is often just nice-to-have, but in some cases required for launch.

            
              This includes debugging capabilities specific to the language feature. For example, catch

            
              prediction should work as expected for `Promise`s.


            This document attempts to list all relevant aspects of V8’s JavaScript debugger that constitute

            
            basic functionality (checkout [this document](https://goo.gle/devtools-wasm-checklist) for V8's

            
            WebAssembly debugger features). Items on the list may not apply to every language feature,

            
            depending on its nature.


            *** note

            
            **IMPORTANT:** Please take a look at the [DevTools UI feature checklist](./ui.md) prior

            
            to changing or extending the DevTools user interface (UI).

            
            ***


            [TOC]


            ## Console printing


            DevTools offers a REPL through the Console panel. Logging a value should print reasonable output.


            ### Affected


            All NLFs that affect how values should be printed in a REPL, such as NLFs that introduce new primitives, new `RegExp` flags, etc.


            ### How to test


            Open DevTools, select the Console panel, and enter a source snippet with the NLF. The printed result should look alright.


            ### Reading material


            [Example CL that adds printing support for a new `RegExp` flag](https://chromium-review.googlesource.com/c/v8/v8/+/2848460)


            ## Syntax highlighting and pretty-printing


            DevTools provides syntax highlighting and pretty-printing for JavaScript sources.


            ### Affected


            All NLFs that introduce new syntax.


            ### How to test


            Open DevTools, select the Sources panel, and create a new source snippet with the NLF. The syntax highlighting of the source

            
            should look alright. This is handled by [CodeMirror](https://codemirror.net/) inside of Chromium DevTools.


            Clicking on the pretty-printing button on the bottom left ("{}") should yield reasonable results. The formatting relies on the

            
            [acorn](https://github.com/acornjs/acorn) parser, which needs to support the NLF in question for this to work.


            ## Stack traces


            Stack traces is the most often used way for developers to debug issues. Aside from the default `Error.stack` string, we also

            
            offer a way for user code to override how to format the `stack` property, and collect a more detailed structured stack trace

            
            for DevTools.


            Sometimes, due to the way a feature is implemented, there may be stack frames that show up on the stack trace when they should

            
            not, or vice versa.


            For runtime exceptions, we look for the closest code position that has a source position attached. That source position is used

            
            as expression position for the exception. For syntax errors, we should report the correct location of the syntax error via

            
            [`MessageHandler::ReportMessage()`](https://source.chromium.org/chromium/chromium/src/+/main:v8/src/execution/messages.h;l=174-176;drc=4e40b002b46c019d18eae68b5e5a342605609d95).


            Note that `Error.stack` is only collected for `Error` objects, at the time the `Error` object is constructed. This may be different

            
            than the stack trace passed to DevTools via [`JSMessageObject`](https://source.chromium.org/chromium/chromium/src/+/main:v8/src/objects/js-objects.h;l=1264-1345;drc=82dff63dbf9db05e9274e11d9128af7b9f51ceaa).


            ### Affected


            NLFs that can cause an exception to be thrown, or can call into another function that throws.


            ### How to test


            When throwing inside the NLF, or with it on the stack, the stack trace including source positions should make sense. Also check

            
            the structured stack trace when the exception is not caught and logged into Chrome's DevTools console.


            Repeat with the "Disable async stack traces" checkbox in the Preferences checked.


            ### Test cases


            Test cases for stack traces is mandatory, if there is any way the NLF can interact with throwing exceptions. For examples look

            
            for `mjsunit` tests with `stack-trace` in their names.


            ### Reading material


            [Design doc for debugging support for tail-calls](https://docs.google.com/a/google.com/document/d/1Bk4QahtaT-XzrMlHTkm0SVol3LoKXTrC9E7INxJBHrE/edit?usp=drive\_web)


            ## Async stack traces


            DevTools offers a way to show async stack traces by stitching together stack traces collected at the

            
            location where the callback is passed, and the actual location of the exception inside the callback.


            The canonical example of this is throwing inside a `setTimeout` callback. The async stack trace will

            
            consist of the stack trace of the error plus a stack trace captured at the `setTimeout` call-site.


