From 59a4ee0cefc975b4e7759fbc664370fa14d0deaf Mon Sep 17 00:00:00 2001
From: cjihrig <cjihrig@gmail.com>
Date: Wed, 21 Dec 2022 13:26:16 -0500
Subject: [PATCH] test_runner: add initial code coverage support

This commit adds code coverage functionality to the node:test
module. When node:test is used in conjunction with the new
--test-coverage CLI flag, a coverage report is created when
the test runner finishes. The coverage summary is forwarded to
any test runner reporters so that the display can be customized
as desired. This new functionality is compatible with the
existing NODE_V8_COVERAGE environment variable as well.

There are still several limitations, which will be addressed in
subsequent pull requests:

- Coverage is only reported for a single process. It is possible
  to merge coverage reports together. Once this is done, the
  --test flag will be supported as well.
- Source maps are not currently supported.
- Excluding specific files or directories from the coverage
  report is not currently supported. Node core modules and
  node_modules/ are excluded though.
---
 doc/api/cli.md                           |  11 +
 doc/api/test.md                          |  59 ++++
 doc/node.1                               |   3 +
 lib/internal/process/pre_execution.js    |  43 +--
 lib/internal/test_runner/coverage.js     | 362 +++++++++++++++++++++++
 lib/internal/test_runner/runner.js       |   2 +-
 lib/internal/test_runner/test.js         |  14 +
 lib/internal/test_runner/tests_stream.js |   4 +
 lib/internal/util.js                     |  22 ++
 lib/test/reporter/tap.js                 |  38 +++
 src/node_options.cc                      |   9 +
 src/node_options.h                       |   1 +
 12 files changed, 537 insertions(+), 31 deletions(-)
 create mode 100644 lib/internal/test_runner/coverage.js

diff --git a/doc/api/cli.md b/doc/api/cli.md
index fa044c442a0138..70e3b5c2176062 100644
--- a/doc/api/cli.md
+++ b/doc/api/cli.md
@@ -1229,6 +1229,16 @@ Starts the Node.js command line test runner. This flag cannot be combined with
 See the documentation on [running tests from the command line][]
 for more details.
 
+### `--test-coverage`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+When used in conjunction with the `node:test` module, a code coverage report is
+generated as part of the test runner output. See the documentation on
+[collecting code coverage from tests][] for more details.
+
 ### `--test-name-pattern`
 
 <!-- YAML
@@ -2358,6 +2368,7 @@ done
 [`unhandledRejection`]: process.md#event-unhandledrejection
 [`v8.startupSnapshot` API]: v8.md#startup-snapshot-api
 [`worker_threads.threadId`]: worker_threads.md#workerthreadid
+[collecting code coverage from tests]: test.md#collecting-code-coverage
 [conditional exports]: packages.md#conditional-exports
 [context-aware]: addons.md#context-aware-addons
 [debugger]: debugger.md
diff --git a/doc/api/test.md b/doc/api/test.md
index f17f43a59fe27d..4783cd6f444565 100644
--- a/doc/api/test.md
+++ b/doc/api/test.md
@@ -368,6 +368,26 @@ Otherwise, the test is considered to be a failure. Test files must be
 executable by Node.js, but are not required to use the `node:test` module
 internally.
 
+## Collecting code coverage
+
+When Node.js is started with the [`--test-coverage`][] command-line flag, code
+coverage is collected and statistics are reported once all tests have completed.
+If the [`NODE_V8_COVERAGE`][] environment variable is used to specify a
+code coverage directory, the generated V8 coverage files are written to that
+directory. Node.js core modules and files within `node_modules/` directories
+are not included in the coverage report. If coverage is enabled, the coverage
+report is sent to any [test reporters][] via the `'test:coverage'` event.
+
+The test runner's code coverage functionality has the following limitations,
+which will be addressed in a future Node.js release:
+
+* Although coverage data is collected for child processes, this information is
+  not included in the coverage report. Because the command line test runner uses
+  child processes to execute test files, it cannot be used with `--test-coverage`.
+* Source maps are not supported.
+* Excluding specific files or directories from the coverage report is not
+  supported.
+
 ## Mocking
 
