syscall/js: enable a JS Promise pattern for Funcs #52581

Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,57 @@
	//go:build js && wasm

	package js

	import "errors"

	// Promisable function that satisfies MakePromise's requirements.
	type PromiseAbleFunc = func(Value, []Value) (interface{}, error)

	// MakePromise makes a promise of a function that takes an array of arguments.
	func PromiseOf(fn PromiseAbleFunc) Func {
	return FuncOf(func(this Value, args []Value) interface{} {
	// Handler for this Promise.
	handler := FuncOf(func(promiseThis Value, promiseArgs []Value) interface{} {
	resolve := promiseArgs[0]
	reject := promiseArgs[1]

	// Run this asynchronously.
	go func() {
	// Recover by rejecting.
	defer func() {
	if err := recover(); err != nil {
	reject.Invoke(PromiseError(err))
	}
	}()

	// Run the PromiseAble func with original this and args.
	res, err := fn(this, args)
	if err != nil {
	reject.Invoke(PromiseError(err))
	return
	}
	resolve.Invoke(res)
	}()

	// Handler doesn't return anything directly.
	return nil
	})

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

	// PromiseError makes sure to return some error that Invoke will understand.
	func PromiseError(e interface{}) (err error) {
	switch x := e.(type) {
	case string:
	err = errors.New(x)
	case error:
	err = x
	default:
	err = errors.New("unknown panic")
	}
	return err
	}

Navigation Menu