            The instrumentation is based on four [`ayncTask*` methods](https://source.chromium.org/chromium/chromium/src/+/main:v8/include/v8-inspector.h;l=370-375;drc=aafd25ffbc72935b52fee731f957e5827c73a096). Namely `asyncTaskScheduled`, `asyncTaskStarted`, `asyncTaskFinished` and `asyncTaskCancelled`.

            
            V8 has a [higher-level interface](https://source.chromium.org/chromium/chromium/src/+/main:v8/src/debug/debug-interface.h;l=351-352;drc=e6fc2038d73ef96ff47deda3146d94d25530e13b) that translates Promise events (such as `await`, `.then`, etc) to these for `asyncTask*` methods.


            ### Affected


            NLFs touching promises or callback scheduling.


            ### How to test


            Throw or pause inside a new language feature and check either the call stack sidebar panel in "Sources"

            
            or use a `console.trace` and check the async stack trace in the console.


            ### Test cases


            Test cases for async stack traces are mandatory, if there is any way the NLF interacts with callback scheduling or

            
            Promises. For examples look in V8 `inspector` tests with `async-stack` in their name (e.g. [here](https://source.chromium.org/chromium/chromium/src/+/main:v8/test/inspector/debugger/async-stack-await.js)).


            ## Catch prediction


            Aside from offering stack traces, V8's debugger supports DevTools' pause-on-exception feature. This comes in two flavors:

            
            pause on all exceptions, and pause on uncaught exceptions. In both cases, we break at the throw site (not at the `catch`,

            
            or any rethrow via `try`-`finally`).


            For the former, this is as easy as breaking in the debugger on `Isolate::Throw()`. For the latter, we have to predict whether

            
            the thrown exception will be caught or otherwise handled (for example by a `Promise`'s catch handler).


            ### Affected


            NLFs that can cause an exception to be thrown, or can call into another function that throws.


            ### How to test


            When pause-on-exception is enabled, and throwing inside the NLF or with it on the stack, the script should pause as expected.


            Repeat with the "pause on caught exception" checkbox checked.


            ### Test cases


            Test cases for exception prediction is mandatory, if there is any way the NLF can interact with exceptions, be it by throwing

            
            exceptions, or by relying on `try`-`catch` or `try`-`finally` in its implementation. Look for `mjsunit` tests that contain the

            
            string `setBreakOnException` or `setBreakOnUncaughtException`.


            ### Reading material


            [Design doc for exception prediction for async-await](https://docs.google.com/a/google.com/document/d/1ef1drN6RTRN7iDB8FXJg\_JJZFNObCFbM1UjZWK\_ORUE/edit?usp=drive\_web)


            ## Breakpoints


            One of the most important features is setting break points. The semantics should be obvious.


            Break locations are function calls, return sites, and statements. Special care are necessary for loops: for example, in `for`-loops

            
            we do want to be able to break separately at the loop entry, condition evaluation, and increment.


            When setting a break point at an arbitrary source position, we actually check for the closest breakable source position, and move

            
            the break point there. Consecutive debug break events at the same source position are ignored by the debugger.


            ### Affected


            NLFs that change generated code, and especially once that introduce new break locations.


            ### How to test


            Open DevTools and set break points in parts of script related to the NLF, then run the script.


            ### Test cases


            Look for `mjsunit` tests with `debug-break` in their names.


            ## Stepping


            Stepping is the logical consequence to breakpoints, and is based on the same mechanism in the debugger. We differentiate between


            * Step out, which takes us to the next break location in the caller.

            
            * Step next, which takes us to the next break location while ignoring calls into other functions. Note that this includes recursive

            
              calls. Step next at a return site is equivalent to a step out.

            
            * Step in, which takes us to the next break location including calls into another function.

            
            * Step frame, which takes us to another function, either a callee or a caller. This is used for framework blackboxing, where the V8

            
              inspector is not interested in stepping in the current function, and wants to be notified once we arrive at another function


            ### Affected


            NLFs that are affect breakpoints


            ### How to test


            Break inside the part of script related to the NLF, and try stepping in, next, and out.


            ### Test cases


            Look for `mjsunit` tests with `debug-step` in their names.


