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Getting Started

For Development of Test262-Harness

For development of Test262-harness, test authoring & execution:

git clone https://github.com/tc39/test262.git --depth 1
cd test262

For Test Authoring & Test Execution:

For test authoring & execution:

npm install -g test262-harness

Run test262-harness --help for details on the various configuration options.


Running tests in hypothetical JavaScript engine "X":

cd test262;
test262-harness --host-type=X --host-path=`which X` test/**/*.js


Option Name Description Required Default
-h, --help Show help & examples n/a n/a
-v, --version Print the current version of test262-harness n/a n/a
--host-type Type of host to run tests in. See eshost's supported hosts for available options. No node
--host-path Path to the host executable. Yes, if host-type is specified process.execPath
--host-args Any additional arguments to pass to the host when invoking it (eg. --harmony, --es6all, etc). No n/a
-t, --threads Run this many tests in parallel. Note that the browser runners don't work great with t > 1. No 1
-r, --reporter Format of data written to standard output. Currently either json or simple. No simple
--features (deprecated) Comma-separated list of features to filter for. Example: --features="BigInt,Atomics". No n/a
--features-include Comma-separated list of features to filter for inclusion. Example: --features-include="BigInt,Atomics" would run only tests that include the features BigInt and Atomics No n/a
--features-exclude Comma-separated list of features to filter for exclusion. Example: --features-exclude="BigInt,Atomics" would run only tests that do not include the features BigInt and Atomics. NOTE exclusions are resolved after inclusions. No n/a
--reporter-keys Comma-separated list of keys to include in output of json reporter. No n/a
--test262-dir Root test262 directory and is used to locate the includes directory. No Relative to test files
--includes-dir Includes directory. No Inferred from test262-dir or else detected by walking upward from the first test found.
--tempDir Directory that eshost will create its temp files in (does not affect location of files created by --save-compiled-tests and --save-only-failed No OS Temp Dir
--prelude Path to a file to include before every test (useful for testing polyfills for example); supports multiple --prelude parameters No n/a
--timeout Set a custom test timeout in milliseconds No 10000
--transformer Path to module which exports a code transformer function No n/a
--preprocessor Path to module which exports a map function that operates on each Test262Test object before it executed. No n/a
--accept-version Execute tests from a version of Test262 that differs from the versions supported by this utility. This may cause the utility to report invalid test results. No Inferred from test262Dir/package.json
--save-compiled-tests Write the compiled version of path/to/test.js as path/to/test.js.<host-type>.<default|strict>.<pass|fail> so that it can be easily re-run under that host. Run test262-harness --help for examples. No n/a
--save-only-failed Only save the compiled version of the test if it failed, to help easily repro failed tests (implies --save-compiled-tests). No n/a
--error-for-failures Return a non-zero exit code if one or more tests fail. No n/a


The --preprocessor feature allows a module that exports a map function that operates on each Test262Test object (ie. the object emitted by Test262-Stream) before their execution.

test.result = Result Object

In some cases, a preprocessor may want to signal to Test262-Harness that a certain result has already been reached, and that it must not further evaluate the test. To create this signal, the preprocessor creates a result property on the Test262Test object, which will indicate to Test262-Harness that it must not evaluate the test, but instead return the value of the result property as though the test had been executed. For example, a preprocessor may attempt to transpile the value of test.contents—which may fail! In the case of failure, the preprocessor can create a result property whose value is a Result Object. This will skip the code evaluation and report the given result object.

module.exports = function(test) {
  try {
    test.contents = babel.transform(test.contents, options).code;
  } catch (error) {
    test.result = {
      stderr: `${error.name}: ${error.message}\n`,
      stdout: '',

  return test;




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