 The `node:test` module supports mocking during testing via a top-level `mock`
@@ -1219,6 +1239,42 @@ A successful call to [`run()`][] method will return a new {TestsStream}
 object, streaming a series of events representing the execution of the tests.
 `TestsStream` will emit events, in the order of the tests definition
 
+### Event: `'test:coverage'`
+
+* `data` {Object}
+  * `summary` {Object} An object containing the coverage report.
+    * `files` {Array} An array of coverage reports for individual files. Each
+      report is an object with the following schema:
+      * `path` {string} The absolute path of the file.
+      * `totalLineCount` {number} The total number of lines.
+      * `totalBranchCount` {number} The total number of branches.
+      * `totalFunctionCount` {number} The total number of functions.
+      * `coveredLineCount` {number} The number of covered lines.
+      * `coveredBranchCount` {number} The number of covered branches.
+      * `coveredFunctionCount` {number} The number of covered functions.
+      * `coveredLinePercent` {number} The percentage of lines covered.
+      * `coveredBranchPercent` {number} The percentage of branches covered.
+      * `coveredFunctionPercent` {number} The percentage of functions covered.
+      * `uncoveredLineNumbers` {Array} An array of integers representing line
+        numbers that are uncovered.
+    * `totals` {Object} An object containing a summary of coverage for all
+      files.
+      * `totalLineCount` {number} The total number of lines.
+      * `totalBranchCount` {number} The total number of branches.
+      * `totalFunctionCount` {number} The total number of functions.
+      * `coveredLineCount` {number} The number of covered lines.
+      * `coveredBranchCount` {number} The number of covered branches.
+      * `coveredFunctionCount` {number} The number of covered functions.
+      * `coveredLinePercent` {number} The percentage of lines covered.
+      * `coveredBranchPercent` {number} The percentage of branches covered.
+      * `coveredFunctionPercent` {number} The percentage of functions covered.
+    * `workingDirectory` {string} The working directory when code coverage
+      began. This is useful for displaying relative path names in case the tests
+      changed the working directory of the Node.js process.
+  * `nesting` {number} The nesting level of the test.
+
+Emitted when code coverage is enabled and all tests have completed.
+
 ### Event: `'test:diagnostic'`
 
 * `data` {Object}
@@ -1589,6 +1645,7 @@ added:
 
 [TAP]: https://testanything.org/
 [`--import`]: cli.md#--importmodule
+[`--test-coverage`]: cli.md#--test-coverage
 [`--test-name-pattern`]: cli.md#--test-name-pattern
 [`--test-only`]: cli.md#--test-only
 [`--test-reporter-destination`]: cli.md#--test-reporter-destination
@@ -1597,6 +1654,7 @@ added:
 [`MockFunctionContext`]: #class-mockfunctioncontext
 [`MockTracker.method`]: #mockmethodobject-methodname-implementation-options
 [`MockTracker`]: #class-mocktracker
+[`NODE_V8_COVERAGE`]: cli.md#node_v8_coveragedir
 [`SuiteContext`]: #class-suitecontext
 [`TestContext`]: #class-testcontext
 [`context.diagnostic`]: #contextdiagnosticmessage
@@ -1607,4 +1665,5 @@ added:
 [describe options]: #describename-options-fn
 [it options]: #testname-options-fn
 [stream.compose]: stream.md#streamcomposestreams
+[test reporters]: #test-reporters
 [test runner execution model]: #test-runner-execution-model
diff --git a/doc/node.1 b/doc/node.1
index 6abee7e4b79a8a..cdbd10edb4f2c6 100644
--- a/doc/node.1
+++ b/doc/node.1
@@ -391,6 +391,9 @@ Specify the minimum allocation from the OpenSSL secure heap. The default is 2. T
 .It Fl -test
 Starts the Node.js command line test runner.
 .
+.It Fl -test-coverage
+Enable code coverage in the test runner.
+.
 .It Fl -test-name-pattern
 A regular expression that configures the test runner to only execute tests
 whose name matches the provided pattern.
diff --git a/lib/internal/process/pre_execution.js b/lib/internal/process/pre_execution.js
index 4c44927fb0c15a..6af480648ce7c7 100644
--- a/lib/internal/process/pre_execution.js
+++ b/lib/internal/process/pre_execution.js
@@ -20,6 +20,7 @@ const {
   exposeInterface,
   exposeLazyInterfaces,
   defineReplaceableLazyAttribute,
+  setupCoverageHooks,
 } = require('internal/util');
 
