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Interpreting Test262 Tests

All tests are declared as text files located within this project's test directory. In order to execute Test262 tests, runtimes must observe the following semantics.

Note When these instructions change in any substantive way, the version property of the JSON-formatted package.json file will be incremented. In this way, consumers who are transitioning between revisions of Test262 can more easily determine the cause of new test failures.

Test Execution

Test262 tests are only valid under the runtime environment conditions described here. Test environments may be further modified according to the metadata contained with each test--refer to the Metadata section for more details.

Realm Isolation

Each test must be executed in a new ECMAScript realm dedicated to that test. Unless configured otherwise (via the module flag), source text must be interpreted as global code.

Test262-Defined Bindings

The contents of the following harness files must be evaluated in the test realm's global scope prior to test execution unless the test uses the raw frontmatter flag:

  1. harness/assert.js
  2. harness/sta.js

Host-Defined Functions

The following values must be defined as writable, configurable, non-enumerable properties of the global scope prior to test execution.

  • print A function that exposes the string value of its first argument to the test runner. This is used as a communication mechanism for asynchronous tests (via the async flag, described below).
  • $262 An ordinary object with the following properties:

    • AbstractModuleSource - a reference to the %AbstractModuleSource% constructor which does not appear as a property of the global object.

    • createRealm - a function which creates a new ECMAScript Realm, defines this API on the new realm's global object, and returns the $262 property of the new realm's global object

    • detachArrayBuffer - a function which implements the DetachArrayBuffer abstract operation

    • evalScript - a function which accepts a string value as its first argument and executes it as an ECMAScript script according to the following algorithm:

      1. Let hostDefined be any host-defined values for the provided
   sourceText (obtained in an implementation dependent manner)
2. Let realm be the current Realm Record.
3. Let s be ParseScript(sourceText, realm, hostDefined).
4. If s is a List of errors, then
   a. Let error be the first element of s.
   b. Return
      Completion{[[Type]]: throw, [[Value]]: error, [[Target]]: empty}.
5. Let status be ScriptEvaluation(s).
6. Return Completion(status).

    • gc - a function that wraps the host's garbage collection invocation mechanism, if such a capability exists. Must throw an exception if no capability exists. This is necessary for testing the semantics of any feature that relies on garbage collection, e.g. the WeakRef API.

    • global - a reference to the global object on which $262 was initially defined

    • IsHTMLDDA - (present only in implementations that can provide it) an object that:

      1. has an [[IsHTMLDDA]] internal slot, and

      2. when called with no arguments or with the first argument "" (an empty string) returns null.

        Note: The peculiar second requirement permits testing algorithms when they also call document.all with such arguments, so that testing for correct behavior requires knowing how the call behaves. This is rarely necessary.

        Use this property to test that ECMAScript algorithms aren't mis-implemented to treat document.all as being undefined or of type Undefined (instead of Object).

        Tests using this function must be tagged with the IsHTMLDDA feature so that only hosts supporting this property will run them.

    • agent - an ordinary object with the following properties:

      • start - a function that takes a script source string and runs the script in a concurrent agent. Will block until that agent is running. The agent has no representation. The agent script will be run in an environment that has an object $262 with a property agent with the following properties:
        • receiveBroadcast - a function that takes a function and calls the function when it has received a broadcast from the parent, passing it the broadcast as two arguments, a SharedArrayBuffer and an Int32 or BigInt. This function may return before a broadcast is received (eg to return to an event loop to await a message) and no code should follow the call to this function.
        • report - a function that accepts a single "message" argument, which is converted to a string* and placed in a transmit queue whence the parent will retrieve it. Messages should be short. (* Note that string conversion has been implicit since the introduction of this host API, but is now explicit.)
        • sleep - a function that takes a millisecond argument and sleeps the agent for approximately that duration.
        • leaving - a function that signals that the agent is done and may be terminated (if possible).
        • monotonicNow - a function that returns a value that conforms to DOMHighResTimeStamp and is produced in such a way that its semantics conform to Monotonic Clock.
      • broadcast - a function that takes a SharedArrayBuffer and an Int32 or BigInt and broadcasts the two values to all concurrent agents. The function blocks until all agents have retrieved the message. Note, this assumes that all agents that were started are still running.
      • getReport - a function that reads an incoming string from any agent, and returns it if it exists, or returns null otherwise.
      • sleep - a function that takes a millisecond argument and sleeps the execution for approximately that duration.
      • monotonicNow - a function that returns a value that conforms to DOMHighResTimeStamp and is produced in such a way that its semantics conform to Monotonic Clock.

