Remove Parens from SequenceExpressions in ForStatements (#1597)

* Remove Parens from SequenceExpressions in ForStatements

Closes #1594

* Add Braces around Assignment Case

src/fast-path.js

@@ -448,7 +448,9 @@ FPp.needsParens = function() {
         parent.object === node
       );
 
-    case "AssignmentExpression":
+    case "AssignmentExpression": {
+      const grandParent = this.getParentNode(1)
+
       if (parent.type === "ArrowFunctionExpression" && parent.body === node) {
         return true;
       } else if (
@@ -468,9 +470,16 @@ FPp.needsParens = function() {
         return node.left.type === "ObjectPattern";
       } else if (parent.type === "AssignmentExpression") {
         return false;
+      } else if (
+        parent.type === "SequenceExpression" &&
+        grandParent &&
+        grandParent.type === "ForStatement" &&
+        (grandParent.init === parent || grandParent.update === parent)
+      ) {
+        return false;
       }
       return true;
-
+    }
     case "ConditionalExpression":
       switch (parent.type) {
         case "TaggedTemplateExpression":

tests/assignment/__snapshots__/jsfmt.spec.js.snap

@@ -20,3 +20,22 @@ computedDescriptionLines =
   (focused && !loading && descriptionLinesFocused) || descriptionLines;
 
 `;
+
+exports[`sequence.js 1`] = `
+for ((i = 0), (len = arr.length); i < len; i++) {
+  console.log(arr[i])
+}
+
+for (i = 0, len = arr.length; i < len; i++) {
+  console.log(arr[i])
+}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+for (i = 0, len = arr.length; i < len; i++) {
+  console.log(arr[i]);
+}
+
+for (i = 0, len = arr.length; i < len; i++) {
+  console.log(arr[i]);
+}
+
+`;

tests/assignment/sequence.js

@@ -0,0 +1,7 @@
+for ((i = 0), (len = arr.length); i < len; i++) {
+  console.log(arr[i])
+}
+
+for (i = 0, len = arr.length; i < len; i++) {
+  console.log(arr[i])
+}