 const {
@@ -66,15 +67,7 @@ function prepareExecution(options) {
   setupFetch();
   setupWebCrypto();
   setupCustomEvent();
-
-  // Resolve the coverage directory to an absolute path, and
-  // overwrite process.env so that the original path gets passed
-  // to child processes even when they switch cwd.
-  if (process.env.NODE_V8_COVERAGE) {
-    process.env.NODE_V8_COVERAGE =
-      setupCoverageHooks(process.env.NODE_V8_COVERAGE);
-  }
-
+  setupCodeCoverage();
   setupDebugEnv();
   // Process initial diagnostic reporting configuration, if present.
   initializeReport();
@@ -304,6 +297,17 @@ function setupWebCrypto() {
   }
 }
 
+function setupCodeCoverage() {
+  // Resolve the coverage directory to an absolute path, and
+  // overwrite process.env so that the original path gets passed
+  // to child processes even when they switch cwd. Don't do anything if the
+  // --test-coverage flag is present, as the test runner will handle coverage.
+  if (process.env.NODE_V8_COVERAGE && !getOptionValue('--test-coverage')) {
+    process.env.NODE_V8_COVERAGE =
+      setupCoverageHooks(process.env.NODE_V8_COVERAGE);
+  }
+}
+
 // TODO(daeyeon): move this to internal/bootstrap/browser when the CLI flag is
 //                removed.
 function setupCustomEvent() {
@@ -315,27 +319,6 @@ function setupCustomEvent() {
   exposeInterface(globalThis, 'CustomEvent', CustomEvent);
 }
 
