Added parens around in operator in for loops 🚀. (#468)

src/fast-path.js

@@ -276,6 +276,13 @@ FPp.needsParens = function(assumeExpressionContext) {
         return true;
       }
 
+      if (
+        node.operator === 'in' &&
+          parent.type === 'AssignmentExpression'
+      ) {
+        return true;
+      }
+
     case "LogicalExpression":
       switch (parent.type) {
         case "CallExpression":

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

@@ -11,9 +11,12 @@ for (var i = 0; i < 10; ++i) {
 
 exports[`test in.js 1`] = `
 "for ((x in a);;) {}
+for (a=(a in b);;) {}
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 for ((x in a); ; ) {
 }
+for (a = (a in b); ; ) {
+}
 "
 `;
 

tests/for/in.js

@@ -1 +1,2 @@
 for ((x in a);;) {}
+for (a=(a in b);;) {}