In addition, consumers may choose to override any of the functions defined by test harness files as they see fit. See the documentation on handling errors and negative test cases for a useful example of this.

Normative references

DOMHighResTimeStamp, Monotonic Clock
Ilya Grigorik, James Simonsen, Jatinder Mann.
High Resolution Time Level 2 March 2018. W3C. URL: https://www.w3.org/TR/hr-time-2/

Strict Mode

Unless configured otherwise (via the noStrict, onlyStrict, module, or raw flags), each test must be executed twice: once in ECMAScript's non-strict mode, and again in ECMAScript's strict mode. To run in strict mode, the test contents must be modified prior to execution--a "use strict" directive must be inserted as the initial character sequence of the file, followed by a semicolon (;) and newline character (\n):

"use strict";

This must precede any additional text modifications described by test metadata.

Modules

Test262 includes tests for ECMAScript 2015 module code, denoted by the "module" metadata flag. Files bearing a name which includes the sequence _FIXTURE MUST NOT be interpreted as standalone tests; they are intended to be referenced by test files. Realm modifications, including Test262-Defined Bindings and Host-Defined Functions, are not applied to code executed from _FIXTURE files. See the Rules For Module _FIXTURE Files section of CONTRIBUTING.md for more information.

All module specifiers used by Test262 begin with the character sequence ./. The remaining characters should be interpreted as the name of a file within the same directory as the file under test. The contents of this file must be interpreted as UTF-8-encoded text and supplied to the Source Text Module Record's ParseModule abstract operation. The result of that operation must be returned by the implementation-defined HostResolveImportedModule directly.

For example, consider a test file located at test/language/import/nested/index.js with the following contents:

import * as ns from './dep.js';

Implementers should attempt to resolve this module specifier by loading a file located at test/language/import/nested/dep.js.

Files bearing a name ending in .json are intended to be interpreted as JSON.

Implementers should resolve the specifier <module source> to a module that provides a valid Module Source, such as a WebAssembly module. Tests use <module source> specifier are guarded with a feature flag source-phase-imports-module-source.

Staging

Tests in the test/staging/ folder are expected to be executed just like all the other tests, in order to promote interoperability as soon as possible.

Implementations without ECMA-402

Tests in the test/intl402/ and test/staging/intl402/ folders assume that the implementation supports ECMA-402, the ECMAScript Internationalization API Specification. When testing an implementation lacking the capabilities in ECMA-402, the tests in those folders should be skipped.

Test Results

By default, tests signal failure by generating an uncaught exception. If execution completes without generating an exception, the test must be interpreted as "passing." Any uncaught exception must be interpreted as test failure. These semantics may be modified by any test according to the metadata declared within the test itself (via the negative attribute and the async flag, described below).

Metadata

Each test file may define metadata that describe additional requirements. This information is delimited by the token sequence /*--- and ---*/ and is structured as YAML.

negative

These tests are expected to generate an uncaught exception. The value of this attribute is a YAML dictonary with two keys:

  • phase - the stage of the test interpretation process that the error is expected to be produced; valid phases are:
    • parse: occurs while parsing the source text, or while checking it for early errors.
    • resolution: occurs during module resolution.
    • runtime: occurs during evaluation.
  • type - the name of the constructor of the expected error

If a test configured with the negative attribute completes without throwing an exception, or if the name of the thrown exception's constructor does not match the specified constructor name, or if the error occurs at a phase that differs from the indicated phase, the test must be interpreted as "failing."

The $DONOTEVALUATE() function is for use in tests that include the following meta data:

negative:
  phase: runtime
  type: ReferenceError

The definition is considered "runner implementation defined" and no guarantees can be made about its behavior, therefore it is restricted to only tests that meet the criteria described above.

Examples:

/*---
negative:
  phase: runtime
  type: ReferenceError
---*/
unresolvable;
/*---
negative:
  phase: parse
  type: SyntaxError
---*/
$DONOTEVALUATE();
var a\u2E2F;
/*---
negative:
  phase: resolution
  type: ReferenceError
flags: [module]
---*/
$DONOTEVALUATE();
export {} from './instn-resolve-empty-export_FIXTURE.js';
// instn-resolve-empty-export_FIXTURE.js contains only:
// 0++;

includes

One or more files whose content must be evaluated in the test realm's global scope prior to test execution. These files are located within the harness/ directory of the Test262 project.