-// Setup User-facing NODE_V8_COVERAGE environment variable that writes
-// ScriptCoverage to a specified file.
-function setupCoverageHooks(dir) {
-  const cwd = require('internal/process/execution').tryGetCwd();
-  const { resolve } = require('path');
-  const coverageDirectory = resolve(cwd, dir);
-  const { sourceMapCacheToObject } =
-    require('internal/source_map/source_map_cache');
-
-  if (process.features.inspector) {
-    internalBinding('profiler').setCoverageDirectory(coverageDirectory);
-    internalBinding('profiler').setSourceMapCacheGetter(sourceMapCacheToObject);
-  } else {
-    process.emitWarning('The inspector is disabled, ' +
-                        'coverage could not be collected',
-                        'Warning');
-    return '';
-  }
-  return coverageDirectory;
-}
-
 function setupStacktracePrinterOnSigint() {
   if (!getOptionValue('--trace-sigint')) {
     return;
diff --git a/lib/internal/test_runner/coverage.js b/lib/internal/test_runner/coverage.js
new file mode 100644
index 00000000000000..0afe84e0ef2532
--- /dev/null
+++ b/lib/internal/test_runner/coverage.js
@@ -0,0 +1,362 @@
+'use strict';
+const {
+  ArrayPrototypeMap,
+  ArrayPrototypePush,
+  JSONParse,
+  MathFloor,
+  NumberParseInt,
+  RegExp,
+  RegExpPrototypeExec,
+  StringPrototypeIncludes,
+  StringPrototypeLocaleCompare,
+  StringPrototypeSplit,
+  StringPrototypeStartsWith,
+} = primordials;
+const {
+  copyFileSync,
+  mkdirSync,
+  mkdtempSync,
+  opendirSync,
+  readFileSync,
+} = require('fs');
+const { setupCoverageHooks } = require('internal/util');
+const { tmpdir } = require('os');
+const { join, resolve } = require('path');
+const { fileURLToPath } = require('url');
+const kCoveragePattern =
+  `^coverage\\-${process.pid}\\-(\\d{13})\\-(\\d+)\\.json$`;
+const kCoverageFileRegex = new RegExp(kCoveragePattern);
+const kIgnoreRegex = /\/\* node:coverage ignore next (?<count>\d+ )?\*\//;
+const kLineEndingRegex = /\r?\n$/u;
+const kLineSplitRegex = /(?<=\r?\n)/u;
+const kStatusRegex = /\/\* node:coverage (?<status>enable|disable) \*\//;
+
+class CoverageLine {
+  #covered;
+
+  constructor(line, src, startOffset) {
+    const newlineLength =
+      RegExpPrototypeExec(kLineEndingRegex, src)?.[0].length ?? 0;
+
+    this.line = line;
+    this.src = src;
+    this.startOffset = startOffset;
+    this.endOffset = startOffset + src.length - newlineLength;
+    this.ignore = false;
+    this.#covered = true;
+  }
+
+  get covered() {
+    return this.#covered;
+  }
+
+  set covered(isCovered) {
+    // V8 can generate multiple ranges that span the same line.
+    if (!this.#covered) {
+      return;
+    }
+
+    this.#covered = isCovered;
+  }
+}
+
+class TestCoverage {
+  constructor(coverageDirectory, originalCoverageDirectory, workingDirectory) {
+    this.coverageDirectory = coverageDirectory;
+    this.originalCoverageDirectory = originalCoverageDirectory;
+    this.workingDirectory = workingDirectory;
+  }
+
+  summary() {
+    internalBinding('profiler').takeCoverage();
+    const coverage = getCoverageFromDirectory(this.coverageDirectory);
+    const coverageSummary = {
+      __proto__: null,
+      workingDirectory: this.workingDirectory,
+      files: [],
+      totals: {
+        __proto__: null,
+        totalLineCount: 0,
+        totalBranchCount: 0,
+        totalFunctionCount: 0,
+        coveredLineCount: 0,
+        coveredBranchCount: 0,
+        coveredFunctionCount: 0,
+        coveredLinePercent: 0,
+        coveredBranchPercent: 0,
+        coveredFunctionPercent: 0,
+      },
+    };
+
+    if (!coverage) {
+      return coverageSummary;
+    }
+
+    for (let i = 0; i < coverage.length; ++i) {
+      const { functions, url } = coverage[i];
+
+      if (StringPrototypeStartsWith(url, 'node:') ||
+          StringPrototypeIncludes(url, '/node_modules/')) {
+        continue;
+      }
+
+      // Split the file source into lines. Make sure the lines maintain their
+      // original line endings because those characters are necessary for
+      // determining offsets in the file.
+      const filePath = fileURLToPath(url);
+      const source = readFileSync(filePath, 'utf8');
+      const linesWithBreaks = StringPrototypeSplit(source, kLineSplitRegex);
+      let ignoreCount = 0;
+      let enabled = true;
+      let offset = 0;
+      let totalBranches = 0;
+      let totalFunctions = 0;
+      let branchesCovered = 0;
+      let functionsCovered = 0;
+
+      const lines = ArrayPrototypeMap(linesWithBreaks, (line, i) => {
+        const startOffset = offset;
+        const coverageLine = new CoverageLine(i + 1, line, startOffset);
+
+        offset += line.length;
+
+        // Determine if this line is being ignored.
+        if (ignoreCount > 0) {
+          ignoreCount--;
+          coverageLine.ignore = true;
+        } else if (!enabled) {
+          coverageLine.ignore = true;
+        }
+
+        if (!coverageLine.ignore) {
+          // If this line is not already being ignored, check for ignore
+          // comments.
+          const match = RegExpPrototypeExec(kIgnoreRegex, line);
+
+          if (match !== null) {
+            const count = NumberParseInt(match.groups?.count ?? 1, 10);
+            ignoreCount = count;
+          }
+        }
+
+        // Check for comments to enable/disable coverage no matter what. These
+        // take precedence over ignore comments.
+        const match = RegExpPrototypeExec(kStatusRegex, line);
+        const status = match?.groups?.status;
+
+        if (status) {
+          ignoreCount = 0;
+          enabled = status === 'enable';
+        }
+
+        return coverageLine;
+      });
+
+      for (let j = 0; j < functions.length; ++j) {
+        const { functionName, isBlockCoverage, ranges } = functions[j];
+
+        for (let k = 0; k < ranges.length; ++k) {
+          const range = ranges[k];
+
+          mapRangeToLines(range, lines);
+
+          if (isBlockCoverage) {
+            if (range.count !== 0 ||
+                range.ignoredLines === range.lines.length) {
+              branchesCovered++;
+            }
+
+            totalBranches++;
+          }
+        }
+
+        if (functionName.length > 0 && ranges.length > 0) {
+          const range = ranges[0];
+
+          if (range.count !== 0 || range.ignoredLines === range.lines.length) {
