Support ES2019 Array.prototype.flat

[midgleyc]

Closes #948.

I'm not sure whether the implementation of the various ids are correct: I just copied the surrounding code.

I took the harmony test from https://github.com/v8/v8/blob/main/test/mjsunit/harmony/array-flat.js, added load("testsrc/assert.js"); to the start and "success"; to the end, and then made adjustments as Rhino doesn't supported redeclared consts in blocks or the Proxy object.

The language version in the test I have set to ES6, but as flat is part of ES2019, perhaps there should be a new version? Perhaps not, and I should just remove the comment?

By spec, we should throw a TypeError if the index j passed to defineElem is greater than 2⁵³ - 1. I have reused the message msg.arraylength.too.big here, but let me know if you'd like me to do something different.

[tuchida]

Thanks for your pull request.

Rhino uses Test262 to test ECMAScript specifications. Could you test the specifications by following these steps?

1. Install Test262
2. Delete the following line
   

   rhino/testsrc/org/mozilla/javascript/tests/Test262SuiteTest.java

   Line 92 in d22447c

   "Array.prototype.flat",

3. Updating the test262.properties file

[tuchida]

This code adds the method to the constructor(Array.flat), not the prototype(Array.prototype.flat).
It is an old specification called Array generic methods.
https://lia.disi.unibo.it/materiale/JS/developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array.html#Array_generic_methods

I do not think this is necessary as it is not in latest ECMA262 specification.

[midgleyc]

Right, I just added these because I saw other functions doing this -- e.g. Array.reduceRight doesn't exist either, just Array.prototype.reduceRight.

I've removed ConstructorId_flat entirely.

[midgleyc]

Thanks, I've updated.

Some of the failing spec tests seem to be failing on something unrelated to flat itself -- I see the error

missing ) after formal parameters

so either I've failed to wire something up correctly, or there's something wrong with a different bit of Rhino.

[tuchida]

missing ) after formal parameters

ref. #652, #817
That is the problem that test262/harness/compareArray.js cannot be loaded because Rest Parameters are not implemented.
I am sorry, but some of the Test262 code does not work because of a specification that is not implemented in Rhino.

[rbri]

Hi @midgleyc,
sorry for the silence here. This looks good to me and i like to merge.

Can you please do

• an rebase, and a regeneration of the test262.properties file (i have done a fix for the compareArray.js). This will show that even more tests are passing
• move the files ArrayFlatTest.java and array-flat.js into the testsrc/org/mozilla/javascript/tests/es6 folder (or if you like create a new es2019 folder

Thanks a lot for providing this

[midgleyc]

Sure, no problem.

As you have merged this already (thanks for this!) I will make those changes in another PR.

[rbri]

sorry, doing it right no. No need for changes.

[rbri]

Thanks for the PR.