            ### Reading material


            [Design doc on stepping in async-await](https://docs.google.com/a/google.com/document/d/1nj3nMlQVEFlq57dA-K8wFxdOB5ovvNuwNbWxBhcBOKE/edit?usp=drive\_web)


            ## Frame inspector


            The frame inspector in V8 offers a way to a way to introspect frames on the call stack at the debug break. For the top-most frame,

            
            the break location is that of the debug break. For frames below the break location is the call site leading to the frame above.


            For each frame, we can


            - inspect the scope chain at the break location with the scope iterator,

            
            - find out whether it's called as constructor,

            
            - find out whether we are at a return position,

            
            - get the function being called,

            
            - get the receiver,

            
            - get the arguments, and

            
            - get the number of stack-allocated locals.


            For optimized code, we use the deoptimizer to get hold of receiver, arguments and stack locals, but this is often not possible, and we

            
            get the `optimzed_out` sentinel value.


            ### Affected


            NLFs that affect the way V8 calls functions.


            ### How to test


            When paused inside the function affected by the NLF, the Call Stack view in the DevTools' Source panel should show useful information.


            ### Test cases


            Take a look at `test/mjsunit/wasm/frame-inspection.js`.


            ## Scope iterator


            The scope iterator in V8 offers a way to introspect the scope chain at the break location. It includes not only the scopes outside of

            
            the current function, but also scopes inside it, for example inner block scopes, catch scopes, with scopes, etc.


            For each scope inside the current function, we can materialize an object representing local variables belonging to it. For scopes

            
            outside the current function this is not possible.


            We can use the scope iterator to alter the value of local variables, unless we are inside an optimized frame.


            ### Affected


            NLFs that introduce new scopes.


            ### How to test


            When paused in DevTools inside the scope introduced by the NLF, the "Scope" view on the Sources panel should show useful information.

            
            Scopes that are introduced by the NLF for desugaring purposes may better be hidden.


            ### Test cases


            Take a look at `test/mjsunit/debug-scopes.js`.


            ### Reading material


            [CL that introduces hidden scopes](https://chromium.googlesource.com/v8/v8/+/672983830f36222d90748ff588831b6dae565c38)


            ## Debug evaluate


            With debug-evaluate, V8 offers a way to evaluate scripts at a break, attempting to behave as if the script code was executed

            
            right at the break location. It is based on the frame inspector and the scope iterator.


            It works by creating a context chain that not only references contexts on the current context chain, but also contains the

            
            materialized stack, including arguments object and the receiver. The script is then compiled and executed inside this context chain.


            There are some limitations, and special attention has to be paid to variable name shadowing.


            Side-effect-free debug-evaluate statically determines whether a function should throw. You should check whether to update the

            
            whitelist in `src/debug/debug-evaluate.cc`.


            ### Affected


            NLFs that are also affected by the scope iterator and frame inspector.


            ### How to test


            Use the DevTools console to run scripts at a debug break. In particular the preview shown in the DevTools console by default indicates

            
            whether the side-effect detection works correctly (i.e. whether you updated the whitelist correctly).


            ### Test cases


            Look for mjsunit tests with "debug-evaluate" in their names.


            ### Reading material


            This [tea-and-crumpets](https://drive.google.com/file/d/0BwPS\_JpKyELWTXV4NGZzS085NVE/view) [presentation](https://docs.google.com/a/google.com/presentation/d/18-c04ri-Whcp1dbteVTqLtK2wqlUzFgfU4kp8KgyF3I/edit?usp=drive\_web)


            Debug-evaluate without side effect [doc](https://docs.google.com/document/d/1l\_JzDbOZ6Cn1k0c5vnyEXHe7B2Xxm7lROs1vYR3nR2I/edit) and [presentation](https://docs.google.com/presentation/d/1u9lDBPMRo-mSQ6mmO03ZmpIiQ-haKyd9O4aFF0nomWs/edit)


            ## Code Coverage


            Code coverage gathers execution counts and exposes them to developers through the Inspector protocol.


            ### Affected


            NLFs that contain control flow (e.g branches, loops, etc.).


            ### How to test


            Run `d8` with `--lcov` and check whether the produced coverage information is correct. E.g. like this:


            ```bash

            
            ./d8 --lcov=cov.info test.js

            
            genhtml cov.info -o coverage

            
            ```


            Then navigate your browser to `coverage/index.html`.


