Implement Array.prototype.flatMap, Array.prototype.flat

[keithamus]

flatMap and flat are now Stage 3. https://github.com/tc39/proposal-flatMap

[rhuanjl]

I'd be keen on implementing this if it's not already planned for. Please let me know if an external contribution would be welcome here.

[digitalinfinity]

Go for it! I would expect these can be written as JS built-ins? @meg-gupta might have pointers here.

[rhuanjl]

OK - I'll make a start and see how I get on, I think that for CC mechanics I can follow the model used for Array.prototype.filter so hopefully won't need much assistance. I'll also endeavour to test against all of the relevant test-262 tests before submitting anything.

[meg-gupta]

ChakraCore/lib/Runtime/Library/JsBuiltIn/JsBuiltIn.js - has our builtins written in Javascript. Array.prototype.indexOf is also a good implementation to follow via the Js route.

[rhuanjl]

One question I could do with help with, implementing a ethodwithin JsBuiltIn.js is it possible to have another JavaScript Function that it calls?

Motivation is that I cannot see a good way to .flat or .flatMap without using the FlattenIntoArray function described in the spec - I've written this function in JavaScript but for some reason including this inside JsBuiltIn.js and then trying to call it from within my Array.Flat implementation results in a hard crash at runtime - is there anything obvious I'm missing?

If I put the below into JsBuiltIn.js, regenerate bytecode, build CC and then call the Array.prototype.flat method at runtime I get a hard crash:

    const FlattenIntoArray = function ( ) {}

    platform.registerFunction(FunctionsEnum.ArrayFlat, function (depth) {
        "use strict";
        FlattenIntoArray(); // hard crash
    }

Deleting the FlattenIntoArray() call results in the code running to completion.

[meg-gupta]

A good example for such a helper function would be CreateArrayIterator in JsBuiltins.js. We register them via registerChakraLibraryFunction and which adds them to the __chakraLibrary object. Functions on __chakraLibrary will be treated internal and will not be visible outside. Unfortunately we have not added documentation on the process of adding a Js builtin, I'll try to make that happen soon.

[rhuanjl]

Thanks for the help, it all seems to be working now, I'll sort out some appropriate tests then submit it.