Fix unstable comments in assignment pattern (#1055)

* Add handleAssignmentPatternComments print function.

* Add new test cases.

src/comments.js

@@ -166,7 +166,8 @@ function attach(comments, ast, text, options) {
           comment
         ) ||
         handleOnlyComments(enclosingNode, ast, comment, isLastComment) ||
-        handleImportDeclarationComments(enclosingNode, precedingNode, comment)
+        handleImportDeclarationComments(enclosingNode, precedingNode, comment) ||
+        handleAssignmentPatternComments(enclosingNode, comment)
       ) {
         // We're good
       } else if (followingNode) {
@@ -657,6 +658,14 @@ function handleImportDeclarationComments(enclosingNode, precedingNode, comment)
   return false;
 }
 
+function handleAssignmentPatternComments(enclosingNode, comment) {
+  if (enclosingNode && enclosingNode.type === "AssignmentPattern") {
+    addLeadingComment(enclosingNode, comment);
+    return true;
+  }
+  return false;
+}
+
 function printComment(commentPath) {
   const comment = commentPath.getValue();
   comment.printed = true;

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

@@ -39,6 +39,22 @@ var fnString = // Comment
 
 var fnString = // Comment
   'some' + 'long' + 'string';
+
+let f = (
+  a =
+  //comment
+  b
+) => {};
+
+let f = (
+  a = //comment
+  b
+) => {};
+
+let f = (
+  a =
+  b //comment
+) => {};
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 fnString =
   // Comment
@@ -88,6 +104,19 @@ var fnString = // Comment
 
 var fnString = // Comment
   \\"some\\" + \\"long\\" + \\"string\\";
+
+let f = (
+  //comment
+  a = b
+) => {};
+
+let f = (
+  a = b //comment
+) => {};
+
+let f = (
+  a = b //comment
+) => {};
 "
 `;
 
@@ -130,6 +159,22 @@ var fnString = // Comment
 
 var fnString = // Comment
   'some' + 'long' + 'string';
+
+let f = (
+  a =
+  //comment
+  b
+) => {};
+
+let f = (
+  a = //comment
+  b
+) => {};
+
+let f = (
+  a =
+  b //comment
+) => {};
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 fnString =
   // Comment
@@ -179,5 +224,18 @@ var fnString = // Comment
 
 var fnString = // Comment
   \\"some\\" + \\"long\\" + \\"string\\";
+
+let f = (
+  //comment
+  a = b
+) => {};
+
+let f = (
+  a = b //comment
+) => {};
+
+let f = (
+  a = b //comment
+) => {};
 "
 `;

tests/assignment_comments/assignment_comments.js

@@ -36,3 +36,19 @@ var fnString = // Comment
 
 var fnString = // Comment
   'some' + 'long' + 'string';
+
+let f = (
+  a =
+  //comment
+  b
+) => {};
+
+let f = (
+  a = //comment
+  b
+) => {};
+
+let f = (
+  a =
+  b //comment
+) => {};