Example

/*---
includes: [propertyHelper.js]
---*/
verifyProperty(this, "Object", {
  value: Object,
  writable: true,
  enumerable: false,
  configurable: true,
});

flags

The flags attribute is an optional value that specifies one or more of the following strings:

  • onlyStrict The test must be executed just once--in strict mode, only. This must be accomplished using the transformation described in the section titled "Strict Mode".

    Example

    /*---
flags: [onlyStrict]
---*/
var thisVal = null;
[null].forEach(function() {
  thisVal = this;
});
assert.sameValue(thisVal, undefined);

  • noStrict The test must be executed just once--in non-strict mode, only. In other words, the transformation described by the section titled "Strict Mode" must not be applied to these tests.

    Example

    /*---
flags: [noStrict]
---*/
var thisVal = null;
[null].forEach(function() {
  thisVal = this;
});
assert.notSameValue(thisVal, undefined);
assert.sameValue(thisVal, this);

  • module The test source code must be interpreted as module code. In addition, this flag negates the default requirement to execute the test both in strict mode and in non-strict mode. In other words, the transformation described by the section titled "Strict Mode" must not be applied to these tests. Refer to the section titled "Modules" for more information on interpreting these tests.

    Example

    /*---
flags: [module]
---*/
export default function* g() {}

  • raw The test source code must not be modified in any way, files from the harness/ directory must not be evaluated, and the test must be executed just once (in non-strict mode, only).

    Example

    /*---
flags: [raw]
---*/
'use strict'
[0]
's'.p = null;

  • async The file harness/doneprintHandle.js must be evaluated in the test realm's global scope prior to test execution. The test must not be considered complete until the implementation-defined print function has been invoked or some length of time has passed without any such invocation. In the event of a passing test run, this function will be invoked with the string 'Test262:AsyncTestComplete'. If invoked with a string that is prefixed with the character sequence Test262:AsyncTestFailure:, the test must be interpreted as failed. The implementation is free to select an appropriate length of time to wait before considering the test "timed out" and failing.

    Example

    /*---
flags: [async]
---*/
Promise.resolve()
  .then(function() {
      print('Test262:AsyncTestComplete');
    }, function(reason) {
      print('Test262:AsyncTestFailure: ' + reason);
    });

  • generated The test file was created procedurally using the project's test generation tool. This flag is specified for informational purposes only and has no bearing on how the test should be interpreted.

  • CanBlockIsFalse The test file should only be run when the [[CanBlock]] property of the Agent Record executing the file is false.

    Example

    /*---
flags: [CanBlockIsFalse]
---*/
var i32 = new Int32Array(new SharedArrayBuffer(Int32Array.BYTES_PER_ELEMENT));
assert.throws(TypeError, function() { Atomics.wait(i32, 0, 0, 1000); });

  • CanBlockIsTrue The test file should only be run when the [[CanBlock]] property of the Agent Record executing the file is true.

    Example

    /*---
flags: [CanBlockIsTrue]
---*/
var i32 = new Int32Array(new SharedArrayBuffer(Int32Array.BYTES_PER_ELEMENT));
Atomics.wait(i32, 0, 0, 1000); // Sleep for one second.

  • non-deterministic Due to implementation defined parts in the matching specs, a test may have more than one path to pass, without failures during the execution. While passing state still refers to a compatible implementation, it doesn't guarantee all the assertions in the test will execute. In the same way, it's not possible to tell the assertions would pass whenever they are executed.

locale

The locale attribute allows tests to declare explicit information regarding locale specificity. Its value is an array of one or more valid language tags or subtags.

Example

/*---
locale: [en, en-US, ar]
---*/

var en = new Intl.PluralRules('en');
assert.sameValue(en.select(1), 'one', 'en.select(1) returns "one"');
assert.sameValue(en.select(2), 'other', 'en.select(2) returns "other"');

var enUS = new Intl.PluralRules('en-US');
assert.sameValue(enUS.select(1), 'one', 'enUS.select(1) returns "one"');
assert.sameValue(enUS.select(2), 'other', 'enUS.select(2) returns "other"');

var ar = new Intl.PluralRules('ar');
assert.sameValue(ar.select(1), 'one', 'ar.select(1) returns "one"');
assert.sameValue(ar.select(2), 'other', 'ar.select(2) returns "two"');