            ### Test cases


            `test/mjsunit/code-coverage-block.js`


            ### Reading material


            Design doc: [go/v8-designdoc-block-code-coverage](http://go/v8-designdoc-block-code-coverage)


            ## Heap profiler


            The heap profiler is a tool usually used to find out what is taking so much memory, and find potential memory leaks. It is an object graph visitor.


            ### Affected


            NLFs that change object layouts or introduce new object types


            ### How to test


            Take a heap Snapshot in DevTools' Profiler panel and inspect the result. Objects related to the NLF should fan out to all objects it references to.


            ### Test cases


            Take a look at `test/cctest/test-heap-profiler.cc`.


            ## Restart frame


            DevTools allows restarting of some stack frames, not just the top-level one. The feature is implemented by throwing a termination exception and unwinding the stack

            
            until after the function we want to restart, and then re-invoking the function. This only works for certain functions, e.g. async functions can't be restarted

            
            as that would require rolling back the underlying generator.


            ### Affected


            NLFs that change function execution or require a function to carry state (e.g. async functions

            
            need a generator that can't be reset).


            ### How to test


            While paused, right click a stack frame with the NLF in the call stack view and select "Restart frame". If the NLF prevents the frame from being restarted, then

            
            the "Restart frame" menu item must be disabled, otherwise the frame must be properly restarted.


            ### Test cases


            Take a look in `test/inspector/debugger/restart-frame/*`.


            ## LiveEdit


            LiveEdit is a feature that allows for script content to be replaced during its execution. While it has many limitations, the most often use case

            
            of editing the function we are paused in and restarting said function afterwards.


            ### Affected


            NLFs that affect code execution


            ### How to test


            Open DevTools and break inside the part of script affected by the NLF. Change the code inside the function with the NLF and save the script.


            ### Test cases


            Look for `mjsunit` tests with `liveedit` in their names.
	# Chromium DevTools support checklist for JavaScript language features

	[goo.gle/v8-checklist](https://goo.gle/v8-checklist)

	Implementation for new language features (NLF) are often intrusive and affect many parts of
	[V8](https://v8.dev). This sometimes causes the debugger to not work seamlessly with the NLF
	out of the box. We generally make the distinction between _Basic functionality_ and _Extended
	functionality_ when talking about debugger support:

	- Basic functionality is required for every NLF in order to launch. The debugger must not crash
	or act in a confusing way when interacting with the NLF. For example, stepping into a `Proxy`
	trap handler should be possible.
	- Extended functionality is often just nice-to-have, but in some cases required for launch.
	This includes debugging capabilities specific to the language feature. For example, catch
	prediction should work as expected for `Promise`s.

	This document attempts to list all relevant aspects of V8’s JavaScript debugger that constitute
	basic functionality (checkout [this document](https://goo.gle/devtools-wasm-checklist) for V8's
	WebAssembly debugger features). Items on the list may not apply to every language feature,
	depending on its nature.

	*** note
	IMPORTANT: Please take a look at the [DevTools UI feature checklist](./ui.md) prior
	to changing or extending the DevTools user interface (UI).
	***

	[TOC]

	## Console printing

	DevTools offers a REPL through the Console panel. Logging a value should print reasonable output.

	### Affected

	All NLFs that affect how values should be printed in a REPL, such as NLFs that introduce new primitives, new `RegExp` flags, etc.

	### How to test

	Open DevTools, select the Console panel, and enter a source snippet with the NLF. The printed result should look alright.

	### Reading material

	[Example CL that adds printing support for a new `RegExp` flag](https://chromium-review.googlesource.com/c/v8/v8/+/2848460)


	## Syntax highlighting and pretty-printing

	DevTools provides syntax highlighting and pretty-printing for JavaScript sources.

	### Affected

	All NLFs that introduce new syntax.

	### How to test

	Open DevTools, select the Sources panel, and create a new source snippet with the NLF. The syntax highlighting of the source
	should look alright. This is handled by [CodeMirror](https://codemirror.net/) inside of Chromium DevTools.

