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Getting Started

For Development of Test262-Harness

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

git clone --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 --hostType=X --hostPath=`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
--hostType Type of host to run tests in. See eshost's supported hosts for available options. No node
--hostPath Path to the host executable. Yes, if hostType is specified process.execPath
--hostArgs 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 Comma-separated list of features to filter for. Example: --features="BigInt,Atomics". No n/a
--reporter-keys Comma-separated list of keys to include in output of json reporter. No n/a
--test262Dir Root test262 directory and is used to locate the includes directory. No Relative to test files
--includesDir Includes directory. No Inferred from test262Dir 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 --saveCompiledTests and --saveOnlyFailed 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
--acceptVersion 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
--saveCompiledTests Write the compiled version of path/to/test.js as path/to/test.js.<hostType>.<default|strict>.<pass|fail> so that it can be easily re-run under that host. Run test262-harness --help for examples. No n/a
--saveOnlyFailed Only save the compiled version of the test if it failed, to help easily repro failed tests (implies --saveCompiledTests). 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.message}\n`,
      stdout: '',

  return test;
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