_js: expose IsIncomplete, don't stringify on throw

For example, ParseError contains more information than just the message,
and we want to have that available.

Errors from the JS layer shouldn't be the Go-style error interface. JS
already has an Error type, whith a message string and its own stack
trace. Use that, which is as simple as calling panic in Go.

_js/main.go

@@ -27,7 +27,7 @@ func main() {
 		}
 		node, ok := v.(syntax.Node)
 		if !ok {
-			throw("NodeType requires a Node argument")
+			panic("NodeType requires a Node argument")
 		}
 		typ := fmt.Sprintf("%T", node)
 		if i := strings.LastIndexAny(typ, "*.]"); i >= 0 {
@@ -47,13 +47,14 @@ func main() {
 		}
 		return jp
 	})
+	stx.Set("IsIncomplete", syntax.IsIncomplete)
 
 	stx.Set("KeepComments", func(v interface{}) {
 		syntax.KeepComments(&v.(*jsParser).Parser)
 	})
 	stx.Set("Variant", func(l syntax.LangVariant) func(interface{}) {
 		if math.IsNaN(float64(l)) {
-			throw("Variant requires a LangVariant argument")
+			panic("Variant requires a LangVariant argument")
 		}
 		return func(v interface{}) {
 			syntax.Variant(l)(&v.(*jsParser).Parser)
@@ -94,8 +95,8 @@ func main() {
 	})
 }
 
-func throw(v interface{}) {
-	js.Global.Call("$throwRuntimeError", fmt.Sprint(v))
+func throw(err error) {
+	js.Global.Call("$throw", js.MakeFullWrapper(err))
 }
 
 // streamReader is an io.Reader wrapper for Node's stream.Readable. See

_js/testmain.js

@@ -34,12 +34,37 @@ const printer = syntax.NewPrinter()
 }
 
 {
-	// parse errors
+	// fatal parse error
+	const src = "echo )"
+	try {
+		parser.Parse(src, "src")
+		assert.fail("did not error")
+	} catch (err) {
+		assert.equal(err.Filename, "src")
+		assert.equal(err.Pos.Line(), 1)
+		assert.equal(syntax.IsIncomplete(err), false)
+	}
+}
+
+{
+	// incomplete parse error
 	const src = "echo ${"
 	try {
 		parser.Parse(src, "src")
 		assert.fail("did not error")
 	} catch (err) {
+		assert.equal(syntax.IsIncomplete(err), true)
+	}
+}
+
+{
+	// js error from the wrapper layer
+	var foo = {}
+	try {
+		syntax.NodeType(foo)
+		assert.fail("did not error")
+	} catch (err) {
+		assert.equal(err.message.includes("requires a Node argument"), true)
 	}
 }