test_runner: add initial test_runner module

This commit adds a new test_runner module that exposes an
API for creating JavaScript tests. As the tests execute, TAP
output is written to standard output. This commit only supports
executing individual test files, and does not implement
command line functionality for a full test runner.

Author: cjihrig

doc/api/errors.md

@@ -2551,6 +2551,14 @@ An unspecified or non-specific system error has occurred within the Node.js
 process. The error object will have an `err.info` object property with
 additional details.
 
+<a id="ERR_TEST_FAILURE"></a>
+
+### `ERR_TEST_FAILURE`
+
+This error represents a failed test. Additional information about the failure
+is available via the `cause` property. The `failureType` property specifies
+what the test was doing when the failure occurred.
+
 <a id="ERR_TLS_CERT_ALTNAME_FORMAT"></a>
 
 ### `ERR_TLS_CERT_ALTNAME_FORMAT`

doc/api/index.md

@@ -54,6 +54,7 @@
 * [Report](report.md)
 * [Stream](stream.md)
 * [String decoder](string_decoder.md)
+* [Test runner](test_runner.md)
 * [Timers](timers.md)
 * [TLS/SSL](tls.md)
 * [Trace events](tracing.md)

doc/api/test_runner.md

@@ -0,0 +1,276 @@
+# Test runner
+
+<!--introduced_in=REPLACEME-->
+
+> Stability: 1 - Experimental
+
+<!-- source_link=lib/test_runner.js -->
+
+The `test_runner` module facilitates the creation of JavaScript tests that
+report results in [TAP][] format. To access it:
+
+```mjs
+import test from 'test_runner';
+```
+
+```cjs
+const test = require('test_runner');
+```
+
+Tests created via the `test_runner` module consist of a single function that
+executes either synchronously or asynchronously. Synchronous tests are
+considered passing if they do not throw an exception. Asynchronous tests return
+a `Promise`, and are considered passing if the returned `Promise` does not
+reject. The following example illustrates how tests are written using the
+`test_runner` module.
+
+```js
+test('synchronous passing test', (t) => {
+  // This test passes because it does not throw an exception.
+  assert.strictEqual(1, 1);
+});
+
+test('synchronous failing test', (t) => {
+  // This test fails because it throws an exception.
+  assert.strictEqual(1, 2);
+});
+
+test('asynchronous passing test', async (t) => {
+  // This test passes because the Promise returned by the async
+  // function is not rejected.
+  assert.strictEqual(1, 1);
+});
+
+test('asynchronous failing test', async (t) => {
+  // This test fails because the Promise returned by the async
+  // function is rejected.
+  assert.strictEqual(1, 2);
+});
+
+test('failing test using Promises', (t) => {
+  // Promises can be used directly as well.
+  return new Promise((resolve, reject) => {
+    setImmediate(() => {
+      reject(new Error('this will cause the test to fail'));
+    });
+  });
+});
+```
+
+As a test file executes, TAP is written to the standard output of the Node.js
+process. This output can be interpreted by any test harness that understands
+the TAP format. If any tests fail, the process exit code is set to `1`.
+
+## Subtests
+
+The test context's `test()` method allows subtests to be created. This method
+behaves identically to the top level `test()` function. The following example
+demonstrates the creation of a top level test with two subtests.
+
+```js
+test('top level test', async (t) => {
+  await t.test('subtest 1', (t) => {
+    assert.strictEqual(1, 1);
+  });
+
+  await t.test('subtest 2', (t) => {
+    assert.strictEqual(2, 2);
+  });
+});
+```
+
+In this example, `await` is used to ensure that both subtests have completed.
+This is necessary because parent tests do not wait for their subtests to
+complete. Any subtests that are still outstanding when their parent finishes
+are cancelled and treated as failures. Any subtest failures cause the parent
+test to fail.
+
+## Skipping tests
+
+Individual tests can be skipped by passing the `skip` option to the test, or by
+calling the test context's `skip()` method. Both of these options support
+including a message that is displayed in the TAP output as shown in the
+following example.
+
+```js
+// The skip option is used, but no message is provided.
+test('skip option', { skip: true }, (t) => {
+  // This code is never executed.
+});
+
+// The skip option is used, and a message is provided.
+test('skip option with message', { skip: 'this is skipped' }, (t) => {
+  // This code is never executed.
+});
+
+test('skip() method', (t) => {
+  // Make sure to return here as well if the test contains additional logic.
+  t.skip();
+});
+
+test('skip() method with message', (t) => {
+  // Make sure to return here as well if the test contains additional logic.
+  t.skip('this is skipped');
+});
+```
+
+## Extraneous asynchronous activity
+
+Once a test function finishes executing, the TAP results are output as quickly
+as possible while maintaining the order of the tests. However, it is possible
+for the test function to generate asynchronous activity that outlives the test
+itself. The test runner handles this type of activity, but does not delay the
+reporting of test results in order to accommodate it.
+
+In the following example, a test completes with two `setImmediate()`
+operations still outstanding. The first `setImmediate()` attempts to create a
+new subtest. Because the parent test has already finished and output its
+results, the new subtest is immediately marked as failed, and reported in the
+top level of the file's TAP output.
+
+The second `setImmediate()` creates an `uncaughtException` event.
+`uncaughtException` and `unhandledRejection` events originating from a completed
+test are handled by the `test_runner` module and reported as diagnostic
+warnings in the top level of the file's TAP output.
+
+```js
+test('a test that creates asynchronous activity', (t) => {
+  setImmediate(() => {
+    t.test('subtest that is created too late', (t) => {