+            functionsCovered++;
+          }
+
+          totalFunctions++;
+        }
+      }
+
+      let coveredCnt = 0;
+      const uncoveredLineNums = [];
+
+      for (let j = 0; j < lines.length; ++j) {
+        const line = lines[j];
+
+        if (line.covered || line.ignore) {
+          coveredCnt++;
+        } else {
+          ArrayPrototypePush(uncoveredLineNums, line.line);
+        }
+      }
+
+      ArrayPrototypePush(coverageSummary.files, {
+        __proto__: null,
+        path: filePath,
+        totalLineCount: lines.length,
+        totalBranchCount: totalBranches,
+        totalFunctionCount: totalFunctions,
+        coveredLineCount: coveredCnt,
+        coveredBranchCount: branchesCovered,
+        coveredFunctionCount: functionsCovered,
+        coveredLinePercent: toPercentage(coveredCnt, lines.length),
+        coveredBranchPercent: toPercentage(branchesCovered, totalBranches),
+        coveredFunctionPercent: toPercentage(functionsCovered, totalFunctions),
+        uncoveredLineNumbers: uncoveredLineNums,
+      });
+
+      coverageSummary.totals.totalLineCount += lines.length;
+      coverageSummary.totals.totalBranchCount += totalBranches;
+      coverageSummary.totals.totalFunctionCount += totalFunctions;
+      coverageSummary.totals.coveredLineCount += coveredCnt;
+      coverageSummary.totals.coveredBranchCount += branchesCovered;
+      coverageSummary.totals.coveredFunctionCount += functionsCovered;
+    }
+
+    coverageSummary.totals.coveredLinePercent = toPercentage(
+      coverageSummary.totals.coveredLineCount,
+      coverageSummary.totals.totalLineCount
+    );
+    coverageSummary.totals.coveredBranchPercent = toPercentage(
+      coverageSummary.totals.coveredBranchCount,
+      coverageSummary.totals.totalBranchCount
+    );
+    coverageSummary.totals.coveredFunctionPercent = toPercentage(
+      coverageSummary.totals.coveredFunctionCount,
+      coverageSummary.totals.totalFunctionCount
+    );
+    coverageSummary.files.sort(sortCoverageFiles);
+
+    return coverageSummary;
+  }
+
+  cleanup() {
+    // Restore the original value of process.env.NODE_V8_COVERAGE. Then, copy
+    // all of the created coverage files to the original coverage directory.
+    if (this.originalCoverageDirectory === undefined) {
+      delete process.env.NODE_V8_COVERAGE;
+      return;
+    }
+
+    process.env.NODE_V8_COVERAGE = this.originalCoverageDirectory;
+
+    try {
+      mkdirSync(this.originalCoverageDirectory, { recursive: true });
+      const dir = opendirSync(this.coverageDirectory);
+
+      for (let entry; (entry = dir.readSync()) !== null;) {
+        const src = join(this.coverageDirectory, entry.name);
+        const dst = join(this.originalCoverageDirectory, entry.name);
+        copyFileSync(src, dst);
+      }
+    } catch {
+      // Ignore.
+    }
+  }
+}
+
+function toPercentage(covered, total) {
+  return total === 0 ? 0 : (covered / total) * 100;
+}
+
+function sortCoverageFiles(a, b) {
+  return StringPrototypeLocaleCompare(a.path, b.path);
+}
+
+function setupCoverage() {
+  let originalCoverageDirectory = process.env.NODE_V8_COVERAGE;
+  const cwd = process.cwd();
+
+  if (originalCoverageDirectory) {
+    // NODE_V8_COVERAGE was already specified. Convert it to an absolute path
+    // and store it for later. The test runner will use a temporary directory
+    // so that no preexisting coverage files interfere with the results of the
+    // coverage report. Then, once the coverage is computed, move the coverage
+    // files back to the original NODE_V8_COVERAGE directory.
+    originalCoverageDirectory = resolve(cwd, originalCoverageDirectory);
+  }
+
+  const coverageDirectory = mkdtempSync(join(tmpdir(), 'node-coverage-'));
+  const enabled = setupCoverageHooks(coverageDirectory);
+
+  if (!enabled) {
+    return null;
+  }
+
+  // Ensure that NODE_V8_COVERAGE is set so that coverage can propagate to
+  // child processes.
+  process.env.NODE_V8_COVERAGE = coverageDirectory;
+
+  return new TestCoverage(coverageDirectory, originalCoverageDirectory, cwd);
+}
+
+function mapRangeToLines(range, lines) {
+  const { startOffset, endOffset, count } = range;
+  const mappedLines = [];
+  let ignoredLines = 0;
+  let start = 0;
+  let end = lines.length;
+  let mid;
+
+  while (start <= end) {
+    mid = MathFloor((start + end) / 2);
+    let line = lines[mid];
+
+    if (startOffset >= line.startOffset && startOffset <= line.endOffset) {
+      while (endOffset > line?.startOffset) {
+        // If the range is not covered, and the range covers the entire line,
+        // then mark that line as not covered.
+        if (count === 0 && startOffset <= line.startOffset &&
+            endOffset >= line.endOffset) {
+          line.covered = false;
+        }
+
+        ArrayPrototypePush(mappedLines, line);
+
+        if (line.ignore) {
+          ignoredLines++;
+        }
+
+        mid++;
+        line = lines[mid];
+      }
+
+      break;
+    } else if (startOffset >= line.endOffset) {
+      start = mid + 1;
+    } else {
+      end = mid - 1;
+    }
+  }
+
+  // Add some useful data to the range. The test runner has read these ranges
+  // from a file, so we own the data structures and can do what we want.
+  range.lines = mappedLines;
+  range.ignoredLines = ignoredLines;
+}
+
+function getCoverageFromDirectory(coverageDirectory) {
+  // TODO(cjihrig): Instead of only reading the coverage file for this process,
+  // combine all coverage files in the directory into a single data structure.
+  try {
+    const dir = opendirSync(coverageDirectory);
+
+    for (let entry; (entry = dir.readSync()) !== null;) {
+      if (RegExpPrototypeExec(kCoverageFileRegex, entry.name) === null) {
+        continue;
+      }
+
+      const coverageFile = join(coverageDirectory, entry.name);
+      const coverage = JSONParse(readFileSync(coverageFile, 'utf8'));
+      return coverage.result;
+    }
+  } catch {
+    // Ignore.
+  }
+}
+
+module.exports = { setupCoverage };
diff --git a/lib/internal/test_runner/runner.js b/lib/internal/test_runner/runner.js
index ff91993ce9df29..37f1957a78016f 100644
--- a/lib/internal/test_runner/runner.js
+++ b/lib/internal/test_runner/runner.js
@@ -49,7 +49,7 @@ const {
   exitCodes: { kGenericUserError },
 } = internalBinding('errors');
 
