js: os now compiles to the JS backend, more builtins & minor codegen fixes (#11302)

vlib/builtin/js/builtin.v

@@ -32,12 +32,38 @@ struct Option {
 	err   Error
 }
 
+// IError holds information about an error instance
+pub interface IError {
+	msg string
+	code int
+}
+
+// Error is the default implementation of IError, that is returned by e.g. `error()`
 pub struct Error {
 pub:
 	msg  string
 	code int
 }
 
+const none__ = IError(&None__{})
+
+struct None__ {
+	msg  string
+	code int
+}
+
+fn (_ None__) str() string {
+	return 'none'
+}
+
+pub fn (err IError) str() string {
+	return match err {
+		None__ { 'none' }
+		Error { err.msg }
+		else { '$err.type_name(): $err.msg' }
+	}
+}
+
 pub fn (o Option) str() string {
 	if o.state == 0 {
 		return 'Option{ ok }'
@@ -48,21 +74,28 @@ pub fn (o Option) str() string {
 	return 'Option{ error: "$o.err" }'
 }
 
-pub fn error(s string) Option {
-	return Option{
-		state: 2
-		err: Error{
-			msg: s
-		}
+[if trace_error ?]
+fn trace_error(x string) {
+	eprintln('> ${@FN} | $x')
+}
+
+// error returns a default error instance containing the error given in `message`.
+// Example: `if ouch { return error('an error occurred') }`
+[inline]
+pub fn error(message string) IError {
+	trace_error(message)
+	return &Error{
+		msg: message
 	}
 }
 
-pub fn error_with_code(s string, code int) Option {
-	return Option{
-		state: 2
-		err: Error{
-			msg: s
-			code: code
-		}
+// error_with_code returns a default error instance containing the given `message` and error `code`.
+// `if ouch { return error_with_code('an error occurred', 1) }`
+[inline]
+pub fn error_with_code(message string, code int) IError {
+	// trace_error('$message | code: $code')
+	return &Error{
+		msg: message
+		code: code
 	}
 }