diff --git a/_js/main.go b/_js/main.go
index 5ea891d9a..13070361a 100644
--- a/_js/main.go
+++ b/_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
diff --git a/_js/testmain.js b/_js/testmain.js
index 73088642d..450f312d6 100644
--- a/_js/testmain.js
+++ b/_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)
 	}
 }