XS: Array.from throws "callback: not a function" when mapfn is an explicit undefined

[d01c2]

Build environment: macOS 26.3 (xs / v8 from jsvu)

Moddable SDK version: Installed via jsvu, so there is no SDK source tree / git describe. Engine reports: XS 17.9.1, slot 32 bytes, ID 4 bytes

Target device: desktop (host xs)

Description

Array.from throws TypeError: callback: not a function when mapfn is an explicit undefined. Per spec, undefined means "no mapping", so the call should complete without error.

Steps to Reproduce

$ xst -e "Array.from.call(0, 0, undefined);"
TypeError: callback: not a function (in Array.from)

Expected behavior

The expression evaluates to [] with no exception thrown.

For comparison, V8 (15.1.166):

$ v8 -e 'print(JSON.stringify(Array.from.call(0, 0, undefined)))'
[]

Other information

This looks like a check on whether the mapfn argument is present rather than what its value is. In Array.from, mapfn being undefined should set mapping to false and skip the callability check, so an explicit undefined must not throw. With this being 0 (not a constructor) and no "length" on ToObject(0), the result should be an empty array.

[phoddie]

Thank you for the report. This does look like a conformance issue. It also appears that Array.fromAsync() has the same issue.

The spec behavior is kind of silly because the only reason to pass undefined for the second argument is to be able to pass something other than undefined for the third argument. But there's no reason to pass a value for the third argument, thisArg, without a valid mapping function. The history of the web is mysterious...

We'll work on fixing those.