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

http-request-wasm.go

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

// MyGoFunc fetches an external resource by making a HTTP request from Go
// The JavaScript method accepts one argument, which is the URL to request
func MyGoFunc() js.Func {
	return js.FuncOf(func(this js.Value, args []js.Value) interface{} {
		// Get the URL as argument
		// args[0] is a js.Value, so we need to get a string out of it
		requestUrl := args[0].String()

		// Handler for the Promise
		// We need to return a Promise because HTTP requests are blocking in Go
		handler := js.FuncOf(func(this js.Value, args []js.Value) interface{} {
			resolve := args[0]
			reject := args[1]

			// Run this code asynchronously
			go func() {
				// Make the HTTP request
				res, err := http.DefaultClient.Get(requestUrl)
				if err != nil {
					// Handle errors: reject the Promise if we have an error
					errorConstructor := js.Global().Get("Error")
					errorObject := errorConstructor.New(err.Error())
					reject.Invoke(errorObject)
					return
				}
				defer res.Body.Close()

				// Read the response body
				data, err := ioutil.ReadAll(res.Body)
				if err != nil {
					// Handle errors here too
					errorConstructor := js.Global().Get("Error")
					errorObject := errorConstructor.New(err.Error())
					reject.Invoke(errorObject)
					return
				}

				// "data" is a byte slice, so we need to convert it to a JS Uint8Array object
				arrayConstructor := js.Global().Get("Uint8Array")
				dataJS := arrayConstructor.New(len(data))
				js.CopyBytesToJS(dataJS, data)

				// Create a Response object and pass the data
				responseConstructor := js.Global().Get("Response")
				response := responseConstructor.New(dataJS)

				// Resolve the Promise
				resolve.Invoke(response)
			}()

			// 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)
	})
}