-const kFilterArgs = ['--test', '--watch'];
+const kFilterArgs = ['--test', '--test-coverage', '--watch'];
 const kFilterArgValues = ['--test-reporter', '--test-reporter-destination'];
 
 // TODO(cjihrig): Replace this with recursive readdir once it lands.
diff --git a/lib/internal/test_runner/test.js b/lib/internal/test_runner/test.js
index b7267491d50c3f..9ef02f94ff007d 100644
--- a/lib/internal/test_runner/test.js
+++ b/lib/internal/test_runner/test.js
@@ -195,6 +195,13 @@ class Test extends AsyncResource {
       this.runOnlySubtests = this.only;
       this.testNumber = 0;
       this.timeout = kDefaultTimeout;
+
+      if (getOptionValue('--test-coverage')) {
+        const { setupCoverage } = require('internal/test_runner/coverage');
+        this.coverage = setupCoverage();
+      } else {
+        this.coverage = null;
+      }
     } else {
       const nesting = parent.parent === null ? parent.nesting :
         parent.nesting + 1;
@@ -206,6 +213,7 @@ class Test extends AsyncResource {
       this.runOnlySubtests = !this.only;
       this.testNumber = parent.subtests.length + 1;
       this.timeout = parent.timeout;
+      this.coverage = null;
     }
 
