Include compile error location in verbose error message format (#235)

Author: dylanahsmith

CHANGELOG.md

@@ -20,6 +20,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Support for compiling a context-dependent UnboundScript which can be run in any context of the isolate it was compiled in.
 - Support for creating a code cache from an UnboundScript which can be used to create an UnboundScript in other isolates
 to run a pre-compiled script in new contexts.
+- Included compile error location in `%+v` formatting of JSError
 
 ### Changed
 - Removed error return value from NewIsolate which never fails

errors.go

@@ -44,7 +44,15 @@ func (e *JSError) Format(s fmt.State, verb rune) {
 	switch verb {
 	case 'v':
 		if s.Flag('+') && e.StackTrace != "" {
+			// The StackTrace starts with the Message, so only the former needs to be printed
 			io.WriteString(s, e.StackTrace)
+
+			// If it was a compile time error, then there wouldn't be a runtime stack trace,
+			// but StackTrace will still include the Message, making them equal. In this case,
+			// we want to include the Location where the compilation failed.
+			if e.StackTrace == e.Message && e.Location != "" {
+				fmt.Fprintf(s, " (at %s)", e.Location)
+			}
 			return
 		}
 		fallthrough

errors_test.go

@@ -11,7 +11,7 @@ import (
 	v8 "rogchap.com/v8go"
 )
 
-func TestErrorFormatting(t *testing.T) {
+func TestJSErrorFormat(t *testing.T) {
 	t.Parallel()
 	tests := [...]struct {
 		name            string
@@ -90,3 +90,30 @@ func TestJSErrorOutput(t *testing.T) {
 		t.Errorf("unexpected error stack trace: %q", e.StackTrace)
 	}
 }
+
+func TestJSErrorFormat_forSyntaxError(t *testing.T) {
+	t.Parallel()
+	iso := v8.NewIsolate()
+	defer iso.Dispose()
+	ctx := v8.NewContext(iso)
+	defer ctx.Close()
+
+	script := `
+		let x = 1;
+		let y = x + ;
+		let z = x + z;
+	`
+	_, err := ctx.RunScript(script, "xyz.js")
+	jsErr := err.(*v8.JSError)
+	if jsErr.StackTrace != jsErr.Message {
+		t.Errorf("unexpected StackTrace %q not equal to Message %q", jsErr.StackTrace, jsErr.Message)
+	}
+	if jsErr.Location == "" {
+		t.Errorf("missing Location")
+	}
+
+	msg := fmt.Sprintf("%+v", err)
+	if msg != "SyntaxError: Unexpected token ';' (at xyz.js:3:15)" {
+		t.Errorf("unexpected verbose error message: %q", msg)
+	}
+}