	Clicking on the pretty-printing button on the bottom left ("{}") should yield reasonable results. The formatting relies on the
	[acorn](https://github.com/acornjs/acorn) parser, which needs to support the NLF in question for this to work.


	## Stack traces

	Stack traces is the most often used way for developers to debug issues. Aside from the default `Error.stack` string, we also
	offer a way for user code to override how to format the `stack` property, and collect a more detailed structured stack trace
	for DevTools.

	Sometimes, due to the way a feature is implemented, there may be stack frames that show up on the stack trace when they should
	not, or vice versa.

	For runtime exceptions, we look for the closest code position that has a source position attached. That source position is used
	as expression position for the exception. For syntax errors, we should report the correct location of the syntax error via
	[`MessageHandler::ReportMessage()`](https://source.chromium.org/chromium/chromium/src/+/main:v8/src/execution/messages.h;l=174-176;drc=4e40b002b46c019d18eae68b5e5a342605609d95).

	Note that `Error.stack` is only collected for `Error` objects, at the time the `Error` object is constructed. This may be different
	than the stack trace passed to DevTools via [`JSMessageObject`](https://source.chromium.org/chromium/chromium/src/+/main:v8/src/objects/js-objects.h;l=1264-1345;drc=82dff63dbf9db05e9274e11d9128af7b9f51ceaa).

	### Affected

	NLFs that can cause an exception to be thrown, or can call into another function that throws.

	### How to test

	When throwing inside the NLF, or with it on the stack, the stack trace including source positions should make sense. Also check
	the structured stack trace when the exception is not caught and logged into Chrome's DevTools console.

	Repeat with the "Disable async stack traces" checkbox in the Preferences checked.

	### Test cases

	Test cases for stack traces is mandatory, if there is any way the NLF can interact with throwing exceptions. For examples look
	for `mjsunit` tests with `stack-trace` in their names.

	### Reading material

	[Design doc for debugging support for tail-calls](https://docs.google.com/a/google.com/document/d/1Bk4QahtaT-XzrMlHTkm0SVol3LoKXTrC9E7INxJBHrE/edit?usp=drive\_web)

	## Async stack traces

	DevTools offers a way to show async stack traces by stitching together stack traces collected at the
	location where the callback is passed, and the actual location of the exception inside the callback.

	The canonical example of this is throwing inside a `setTimeout` callback. The async stack trace will
	consist of the stack trace of the error plus a stack trace captured at the `setTimeout` call-site.

	The instrumentation is based on four [`ayncTask*` methods](https://source.chromium.org/chromium/chromium/src/+/main:v8/include/v8-inspector.h;l=370-375;drc=aafd25ffbc72935b52fee731f957e5827c73a096). Namely `asyncTaskScheduled`, `asyncTaskStarted`, `asyncTaskFinished` and `asyncTaskCancelled`.
	V8 has a [higher-level interface](https://source.chromium.org/chromium/chromium/src/+/main:v8/src/debug/debug-interface.h;l=351-352;drc=e6fc2038d73ef96ff47deda3146d94d25530e13b) that translates Promise events (such as `await`, `.then`, etc) to these for `asyncTask*` methods.

	### Affected

	NLFs touching promises or callback scheduling.

	### How to test

	Throw or pause inside a new language feature and check either the call stack sidebar panel in "Sources"
	or use a `console.trace` and check the async stack trace in the console.

	### Test cases

	Test cases for async stack traces are mandatory, if there is any way the NLF interacts with callback scheduling or
	Promises. For examples look in V8 `inspector` tests with `async-stack` in their name (e.g. [here](https://source.chromium.org/chromium/chromium/src/+/main:v8/test/inspector/debugger/async-stack-await.js)).

	## Catch prediction

	Aside from offering stack traces, V8's debugger supports DevTools' pause-on-exception feature. This comes in two flavors:
	pause on all exceptions, and pause on uncaught exceptions. In both cases, we break at the throw site (not at the `catch`,
	or any rethrow via `try`-`finally`).

	For the former, this is as easy as breaking in the debugger on `Isolate::Throw()`. For the latter, we have to predict whether
	the thrown exception will be caught or otherwise handled (for example by a `Promise`'s catch handler).

