From f3864a59ca6e22fc8c44f359abeeeac30493cb98 Mon Sep 17 00:00:00 2001
From: Chan Wen Xu <lutherchanpublic@gmail.com>
Date: Sat, 20 Mar 2021 20:44:04 +0800
Subject: [PATCH] feat: Implement Promise

---
 wasm/error.go   |  13 +++++
 wasm/promise.go | 145 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 158 insertions(+)
 create mode 100644 wasm/error.go
 create mode 100644 wasm/promise.go

diff --git a/wasm/error.go b/wasm/error.go
new file mode 100644
index 0000000..063043a
--- /dev/null
+++ b/wasm/error.go
@@ -0,0 +1,13 @@
+package wasm
+
+import "syscall/js"
+
+// NewError returns a JS Error with the provided Go error's error message.
+func NewError(goErr error) js.Value {
+	errConstructor, err := Global().Expect(js.TypeFunction, "Error")
+	if err != nil {
+		panic("Error constructor not found")
+	}
+
+	return errConstructor.New(goErr.Error())
+}
diff --git a/wasm/promise.go b/wasm/promise.go
new file mode 100644
index 0000000..7c3d8d2
--- /dev/null
+++ b/wasm/promise.go
@@ -0,0 +1,145 @@
+package wasm
+
+import "syscall/js"
+
+// Promise is an instance of a JS promise.
+// The zero value of this struct is not a valid Promise.
+type Promise struct {
+	Object
+}
+
+// FromJSValue turns a JS value to a Promise.
+func (p *Promise) FromJSValue(value js.Value) error {
+	var err error
+	p.Object, err = NewObject(value)
+	return err
+}
+
+// NewPromise returns a promise that is fulfilled or rejected when the provided handler returns.
+// The handler is spawned in its own goroutine.
+func NewPromise(handler func() (interface{}, error)) Promise {
+	resultChan := make(chan interface{})
+	errChan := make(chan error)
+
+	// Invoke the handler in a new goroutine.
+	go func() {
+		result, err := handler()
+		if err != nil {
+			errChan <- err
+			return
+		}
+		resultChan <- result
+	}()
+
+	// Create a JS promise handler.
+	var jsHandler js.Func
+	jsHandler = js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		if len(args) < 2 {
+			panic("not enough arguments are passed to the Promise constructor handler")
+		}
+
+		resolve := args[0]
+		reject := args[1]
+
+		if resolve.Type() != js.TypeFunction || reject.Type() != js.TypeFunction {
+			panic("invalid type passed to Promise constructor handler")
+		}
+
+		go func() {
+			select {
+			case r := <-resultChan:
+				resolve.Invoke(ToJSValue(r))
+			case err := <-errChan:
+				reject.Invoke(NewError(err))
+			}
+
+			// Free up resources now that we are done.
+			jsHandler.Release()
+		}()
+
+		return nil
+	})
+
+	promise, err := Global().Expect(js.TypeFunction, "Promise")
+	if err != nil {
+		panic("Promise constructor not found")
+	}
+
+	return mustJSValueToPromise(promise.New(jsHandler))
+}
+
+// PromiseAll creates a promise that is fulfilled when all the provided promises have been fulfilled.
+// The promise is rejected when any of the promises provided rejects.
+// It is implemented by calling Promise.all on JS.
+func PromiseAll(promise ...Promise) Promise {
+	promiseAll, err := Global().Expect(js.TypeFunction, "Promise", "all")
+	if err != nil {
+		panic("Promise.all not found")
+	}
+
+	pInterface := make([]interface{}, 0, len(promise))
+	for _, v := range promise {
+		pInterface = append(pInterface, v)
+	}
+
+	return mustJSValueToPromise(promiseAll.Invoke(pInterface))
+}
+
+// PromiseAllSettled creates a promise that is fulfilled when all the provided promises have been fulfilled or rejected.
+// It is implemented by calling Promise.allSettled on JS.
+func PromiseAllSettled(promise ...Promise) Promise {
+	promiseAllSettled, err := Global().Expect(js.TypeFunction, "Promise", "allSettled")
+	if err != nil {
+		panic("Promise.allSettled not found")
+	}
+
+	pInterface := make([]interface{}, 0, len(promise))
+	for _, v := range promise {
+		pInterface = append(pInterface, v)
+	}
+
+	return mustJSValueToPromise(promiseAllSettled.Invoke(pInterface))
+}
+
+// PromiseAny creates a promise that is fulfilled when any of the provided promises have been fulfilled.
+// The promise is rejected when all of the provided promises gets rejected.
+// It is implemented by calling Promise.any on JS.
+func PromiseAny(promise ...Promise) Promise {
+	promiseAny, err := Global().Expect(js.TypeFunction, "Promise", "any")
+	if err != nil {
+		panic("Promise.any not found")
+	}
+
+	pInterface := make([]interface{}, 0, len(promise))
+	for _, v := range promise {
+		pInterface = append(pInterface, v)
+	}
+
+	return mustJSValueToPromise(promiseAny.Invoke(pInterface))
+}
+
+// PromiseRace creates a promise that is fulfilled or rejected when one of the provided promises fulfill or reject.
+// It is implemented by calling Promise.race on JS.
+func PromiseRace(promise ...Promise) Promise {
+	promiseRace, err := Global().Expect(js.TypeFunction, "Promise", "race")
+	if err != nil {
+		panic("Promise.race not found")
+	}
+
+	pInterface := make([]interface{}, 0, len(promise))
+	for _, v := range promise {
+		pInterface = append(pInterface, v)
+	}
+
+	return mustJSValueToPromise(promiseRace.Invoke(pInterface))
+}
+
+func mustJSValueToPromise(v js.Value) Promise {
+	var p Promise
+	err := p.FromJSValue(v)
+	if err != nil {
+		panic("Expected a Promise from JS standard library")
+	}
+
+	return p
+}