wasm: global functions from wasm not working from node

[mattetti]

What version of Go are you using (go version)?

$ go version
go version go1.12beta2 Darwin/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

I am trying to call registered functions (aka callbacks) defined in the WebAssembly file from node js. I use the provided wasm_exec.js which does call into the main function without any issues. From what I could gather, we are missing a way to expose the registered functions in JS. But unfortunately I am not familiar enough with the implementation details nor did I find a lot of information online on the matter.

What did you expect to see?

I expected to be able to call my exposed functions from js outside of the browser.

What did you see instead?

A js exception

[mattetti]

@neelance you might know if that's a user error, a missing feature or a bug. Any feedback or pointer to solve the challenge for would be greatly appreciated.

[agnivade]

I believe this is something that you are trying to do ?

fn := js.FuncOf(func(this js.Value, args []js.Value) interface{} {
    // do your stuff here
})

js.Global().Set("myFunc", fn)

Now from JS, call myFunc().

[mattetti]

@agnivade that's exact what I do, but myFunc isn't defined in the scope. At least not when try to call it after the wasm binary was instantiated. Some languages seem to out the functions in the instance.exports but I am a bit lost with what we are doing in Go and my node knowledge is limited. Thanks for looking into it and trying to help though, very much appreciated!

[agnivade]

Ah yes, that just might be the case. In the browser, we have window as global. I am not sure how to define functions in the global scope for node from wasm.

[neelance]

In Node.js the global object is called global instead of window.

[agnivade]

So then shouldn't js.Global().Set("myFunc", fn) work transparently for the browser and node both ?

[mikioh]

@agnivade,

It might be better to have a look at https://github.com/tc39/proposal-global.

[neelance]

So then shouldn't js.Global().Set("myFunc", fn) work transparently for the browser and node both ?

Yes.

[agnivade]

But that's not what's happening

but myFunc isn't defined in the scope.

That is why I thought setting an object in the global scope might not work for Node.js.

But if that's supposed to work, then I am not sure why myFunc doesn't get defined in the global scope. Or probably I have misunderstood the question somehow.

[neelance]

It should definitely be possible to call global.myFunc();. Someone please post some concrete code that is failing, otherwise we're just guessing about the issue.

[mattetti]

I was traveling with very limited internet and could not upload a repro case. I did test calling global.myFunc() but it wasn't defined. I'm back home and will out together a quick example ASAP. Thanks everyone

[mattetti]

@neelance here is the repro example: https://github.com/mattetti/go-node-wasm

The main Go file is here: https://github.com/mattetti/go-node-wasm/blob/master/src/wasm/main.go
JS code change: https://github.com/mattetti/go-node-wasm/blob/master/src/node/wasm_exec.js#L506

To run the code I use the following command: GOOS=js GOARCH=wasm go run -exec=./src/node/go_js_wasm_exec ./src/wasm/main.go

Error:

(node:40122) UnhandledPromiseRejectionWarning: TypeError: global.add is not a function
    at WebAssembly.instantiate.then (/Users/mattetti/Code/go-node-wasm/src/node/wasm_exec.js:506:28)