Problematic JS BigInt coercion

[dcodeIO]

It has recently been reported to me that the JS API for BigInt integration can cause unexpected problems when omitting arguments. Say one has the following Wasm module

export function test(a: i64 = 0): void { }

and wants to call exports.test() externally, with the a argument omitted for some reason, then the call will fail with

Uncaught TypeError: Cannot convert undefined to a BigInt

Without some sort of bindings generation, the JS module does not know however that a is of type i64 and should be passed a 0n default value. Something similar also happens when the argument is 0, or another number:

Uncaught TypeError: Cannot convert number to a BigInt

So I wonder: Would it be useful to define a coercion from undefined to 0n when calling Wasm exports, and perhaps also a coercion from number? I guess this is not a V8 bug?

[Ms2ger]

Spec context: https://webassembly.github.io/spec/js-api/#call-an-exported-function calls ToWebAssemblyValue → ToBigInt64 → ToBigInt which throws on undefined. (i32 gets zero, the floats get NaN.)

export function test(a: i64 = 0): void { }

The = 0 here doesn't refer to anything in WebAssembly, does it? You mean a function with the type [i64] → []?

[dcodeIO]

Yes, in Wasm this would be an [i64] -> [] function, called externally with the argument omitted where a compiler allows to do this. AssemblyScript for example has a mechanism for filling in default values of omitted arguments post-call, by hinting arguments.length pre-call.

[MaxGraey]

I guess better address this issue to this discussion: https://es.discourse.group/t/why-is-bigint-broken/567/18

[devsnek]

I don't think that this issue is as specific as the issue OP implies:

For example:

export function test(a: i64 = 3): void { }

or for that matter,

export function test(a: f64 = 0): void { } // will be NaN if you call test() in js, not 0

[dcodeIO]

Thanks for noting, I overlooked that, since in the specific use case I am describing the value does not matter, as it is only important that the call succeeds (the default value replaces what's is passed). I agree that there is more to think about here.

Also, in https://github.com/WebAssembly/design/issues/1402#issuecomment-789758084 it was noted that Wasm could allow duplicate exports, which seems like a preferable solution over what I suggested here, at least for that specific use case. With duplicate exports, I'd expect the JS/Wasm binding to pick the export with matching arity (not sure if that's a realistic expectation?), so the problem can be worked around fairly easily by exporting both a test() and a test(a: T).