1.2.0 • Public • Published

Convert test262 tests to mjsunit format

V8 hasn't quite gotten around to letting its contributors write test262 tests directly, as I found out recently. Instead they prefer to check in tests in their custom mjsunit format.

But what if you've already written a bunch of test262 tests? Or what if you want to be a good citizen and contribute to the general cross-browser testing cause that is test262, and yet you'd like to be able to land features in V8? This project will solve your problems, by converting test262-format tests to mjsunit-format tests destined for V8.


Pass the tests you'd like to convert to the CLI utility. The resulting mjsunit files will be written to standard out.

$ cd Array.prototype.includes
$ test262-to-mjsunit test/*.js > ../v8/test/mjsunit/array-includes.js


By default, if test262-to-mjsunit can't convert something, it will output a comment block with an error:

// !!! Test failed to convert:
// Cannot convert negative tests.
// !!!

If you'd rather have it fail, pass the --fail-hard option.


If you're testing a feature that needs a given V8 flag to run, use the --flags option to make the converter output the appropriate comment:

$ test262-to-mjsunit --flags="--harmony-array-includes" test/*.js > ../v8/test/mjsunit/array-includes.js

Conversion Details

Per-Test Changes

The process of converting to mjsunit consists of these steps:

First, test262-to-mjsunit removes the copyright header and YAML frontmatter. It reuses the description key from the frontmatter to build a header comment for the test.

Then, it wraps the test body in an IIFE.

If the test has the onlyStrict flag set, it prepends "use strict"; to the test body (inside the IIFE).

If the test includes Test262Error.js, it prepends a quick function Test262Error() {}. (This ensures that any assert.throws that are converted to assertThrows work correctly.)

Next, it performs a series of substitutions on the test body (done at the abstract syntax-tree level, so it should be fairly robust):

test262 construct mjsunit construct
assert(value, "message") assertTrue(value)
assert.sameValue(value, true, "message") assertTrue(value)
assert.sameValue(value, false, "message") assertFalse(value)
assert.sameValue(value, any other literal, "message") assertEquals(any other literal, value)
assert.throws(ErrorConstructor, functionToRun) assertThrows(functionToRun, ErrorConstructor)
$ERROR("message") assertUnreachable("message")
new Test262Error("message") new Test262Error()

Finally, it reformats the test to be two-space indents, double quotes, and whatever else Recast does with its default pretty-printer.

Unsupported features

Although there are presumably many features of test262 tests that this converter doesn't yet support, two are caught explicitly as early errors: negative tests and tests that include other files. Both of these are detected via the YAML frontmatter.

If either of these features is used in a test, the resulting output will be a three-line comment with an appropriate error message. For example:

// !!! Test failed to convert:
// Cannot convert tests with includes.
// !!!

(However, there is one exception to the no-includes rule: Test262Error.js, as noted above.)

You can bypass the check for includes via the --ignore-includes option. Presumably in this case you're planning to supply the relevant includes by manually editing the output.


Each test is processed separately according to the above process. Then, they are stitched together with double-newlines between them. The appropriate V8 copyright header is added to the top of the output, and if specified, the appropriate V8 flags comment is added below that.

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npm i test262-to-mjsunit

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