Minor update. I was correct that the node example was not working, but it was a separate issue. The node example works if I change add() in ./example/node/src/main.go to:

func add(this js.Value, args []js.Value) (interface{}, error) {
	ret := 0

	for _, item := range args {
		// This fails because item.String() is "<number: 1>" not "1"
		// val, err := strconv.Atoi(item.String())
		// if err != nil {
		// 	return 0, err
		// }

		val := item.Int()
		ret += val
	}

	return ret, nil
}

I found a similar issue ("syscall/js: make wasm_exec.js compatible with Webpack" here) related to fs.writeSync() but applying the fix for that does not fix the web example problem in the OP. Nor is this fix needed for the node example to work.

I found a partial fix, at least it avoids the above error which occurs when executing println("Web Assembly is ready") in web/src/main.go and this is printed to the browser console when the page loads. The fix is another edit to wasm_exec.js to change the test on global.fs to detect the case where it is an object with an empty keys array, so that it creates an implementation of fs.writeSync() etc. So at line 37-ish I changed:

if (!global.fs) {

To:

if (!global.fs || Object.keys(global.fs) === 0 || Object.keys(global.fs).length === 0) {

The page displays ok, and clicking on "simulate an error" works, but changing integer values does not affect the "Value is now" value, which is always '0'.

Fixed!

Applying the main.go / add() fix from the node example to the web example, both examples now work.

I'm going to add a Svelte based example.

Let me know if you would like a PR.

I've published a fork as a new plugin which works with golang v1.13:

https://www.npmjs.com/package/golang-wasm-async-loader2