diff --git a/_js/main.go b/_js/main.go
index ce6eabafb..5ea891d9a 100644
--- a/_js/main.go
+++ b/_js/main.go
@@ -116,6 +116,9 @@ func (r streamReader) Read(p []byte) (n int, err error) {
 
 type jsParser struct {
 	syntax.Parser
+
+	accumulated []*syntax.Stmt
+	incomplete  bytes.Buffer
 }
 
 func adaptReader(src *js.Object) io.Reader {
@@ -151,6 +154,33 @@ func (p *jsParser) Interactive(src *js.Object, jsFn func([]*js.Object) bool) {
 	}
 }
 
+func (p *jsParser) InteractiveStep(line string) []*syntax.Stmt {
+	// pick up previous chunks of the incomplete statement
+	r := strings.NewReader(p.incomplete.String() + line)
+	lastEnd := uint(0)
+	err := p.Parser.Interactive(r, func(stmts []*syntax.Stmt) bool {
+		if len(stmts) > 0 {
+			// don't re-parse finished statements
+			lastEnd = stmts[len(stmts)-1].End().Offset()
+		}
+		p.accumulated = append(p.accumulated, stmts...)
+		return false
+	})
+	if syntax.IsIncomplete(err) {
+		// starting or continuing an incomplete statement
+		p.incomplete.WriteString(line[lastEnd:])
+		return p.accumulated
+	}
+	// complete; empty both fields and return
+	p.incomplete.Reset()
+	if err != nil {
+		throw(err)
+	}
+	acc := p.accumulated
+	p.accumulated = p.accumulated[:0]
+	return acc
+}
+
 type jsPrinter struct {
 	*syntax.Printer
 }
diff --git a/_js/testmain.js b/_js/testmain.js
index 07a1f1290..73088642d 100644
--- a/_js/testmain.js
+++ b/_js/testmain.js
@@ -174,6 +174,38 @@ const printer = syntax.NewPrinter()
 	assert.deepEqual(gotCallbacks, wantCallbacks)
 }
 
+{
+	// using the parser interactively with steps
+	const lines = [
+		"foo\n",
+		"bar; baz\n",
+		"\n",
+		"foo; 'incom\n",
+		" \n",
+		"plete'\n",
+	]
+	const wantResults = [
+		{"count": 1, "incomplete": false},
+		{"count": 2, "incomplete": false},
+		{"count": 0, "incomplete": false},
+		{"count": 1, "incomplete": true},
+		{"count": 1, "incomplete": true},
+		{"count": 2, "incomplete": false},
+	]
+	var gotResults = []
+	for (var i = 0; i < lines.length; i++) {
+		var line = lines[i]
+		var want = wantResults[i]
+
+		var stmts = parser.InteractiveStep(line)
+		gotResults.push({
+			"count":      stmts.length,
+			"incomplete": parser.Incomplete(),
+		})
+	}
+	assert.deepEqual(gotResults, wantResults)
+}
+
 {
 	// splitting brace expressions
 	const parser = syntax.NewParser()