fix: do-statementlist behavior

packages/babel-plugin-proposal-do-expressions/test/fixtures/do-expressions/semicolons.js

@@ -0,0 +1,15 @@
+expect(do {
+  x = do { 1; };
+}).toBe(1);
+
+expect(do {
+  z = do { 1;;;; };
+}).toBe(1)
+
+expect(do {
+  w = (do { 1;;;; });
+}).toBe(1);
+
+expect(do {
+  k = do { ; };
+}).toBe(undefined);

packages/babel-types/src/converters/gatherSequenceExpressions.js

@@ -25,7 +25,11 @@ export default function gatherSequenceExpressions(
   let ensureLastUndefined = true;
 
   for (const node of nodes) {
-    ensureLastUndefined = false;
+    // if we encounter emptyStatement before a non-emptyStatement
+    // we want to disregard that
+    if (!isEmptyStatement(node)) {
+      ensureLastUndefined = false;
+    }
 
     if (isExpression(node)) {
       exprs.push(node);
@@ -66,7 +70,10 @@ export default function gatherSequenceExpressions(
       exprs.push(body);
     } else if (isEmptyStatement(node)) {
       // empty statement so ensure the last item is undefined if we're last
-      ensureLastUndefined = true;
+      // checks if emptyStatement is first
+      if (nodes.indexOf(node) === 0) {
+        ensureLastUndefined = true;
+      }
     } else {
       // bailed, we can't turn this statement into an expression
       return;

packages/babel-types/test/converters.js

@@ -317,10 +317,10 @@ describe("converters", function () {
       const sequence = t.toSequenceExpression([undefinedNode, node], scope);
       expect(sequence).toBeUndefined();
     });
-    it("gathers empty statements", function () {
+    it("gathers empty statements if first element", function () {
       const node = parseCode(";");
       const sequence = t.toSequenceExpression([undefinedNode, node], scope);
-      expect(generateCode(sequence.expressions[1])).toBe("undefined");
+      expect(generateCode(sequence)).toBe("undefined");
     });
     it("skips empty statement if expression afterwards", function () {
       const node = parseCode("{ ; true }");