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

js-promise-from-wasm.go

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

// MyGoFunc returns a Promise that resolves after 3 seconds with a message
func MyGoFunc() js.Func {
	return js.FuncOf(func(this js.Value, args []js.Value) interface{} {
		// Handler for the Promise: this is a JS function
		// It receives two arguments, which are JS functions themselves: resolve and reject
		handler := js.FuncOf(func(this js.Value, args []js.Value) interface{} {
			resolve := args[0]
			// Commented out because this Promise never fails
			//reject := args[1]

			// Now that we have a way to return the response to JS, spawn a goroutine
			// This way, we don't block the event loop and avoid a deadlock
			go func() {
				// Block the goroutine for 3 seconds
				time.Sleep(3 * time.Second)
				// Resolve the Promise, passing anything back to JavaScript
				// This is done by invoking the "resolve" function passed to the handler
				resolve.Invoke("Trentatré Trentini entrarono a Trento, tutti e trentatré trotterellando")
			}()

			// 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)
	})
}