diff --git a/src/language-js/comments.js b/src/language-js/comments.js
index 4ad90155..ff4ef1da 100644
--- a/src/language-js/comments.js
+++ b/src/language-js/comments.js
@@ -135,6 +135,13 @@ function handleRemainingComment(comment, text, options, ast, isLastComment) {
       comment,
       options
     ) ||
+    handleTSMappedTypeComments(
+      text,
+      enclosingNode,
+      precedingNode,
+      followingNode,
+      comment
+    ) ||
     handleBreakAndContinueStatementComments(enclosingNode, comment)
   ) {
     return true;
@@ -700,6 +707,38 @@ function handleVariableDeclaratorComments(
   return false;
 }
 
+function handleTSMappedTypeComments(
+  text,
+  enclosingNode,
+  precedingNode,
+  followingNode,
+  comment
+) {
+  if (!enclosingNode || enclosingNode.type !== "TSMappedType") {
+    return false;
+  }
+
+  if (
+    followingNode &&
+    followingNode.type === "TSTypeParameter" &&
+    followingNode.name
+  ) {
+    addLeadingComment(followingNode.name, comment);
+    return true;
+  }
+
+  if (
+    precedingNode &&
+    precedingNode.type === "TSTypeParameter" &&
+    precedingNode.constraint
+  ) {
+    addTrailingComment(precedingNode.constraint, comment);
+    return true;
+  }
+
+  return false;
+}
+
 function isBlockComment(comment) {
   return comment.type === "Block" || comment.type === "CommentBlock";
 }
diff --git a/src/language-js/printer-estree.js b/src/language-js/printer-estree.js
index e301bd9f..762ce04a 100644
--- a/src/language-js/printer-estree.js
+++ b/src/language-js/printer-estree.js
@@ -2559,10 +2559,11 @@ function printPathNoParens(path, options, print, args) {
     case "TypeParameter": {
       const parent = path.getParentNode();
       if (parent.type === "TSMappedType") {
-        parts.push(path.call(print, "name"));
+        parts.push("[", path.call(print, "name"));
         if (n.constraint) {
           parts.push(" in ", path.call(print, "constraint"));
         }
+        parts.push("]");
         return concat(parts);
       }
 
@@ -2788,9 +2789,7 @@ function printPathNoParens(path, options, print, args) {
                   ])
                 : "",
               printTypeScriptModifiers(path, options, print),
-              "[",
               path.call(print, "typeParameter"),
-              "]",
               n.questionToken
                 ? getTypeScriptMappedTypeModifier(n.questionToken, "?")
                 : "",
diff --git a/tests/typescript_comments/__snapshots__/jsfmt.spec.js.snap b/tests/typescript_comments/__snapshots__/jsfmt.spec.js.snap
index f68bb8c5..4358de8b 100644
--- a/tests/typescript_comments/__snapshots__/jsfmt.spec.js.snap
+++ b/tests/typescript_comments/__snapshots__/jsfmt.spec.js.snap
@@ -116,6 +116,55 @@ export type WrappedFormUtils = {
 
 `;
 
+exports[`mapped_types.ts 1`] = `
+type A = {
+  // commentA
+  [a in A]: string;
+}
+
+type B = {
+  /* commentB */ [b in B]: string
+}
+
+type C = {
+  [/* commentC */ c in C]: string
+}
+
+type D = {
+  [d /* commentD */ in D]: string
+}
+
+type E = {
+  [e in /* commentE */ E]: string
+}
+
+type F = {
+  [f in F /* commentF */]: string
+}
+
+type G = {
+  [g in G] /* commentG */: string
+}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+type A = {
+  // commentA
+  [a in A]: string
+};
+
+type B = { /* commentB */ [b in B]: string };
+
+type C = { [/* commentC */ c in C]: string };
+
+type D = { [d /* commentD */ in D]: string };
+
+type E = { [e in /* commentE */ E]: string };
+
+type F = { [f in F /* commentF */]: string };
+
+type G = { [g in G /* commentG */]: string };
+
+`;
+
 exports[`methods.ts 1`] = `
 export class Point {
 /**
diff --git a/tests/typescript_comments/mapped_types.ts b/tests/typescript_comments/mapped_types.ts
new file mode 100644
index 00000000..67dcc092
--- /dev/null
+++ b/tests/typescript_comments/mapped_types.ts
@@ -0,0 +1,28 @@
+type A = {
+  // commentA
+  [a in A]: string;
+}
+
+type B = {
+  /* commentB */ [b in B]: string
+}
+
+type C = {
+  [/* commentC */ c in C]: string
+}
+
+type D = {
+  [d /* commentD */ in D]: string
+}
+
+type E = {
+  [e in /* commentE */ E]: string
+}
+
+type F = {
+  [f in F /* commentF */]: string
+}
+
+type G = {
+  [g in G] /* commentG */: string
+}