	### Affected

	NLFs that can cause an exception to be thrown, or can call into another function that throws.

	### How to test

	When pause-on-exception is enabled, and throwing inside the NLF or with it on the stack, the script should pause as expected.

	Repeat with the "pause on caught exception" checkbox checked.

	### Test cases

	Test cases for exception prediction is mandatory, if there is any way the NLF can interact with exceptions, be it by throwing
	exceptions, or by relying on `try`-`catch` or `try`-`finally` in its implementation. Look for `mjsunit` tests that contain the
	string `setBreakOnException` or `setBreakOnUncaughtException`.

	### Reading material

	[Design doc for exception prediction for async-await](https://docs.google.com/a/google.com/document/d/1ef1drN6RTRN7iDB8FXJg\_JJZFNObCFbM1UjZWK\_ORUE/edit?usp=drive\_web)


	## Breakpoints

	One of the most important features is setting break points. The semantics should be obvious.

	Break locations are function calls, return sites, and statements. Special care are necessary for loops: for example, in `for`-loops
	we do want to be able to break separately at the loop entry, condition evaluation, and increment.

	When setting a break point at an arbitrary source position, we actually check for the closest breakable source position, and move
	the break point there. Consecutive debug break events at the same source position are ignored by the debugger.

	### Affected

	NLFs that change generated code, and especially once that introduce new break locations.

	### How to test

	Open DevTools and set break points in parts of script related to the NLF, then run the script.

	### Test cases

	Look for `mjsunit` tests with `debug-break` in their names.


	## Stepping

	Stepping is the logical consequence to breakpoints, and is based on the same mechanism in the debugger. We differentiate between

	* Step out, which takes us to the next break location in the caller.
	* Step next, which takes us to the next break location while ignoring calls into other functions. Note that this includes recursive
	calls. Step next at a return site is equivalent to a step out.
	* Step in, which takes us to the next break location including calls into another function.
	* Step frame, which takes us to another function, either a callee or a caller. This is used for framework blackboxing, where the V8
	inspector is not interested in stepping in the current function, and wants to be notified once we arrive at another function

	### Affected

	NLFs that are affect breakpoints

	### How to test

	Break inside the part of script related to the NLF, and try stepping in, next, and out.

	### Test cases

	Look for `mjsunit` tests with `debug-step` in their names.

	### Reading material

	[Design doc on stepping in async-await](https://docs.google.com/a/google.com/document/d/1nj3nMlQVEFlq57dA-K8wFxdOB5ovvNuwNbWxBhcBOKE/edit?usp=drive\_web)


	## Frame inspector

	The frame inspector in V8 offers a way to a way to introspect frames on the call stack at the debug break. For the top-most frame,
	the break location is that of the debug break. For frames below the break location is the call site leading to the frame above.

	For each frame, we can

	- inspect the scope chain at the break location with the scope iterator,
	- find out whether it's called as constructor,
	- find out whether we are at a return position,
	- get the function being called,
	- get the receiver,
	- get the arguments, and
	- get the number of stack-allocated locals.

	For optimized code, we use the deoptimizer to get hold of receiver, arguments and stack locals, but this is often not possible, and we
	get the `optimzed_out` sentinel value.

	### Affected

	NLFs that affect the way V8 calls functions.

	### How to test

	When paused inside the function affected by the NLF, the Call Stack view in the DevTools' Source panel should show useful information.

	### Test cases

	Take a look at `test/mjsunit/wasm/frame-inspection.js`.


	## Scope iterator

	The scope iterator in V8 offers a way to introspect the scope chain at the break location. It includes not only the scopes outside of
	the current function, but also scopes inside it, for example inner block scopes, catch scopes, with scopes, etc.

	For each scope inside the current function, we can materialize an object representing local variables belonging to it. For scopes
	outside the current function this is not possible.

	We can use the scope iterator to alter the value of local variables, unless we are inside an optimized frame.

	### Affected

	NLFs that introduce new scopes.

	### How to test

	When paused in DevTools inside the scope introduced by the NLF, the "Scope" view on the Sources panel should show useful information.
	Scopes that are introduced by the NLF for desugaring purposes may better be hidden.

