Node.js v14.19.0 documentation

v8.cachedDataVersionTag()#

• Returns: <integer>

Returns an integer representing a version tag derived from the V8 version, command-line flags, and detected CPU features. This is useful for determining whether a vm.Script cachedData buffer is compatible with this instance of V8.

console.log(v8.cachedDataVersionTag()); // 3947234607
// The value returned by v8.cachedDataVersionTag() is derived from the V8
// version, command-line flags, and detected CPU features. Test that the value
// does indeed update when flags are toggled.
v8.setFlagsFromString('--allow_natives_syntax');
console.log(v8.cachedDataVersionTag()); // 183726201

v8.getHeapCodeStatistics()#

• Returns: <Object>

Returns an object with the following properties:

• code_and_metadata_size <number>
• bytecode_and_metadata_size <number>
• external_script_source_size <number>

{
  code_and_metadata_size: 212208,
  bytecode_and_metadata_size: 161368,
  external_script_source_size: 1410794
}

v8.getHeapSnapshot()#

• Returns: <stream.Readable> A Readable Stream containing the V8 heap snapshot

Generates a snapshot of the current V8 heap and returns a Readable Stream that may be used to read the JSON serialized representation. This JSON stream format is intended to be used with tools such as Chrome DevTools. The JSON schema is undocumented and specific to the V8 engine. Therefore, the schema may change from one version of V8 to the next.

// Print heap snapshot to the console
const v8 = require('v8');
const stream = v8.getHeapSnapshot();
stream.pipe(process.stdout);

v8.getHeapSpaceStatistics()#

• Returns: <Object[]>

Returns statistics about the V8 heap spaces, i.e. the segments which make up the V8 heap. Neither the ordering of heap spaces, nor the availability of a heap space can be guaranteed as the statistics are provided via the V8 GetHeapSpaceStatistics function and may change from one V8 version to the next.

The value returned is an array of objects containing the following properties:

• space_name <string>
• space_size <number>
• space_used_size <number>
• space_available_size <number>
• physical_space_size <number>

[
  {
    "space_name": "new_space",
    "space_size": 2063872,
    "space_used_size": 951112,
    "space_available_size": 80824,
    "physical_space_size": 2063872
  },
  {
    "space_name": "old_space",
    "space_size": 3090560,
    "space_used_size": 2493792,
    "space_available_size": 0,
    "physical_space_size": 3090560
  },
  {
    "space_name": "code_space",
    "space_size": 1260160,
    "space_used_size": 644256,
    "space_available_size": 960,
    "physical_space_size": 1260160
  },
  {
    "space_name": "map_space",
    "space_size": 1094160,
    "space_used_size": 201608,
    "space_available_size": 0,
    "physical_space_size": 1094160
  },
  {
    "space_name": "large_object_space",
    "space_size": 0,
    "space_used_size": 0,
    "space_available_size": 1490980608,
    "physical_space_size": 0
  }
]

v8.getHeapStatistics()#

• Returns: <Object>

Returns an object with the following properties:

• total_heap_size <number>
• total_heap_size_executable <number>
• total_physical_size <number>
• total_available_size <number>
• used_heap_size <number>
• heap_size_limit <number>
• malloced_memory <number>
• peak_malloced_memory <number>
• does_zap_garbage <number>
• number_of_native_contexts <number>
• number_of_detached_contexts <number>

does_zap_garbage is a 0/1 boolean, which signifies whether the --zap_code_space option is enabled or not. This makes V8 overwrite heap garbage with a bit pattern. The RSS footprint (resident set size) gets bigger because it continuously touches all heap pages and that makes them less likely to get swapped out by the operating system.

number_of_native_contexts The value of native_context is the number of the top-level contexts currently active. Increase of this number over time indicates a memory leak.

number_of_detached_contexts The value of detached_context is the number of contexts that were detached and not yet garbage collected. This number being non-zero indicates a potential memory leak.

{
  total_heap_size: 7326976,
  total_heap_size_executable: 4194304,
  total_physical_size: 7326976,
  total_available_size: 1152656,
  used_heap_size: 3476208,
  heap_size_limit: 1535115264,
  malloced_memory: 16384,
  peak_malloced_memory: 1127496,
  does_zap_garbage: 0,
  number_of_native_contexts: 1,
  number_of_detached_contexts: 0
}

v8.setFlagsFromString(flags)#

• flags <string>

The v8.setFlagsFromString() method can be used to programmatically set V8 command-line flags. This method should be used with care. Changing settings after the VM has started may result in unpredictable behavior, including crashes and data loss; or it may simply do nothing.

The V8 options available for a version of Node.js may be determined by running node --v8-options.

Usage:

// Print GC events to stdout for one minute.
const v8 = require('v8');
v8.setFlagsFromString('--trace_gc');
setTimeout(() => { v8.setFlagsFromString('--notrace_gc'); }, 60e3);

v8.stopCoverage()#

The v8.stopCoverage() method allows the user to stop the coverage collection started by NODE_V8_COVERAGE, so that V8 can release the execution count records and optimize code. This can be used in conjunction with v8.takeCoverage() if the user wants to collect the coverage on demand.

v8.takeCoverage()#

The v8.takeCoverage() method allows the user to write the coverage started by NODE_V8_COVERAGE to disk on demand. This method can be invoked multiple times during the lifetime of the process. Each time the execution counter will be reset and a new coverage report will be written to the directory specified by NODE_V8_COVERAGE.

When the process is about to exit, one last coverage will still be written to disk unless v8.stopCoverage() is invoked before the process exits.

v8.writeHeapSnapshot([filename])#

• filename <string> The file path where the V8 heap snapshot is to be saved. If not specified, a file name with the pattern 'Heap-${yyyymmdd}-${hhmmss}-${pid}-${thread_id}.heapsnapshot' will be generated, where {pid} will be the PID of the Node.js process, {thread_id} will be 0 when writeHeapSnapshot() is called from the main Node.js thread or the id of a worker thread.
• Returns: