This specification describes an API that allows web applications to control a sampling profiler for measuring client JavaScript execution times.

Definitions

A sample is a descriptor of the instantaneous state of execution at a given point in time. Each sample is associated with a stack.

A stack is a list of frames that MUST be ordered sequentially from outermost to innermost frame.

A frame is an element in the context of a stack containing information about the current execution state.

Profiling Sessions

A profiling session is an abstract producer of samples. Each session has:

1. A state, which is one of {started, paused, stopped}.
2. A sample interval, defined as the periodicity at which the session obtains samples.

   The UA is NOT REQUIRED to take samples at this rate. However, it is RECOMMENDED that sampling is prioritized to take samples at this rate to produce higher quality traces.

3. An agent to profile.
4. A realm to profile.
5. A time origin that samples' timestamps are measured relative to.
6. A sample buffer size limit.
7. A ProfilerTrace storing captured samples.

Multiple profiling sessions on the same page SHOULD be supported.

Processing Model

To take a sample given a profiling session, perform the following steps:

1. If the length of ProfilerTrace.samples is greater than or equal to the sample buffer size limit associated with the profiling session, fire a new event of type samplebufferfull to the associated Profiler, move the state to stopped, and return.
2. Let sample be a new ProfilerSample.
3. Set the ProfilerSample.timestamp property of sample to the current high resolution time relative to the profiling session's time origin.
4. Let stack be the execution context stack associated with the profiling session's agent.
5. Set the ProfilerSample.stackId property of sample to the result of the get a stack ID algorithm on stack.
6. Add sample to the ProfilerTrace.samples associated with the session's ProfilerTrace.

To get a stack ID given an execution context stack bound to stack, perform the following steps:

1. If stack is empty, return undefined.
2. Let head be the top element of stack, and tail be the remainder of stack after removing its top element.
3. Let parentId be the result of calling get a stack ID recursively on tail.
4. Let frameId be the result of calling get a frame ID on head.
5. If frameId is undefined, return parentId.
6. Let profilerStack be a new ProfilerStack with ProfilerStack.frameId equal to frameId, and ProfilerStack.parentId equal to parentId.
7. Return the result of running get an element ID on profilerStack and ProfilerTrace.stacks.

To get a frame ID given an execution context bound to context, perform the following steps:

1. If the [= realm =] associated with context does not match the realm associated with the profiling session, return undefined.
2. Let frame be a new ProfilerFrame.
3. Let instance be equal to the function instance associated with context.
4. Set ProfilerFrame.name of frame to the function instance name associated with |instance|.
5. Let scriptOrModule be equal to the ScriptOrModule associated with context.
6. If |scriptOrModule| is non-null:

   Builtin functions have a null ScriptOrModule.

   1. Let script be the script obtained from scriptOrModule.[[\HostDefined]].
   2. If script is a classic script and its muted errors boolean is equal to true, return null.

      This check ensures that we avoid including stack frames from cross-origin scripts served in a CORS-cross-origin response. We may want to consider renaming muted errors to better reflect this use case.

   3. Let resourceString be equal to the base URL of script.
   4. Set ProfilerFrame.resourceId to the result of running get an element ID on resourceString and ProfilerTrace.resources.
   5. Set ProfilerFrame.line of frame to the 1-based index of the line at which instance is defined in its containing script.
   6. Set ProfilerFrame.column of frame to the 1-based index of the column at which instance is defined in its containing script.
7. Return the result of running get an element ID on frame and ProfilerTrace.frames.

To get an element ID for an item in a list, run the following steps:

1. If there exists an element in list component-wise equal to item, return its index.
2. Otherwise, append item to the end of list and return its index.

The Profiler Interface

      [Exposed=Window]
      interface Profiler : EventTarget {
        readonly attribute DOMHighResTimeStamp sampleInterval;
        readonly attribute boolean stopped;

        constructor(ProfilerInitOptions options);
        Promise<ProfilerTrace> stop();
      };
      

Each Profiler MUST be associated with exactly one profiling session.

The sampleInterval attribute MUST reflect the sample interval of the associated profiling session expressed as a DOMHighResTimeStamp.

The stopped attribute MUST be true if and only if the profiling session has state stopped.

{{Profiler}} is only exposed on {{Window}} until consensus is reached on [[Permissions-Policy]] and {{Worker}} integration.

The ProfilerTrace Dictionary

      typedef DOMString ProfilerResource;

      dictionary ProfilerTrace {
        required sequence<ProfilerResource> resources;
        required sequence<ProfilerFrame> frames;
        required sequence<ProfilerStack> stacks;
        required sequence<ProfilerSample> samples;
      };
      

The resources attribute MUST return the ProfilerResource list set by the take a sample algorithm.

The frames attribute MUST return the ProfilerFrame list set by the take a sample algorithm.

The stacks attribute MUST return the ProfilerStack list set by the take a sample algorithm.

The samples attribute MUST return the ProfilerSample list set by the take a sample algorithm.

Inspired by the V8 trace event format and Gecko profile format, this representation is designed to be easily and efficiently serializable.

        dictionary ProfilerSample {
          required DOMHighResTimeStamp timestamp;
          unsigned long long stackId;
        };
        

        dictionary ProfilerStack {
          unsigned long long parentId;
          required unsigned long long frameId;
        };
        

        dictionary ProfilerFrame {
          required DOMString name;
          unsigned long long resourceId;
          unsigned long long line;
          unsigned long long column;
        };
        

The ProfilerInitOptions dictionary

      dictionary ProfilerInitOptions {
        required DOMHighResTimeStamp sampleInterval;
        required unsigned long maxBufferSize;
      };
      

ProfilerInitOptions MUST support the following fields:

• sampleInterval is the application's desired sample interval. This value MUST be greater than or equal to zero.
• maxBufferSize is the desired sample buffer size limit, in samples.

Document Policy

This spec defines a configuration point in Document Policy with name js-profiling. Its type is boolean with default value false.

Document policy is leveraged to give UAs the ability to avoid storing required metadata for profiling when the document does not explicitly request it. While this metadata could conceivably be generated in response to a profiler being started, we store this bit on the document in order to signal to the engine as early as possible (as profiling early in page load is a common use case). This overhead may be non-trivial depending on the implementation, and therefore we default to false.

Automation

For the purposes of user-agent automation and application testing, this document defines the following [[WebDriver]] extension command.