	### Test cases

	Take a look at `test/mjsunit/debug-scopes.js`.

	### Reading material

	[CL that introduces hidden scopes](https://chromium.googlesource.com/v8/v8/+/672983830f36222d90748ff588831b6dae565c38)


	## Debug evaluate

	With debug-evaluate, V8 offers a way to evaluate scripts at a break, attempting to behave as if the script code was executed
	right at the break location. It is based on the frame inspector and the scope iterator.

	It works by creating a context chain that not only references contexts on the current context chain, but also contains the
	materialized stack, including arguments object and the receiver. The script is then compiled and executed inside this context chain.

	There are some limitations, and special attention has to be paid to variable name shadowing.

	Side-effect-free debug-evaluate statically determines whether a function should throw. You should check whether to update the
	whitelist in `src/debug/debug-evaluate.cc`.

	### Affected

	NLFs that are also affected by the scope iterator and frame inspector.

	### How to test

	Use the DevTools console to run scripts at a debug break. In particular the preview shown in the DevTools console by default indicates
	whether the side-effect detection works correctly (i.e. whether you updated the whitelist correctly).

	### Test cases

	Look for mjsunit tests with "debug-evaluate" in their names.

	### Reading material

	This [tea-and-crumpets](https://drive.google.com/file/d/0BwPS\_JpKyELWTXV4NGZzS085NVE/view) [presentation](https://docs.google.com/a/google.com/presentation/d/18-c04ri-Whcp1dbteVTqLtK2wqlUzFgfU4kp8KgyF3I/edit?usp=drive\_web)

	Debug-evaluate without side effect [doc](https://docs.google.com/document/d/1l\_JzDbOZ6Cn1k0c5vnyEXHe7B2Xxm7lROs1vYR3nR2I/edit) and [presentation](https://docs.google.com/presentation/d/1u9lDBPMRo-mSQ6mmO03ZmpIiQ-haKyd9O4aFF0nomWs/edit)


	## Code Coverage

	Code coverage gathers execution counts and exposes them to developers through the Inspector protocol.

	### Affected

	NLFs that contain control flow (e.g branches, loops, etc.).

	### How to test

	Run `d8` with `--lcov` and check whether the produced coverage information is correct. E.g. like this:

	```bash
	./d8 --lcov=cov.info test.js
	genhtml cov.info -o coverage
	```

	Then navigate your browser to `coverage/index.html`.

	### Test cases

	`test/mjsunit/code-coverage-block.js`

	### Reading material

	Design doc: [go/v8-designdoc-block-code-coverage](http://go/v8-designdoc-block-code-coverage)


	## Heap profiler

	The heap profiler is a tool usually used to find out what is taking so much memory, and find potential memory leaks. It is an object graph visitor.

	### Affected

	NLFs that change object layouts or introduce new object types

	### How to test

	Take a heap Snapshot in DevTools' Profiler panel and inspect the result. Objects related to the NLF should fan out to all objects it references to.

	### Test cases

	Take a look at `test/cctest/test-heap-profiler.cc`.

	## Restart frame

	DevTools allows restarting of some stack frames, not just the top-level one. The feature is implemented by throwing a termination exception and unwinding the stack
	until after the function we want to restart, and then re-invoking the function. This only works for certain functions, e.g. async functions can't be restarted
	as that would require rolling back the underlying generator.

	### Affected

	NLFs that change function execution or require a function to carry state (e.g. async functions
	need a generator that can't be reset).

	### How to test

	While paused, right click a stack frame with the NLF in the call stack view and select "Restart frame". If the NLF prevents the frame from being restarted, then
	the "Restart frame" menu item must be disabled, otherwise the frame must be properly restarted.

	### Test cases

	Take a look in `test/inspector/debugger/restart-frame/*`.

	## LiveEdit

	LiveEdit is a feature that allows for script content to be replaced during its execution. While it has many limitations, the most often use case
	of editing the function we are paused in and restarting said function afterwards.

	### Affected

	NLFs that affect code execution

	### How to test

	Open DevTools and break inside the part of script affected by the NLF. Change the code inside the function with the NLF and save the script.

	### Test cases

	Look for `mjsunit` tests with `liveedit` in their names.