     switch (typeof concurrency) {
@@ -645,6 +653,12 @@ class Test extends AsyncResource {
       this.reporter.diagnostic(this.nesting, `skipped ${counters.skipped}`);
       this.reporter.diagnostic(this.nesting, `todo ${counters.todo}`);
       this.reporter.diagnostic(this.nesting, `duration_ms ${this.#duration()}`);
+
+      if (this.coverage) {
+        this.reporter.coverage(this.nesting, this.coverage.summary());
+        this.coverage.cleanup();
+      }
+
       this.reporter.push(null);
     }
   }
diff --git a/lib/internal/test_runner/tests_stream.js b/lib/internal/test_runner/tests_stream.js
index b016d316154807..e2ce0afe055e97 100644
--- a/lib/internal/test_runner/tests_stream.js
+++ b/lib/internal/test_runner/tests_stream.js
@@ -55,6 +55,10 @@ class TestsStream extends Readable {
     this.#emit('test:diagnostic', { __proto__: null, nesting, message });
   }
 
+  coverage(nesting, summary) {
+    this.#emit('test:coverage', { __proto__: null, nesting, summary });
+  }
+
   #emit(type, data) {
     this.emit(type, data);
     this.#tryPush({ type, data });
diff --git a/lib/internal/util.js b/lib/internal/util.js
index 0d769e061fe448..9117611f8d3560 100644
--- a/lib/internal/util.js
+++ b/lib/internal/util.js
@@ -715,6 +715,27 @@ function getLazy(initializer) {
   };
 }
 
+// Setup user-facing NODE_V8_COVERAGE environment variable that writes
+// ScriptCoverage objects to a specified directory.
+function setupCoverageHooks(dir) {
+  const cwd = require('internal/process/execution').tryGetCwd();
+  const { resolve } = require('path');
+  const coverageDirectory = resolve(cwd, dir);
+  const { sourceMapCacheToObject } =
+    require('internal/source_map/source_map_cache');
+
+  if (process.features.inspector) {
+    internalBinding('profiler').setCoverageDirectory(coverageDirectory);
+    internalBinding('profiler').setSourceMapCacheGetter(sourceMapCacheToObject);
+  } else {
+    process.emitWarning('The inspector is disabled, ' +
+                        'coverage could not be collected',
+                        'Warning');
+    return '';
+  }
+  return coverageDirectory;
+}
+
 module.exports = {
   getLazy,
   assertCrypto,
@@ -750,6 +771,7 @@ module.exports = {
   SideEffectFreeRegExpPrototypeSymbolSplit,
   sleep,
   spliceOne,
+  setupCoverageHooks,
   toUSVString,
   removeColors,
 
