Blog: https://withblue.ink/2020/10/03/go-webassembly-http-requests-and-promises.html

js-promise-with-errors-from-wasm.go

// Copyright (C) 2020 Alessandro Segala (ItalyPaleAle)
// License: MIT

// MyGoFunc returns a Promise that fails with an exception about 50% of times
func MyGoFunc() js.Func {
	return js.FuncOf(func(this js.Value, args []js.Value) interface{} {
		// Handler for the Promise
		handler := js.FuncOf(func(this js.Value, args []js.Value) interface{} {
			resolve := args[0]
			reject := args[1]

			// Run this code asynchronously
			go func() {
				// Cause a failure 50% of times
				if rand.Int()%2 == 0 {
					// Invoke the resolve function passing a plain JS object/dictionary
					resolve.Invoke(map[string]interface{}{
						"message": "Hooray, it worked!",
						"error":   nil,
					})
				} else {
					// Assume this were a Go error object
					err := errors.New("Nope, it failed")

					// Create a JS Error object and pass it to the reject function
					// The constructor for Error accepts a string,
					// so we need to get the error message as string from "err"
					errorConstructor := js.Global().Get("Error")
					errorObject := errorConstructor.New(err.Error())
					reject.Invoke(errorObject)
				}
			}()

			// The handler of a Promise doesn't return any value
			return nil
		})

		// Create and return the Promise object
		promiseConstructor := js.Global().Get("Promise")
		return promiseConstructor.New(handler)
	})
}