Added a Go template playground

cmds/gotemplate/main.go

@@ -0,0 +1,92 @@
+//go:build js
+
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"syscall/js"
+	"text/template"
+)
+
+func main() {
+	regenChan := make(chan struct{})
+	go executeTemplate(regenChan)
+
+	eventListener := func(this js.Value, args []js.Value) interface{} {
+		ev := args[0]
+		ev.Call("preventDefault")
+
+		regenChan <- struct{}{}
+
+		return nil
+	}
+
+	querySelector("#template").Call("addEventListener", "keyup", js.FuncOf(eventListener))
+	querySelector("#data").Call("addEventListener", "keyup", js.FuncOf(eventListener))
+
+	regenChan <- struct{}{}
+	<-make(chan struct{})
+}
+
+func executeTemplate(regenChan chan struct{}) {
+	templateEl := querySelector("#template")
+	dataEl := querySelector("#data")
+	outputEl := querySelector("#output")
+
+	tmplMessageEl := querySelector("#template-message")
+	dataMessageEl := querySelector("#data-message")
+	displayErrors := func(dataErr error, tmplErr error) {
+		if dataErr != nil {
+			dataMessageEl.Set("innerText", dataErr.Error())
+		} else {
+			dataMessageEl.Set("innerText", "")
+		}
+		if tmplErr != nil {
+			tmplMessageEl.Set("innerText", tmplErr.Error())
+		} else {
+			tmplMessageEl.Set("innerText", "")
+		}
+	}
+
+	var templateSource string
+	var tmpl *template.Template
+	setTemplateSource := func(newTemplateSource string) (*template.Template, error) {
+		if tmpl == nil || templateSource != newTemplateSource {
+			newTmpl := template.New("")
+			if _, err := newTmpl.Parse(newTemplateSource); err != nil {
+				return nil, err
+			}
+			tmpl = newTmpl
+		}
+		return tmpl, nil
+	}
+
+	var outBuf bytes.Buffer
+	for range regenChan {
+		templateValue := templateEl.Get("value").String()
+		dataValue := dataEl.Get("value").String()
+
+		var dataJson interface{}
+		dataErr := json.Unmarshal([]byte(dataValue), &dataJson)
+		tmplInst, tmplErr := setTemplateSource(templateValue)
+
+		if dataErr != nil || tmplErr != nil {
+			displayErrors(dataErr, tmplErr)
+			continue
+		}
+
+		outBuf.Reset()
+		if err := tmplInst.Execute(&outBuf, dataJson); err != nil {
+			displayErrors(nil, err)
+			continue
+		}
+
+		outputEl.Set("innerText", outBuf.String())
+		displayErrors(nil, nil)
+	}
+}
+
+func querySelector(query string) js.Value {
+	return js.Global().Get("document").Call("querySelector", query)
+}