+      throw new Error('error1');
+    });
+  });
+
+  setImmediate(() => {
+    throw new Error('error2');
+  });
+
+  // The test finishes after this line.
+});
+```
+
+## `test([name][, options][, fn])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `name` {string} The name of the test, which is displayed when reporting test
+  results. **Default:** The `name` property of `fn`, or `'<anonymous>'` if `fn`
+  does not have a name.
+* `options` {Object} Configuration options for the test. The following
+  properties are supported:
+  * `concurrency` {number} The number of tests that can be run at the same time.
+    If unspecified, subtests inherit this value from their parent.
+    **Default:** `1`.
+  * `skip` {boolean|string} If truthy, the test is skipped. If a string is
+    provided, that string is displayed in the test results as the reason for
+    skipping the test. **Default:** `false`.
+* `fn` {Function|AsyncFunction} The function under test. The test fails if this
+  function throws an exception or returns a `Promise` that rejects. This
+  function is invoked with a single [`TestContext`][] argument.
+  **Default:** A no-op function.
+* Returns: {Promise} Resolved with `undefined` once the test completes.
+
+The `test()` function is the value imported from the `test_runner` module. Each
+invocation of this function results in the creation of a test point in the TAP
+output.
+
+The `TestContext` object passed to the `fn` argument can be used to perform
+actions related to the current test. Examples include skipping the test, adding
+additional TAP diagnostic information, or creating subtests.
+
+`test()` returns a `Promise` that resolves once the test completes. The return
+value can usually be discarded for top level tests. However, the return value
+from subtests should be used to prevent the parent test from finishing first
+and cancelling the subtest as shown in the following example.
+
+```js
+test('top level test', async (t) => {
+  // The setTimeout() in the following subtest would cause it to outlive its
+  // parent test if 'await' is removed on the next line. Once the parent test
+  // completes, it will cancel any outstanding subtests.
+  await t.test('longer running subtest', async (t) => {
+    return new Promise((resolve, reject) => {
+      setTimeout(resolve, 1000);
+    });
+  });
+});
+```
+
+## Class: `TestContext`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+An instance of `TestContext` is passed to each test function in order to
+interact with the test runner. However, the `TestContext` constructor is not
+exposed as part of the API.
+
+### `context.diagnostic(message)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `message` {string} Message to be displayed as a TAP diagnostic.
+
+This function is used to write TAP diagnostics to the output. Any diagnostic
+information is included at the end of the test's results. This function does
+not return a value.
+
+### `context.skip([message])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `message` {string} Optional skip message to be displayed in TAP output.
+
+This function causes the test's output to indicate the test as skipped. If
+`message` is provided, it is included in the TAP output. Calling `skip()` does
+not terminate execution of the test function. This function does not return a
+value.
+
+### `context.todo([message])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `message` {string} Optional `TODO` message to be displayed in TAP output.
+
+This function adds a `TODO` directive to the test's output. If `message` is
+provided, it is included in the TAP output. Calling `todo()` does not terminate
+execution of the test function. This function does not return a value.
+
+### `context.test([name][, options][, fn])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `name` {string} The name of the subtest, which is displayed when reporting
+  test results. **Default:** The `name` property of `fn`, or `'<anonymous>'` if
+  `fn` does not have a name.
+* `options` {Object} Configuration options for the subtest. The following
+  properties are supported:
+  * `concurrency` {number} The number of tests that can be run at the same time.
+    If unspecified, subtests inherit this value from their parent.
+    **Default:** `1`.
+  * `skip` {boolean|string} If truthy, the test is skipped. If a string is
+    provided, that string is displayed in the test results as the reason for
+    skipping the test. **Default:** `false`.
+* `fn` {Function|AsyncFunction} The function under test. The test fails if this
+  function throws an exception or returns a `Promise` that rejects. This
+  function is invoked with a single [`TestContext`][] argument.
+  **Default:** A no-op function.
+* Returns: {Promise} Resolved with `undefined` once the test completes.
+
+This function is used to create subtests under the current test. This function
+behaves in the same fashion as the top level [`test()`][] function.
+
+[TAP]: https://testanything.org/
+[`TestContext`]: #class-testcontext
+[`test()`]: #testname-options-fn

lib/internal/errors.js

@@ -1544,6 +1544,18 @@ E('ERR_STREAM_WRAP', 'Stream has StringDecoder set or is in objectMode', Error);
 E('ERR_STREAM_WRITE_AFTER_END', 'write after end', Error);
 E('ERR_SYNTHETIC', 'JavaScript Callstack', Error);
 E('ERR_SYSTEM_ERROR', 'A system error occurred', SystemError);
+E('ERR_TEST_FAILURE', function(error, failureType) {
+  hideInternalStackFrames(this);
+  assert(typeof failureType === 'string',
+         "The 'failureType' argument must be of type string.");
+
+  const msg = error?.message ?? lazyInternalUtilInspect().inspect(error);
+
+  this.failureType = failureType;
+  this.cause = error;
+
+  return msg;
+}, Error);
 E('ERR_TLS_CERT_ALTNAME_FORMAT', 'Invalid subject alternative name string',
   SyntaxError);
 E('ERR_TLS_CERT_ALTNAME_INVALID', function(reason, host, cert) {