diff --git a/lib/test/reporter/tap.js b/lib/test/reporter/tap.js
index fa5d4684fbb9e3..ebf7be80e2a2f5 100644
--- a/lib/test/reporter/tap.js
+++ b/lib/test/reporter/tap.js
@@ -3,6 +3,7 @@ const {
   ArrayPrototypeForEach,
   ArrayPrototypeJoin,
   ArrayPrototypePush,
+  NumberPrototypeToFixed,
   ObjectEntries,
   RegExpPrototypeSymbolReplace,
   SafeMap,
@@ -12,6 +13,7 @@ const {
 } = primordials;
 const { inspectWithNoCustomRetry } = require('internal/errors');
 const { isError, kEmptyObject } = require('internal/util');
+const { relative } = require('path');
 const kDefaultIndent = '    '; // 4 spaces
 const kFrameStartRegExp = /^ {4}at /;
 const kLineBreakRegExp = /\n|\r\n/;
@@ -46,6 +48,9 @@ async function * tapReporter(source) {
       case 'test:diagnostic':
         yield `${indent(data.nesting)}# ${tapEscape(data.message)}\n`;
         break;
+      case 'test:coverage':
+        yield reportCoverage(data.nesting, data.summary);
+        break;
     }
   }
 }
@@ -68,6 +73,39 @@ function reportTest(nesting, testNumber, status, name, skip, todo) {
   return line;
 }
 
+function reportCoverage(nesting, summary) {
+  const pad = indent(nesting);
+  let report = `${pad}# start of coverage report\n`;
+
+  report += `${pad}# file | line % | branch % | funcs % | uncovered lines\n`;
+
+  for (let i = 0; i < summary.files.length; ++i) {
+    const {
+      path,
+      coveredLinePercent,
+      coveredBranchPercent,
+      coveredFunctionPercent,
+      uncoveredLineNumbers,
+    } = summary.files[i];
+    const relativePath = relative(summary.workingDirectory, path);
+    const lines = NumberPrototypeToFixed(coveredLinePercent, 2);
+    const branches = NumberPrototypeToFixed(coveredBranchPercent, 2);
+    const functions = NumberPrototypeToFixed(coveredFunctionPercent, 2);
+    const uncovered = ArrayPrototypeJoin(uncoveredLineNumbers, ', ');
+
+    report += `${pad}# ${relativePath} | ${lines} | ${branches} | ` +
+              `${functions} | ${uncovered}\n`;
+  }
+
+  const { totals } = summary;
+  report += `${pad}# all files | ` +
+            `${NumberPrototypeToFixed(totals.coveredLinePercent, 2)} | ` +
+            `${NumberPrototypeToFixed(totals.coveredBranchPercent, 2)} | ` +
+            `${NumberPrototypeToFixed(totals.coveredFunctionPercent, 2)} |\n`;
+
+  report += `${pad}# end of coverage report\n`;
+  return report;
+}
 
 function reportDetails(nesting, data = kEmptyObject) {
   const { error, duration_ms } = data;
diff --git a/src/node_options.cc b/src/node_options.cc
index 3a064ce8282d05..0b19c859cdffe0 100644
--- a/src/node_options.cc
+++ b/src/node_options.cc
@@ -140,6 +140,12 @@ void EnvironmentOptions::CheckOptions(std::vector<std::string>* errors,
   }
 
   if (test_runner) {
+    if (test_runner_coverage) {
+      // TODO(cjihrig): This restriction can be removed once multi-process
+      // code coverage is supported.
+      errors->push_back("--test-coverage cannot be used with --test");
+    }
+
     if (syntax_check_only) {
       errors->push_back("either --test or --check can be used, not both");
     }
@@ -549,6 +555,9 @@ EnvironmentOptionsParser::EnvironmentOptionsParser() {
   AddOption("--test",
             "launch test runner on startup",
             &EnvironmentOptions::test_runner);
+  AddOption("--test-coverage",
+            "enable code coverage in the test runner",
+            &EnvironmentOptions::test_runner_coverage);
   AddOption("--test-name-pattern",
             "run tests whose name matches this regular expression",
             &EnvironmentOptions::test_name_pattern);
diff --git a/src/node_options.h b/src/node_options.h
index e145868763a51b..8f43fc106fe805 100644
--- a/src/node_options.h
+++ b/src/node_options.h
@@ -155,6 +155,7 @@ class EnvironmentOptions : public Options {
   std::string redirect_warnings;
   std::string diagnostic_dir;
   bool test_runner = false;
+  bool test_runner_coverage = false;
   std::vector<std::string> test_name_pattern;
   std::vector<std::string> test_reporter;
   std::vector<std::string> test_reporter_destination;