fix(formatter): switch case comments (#9303)

ematipico · web-flow · commit 464910c06058 · 2026-03-02T22:01:02.000Z
diff --git a/.changeset/fix-switch-case-comment-idempotence.md b/.changeset/fix-switch-case-comment-idempotence.md
@@ -0,0 +1,5 @@
+---
+"@biomejs/biome": patch
+---
+
+Fixed [#2786](https://github.com/biomejs/biome/issues/2786): The formatter no longer produces different output on subsequent runs when a `case` clause has a trailing line comment followed by a single block statement.
diff --git a/crates/biome_js_formatter/src/js/auxiliary/case_clause.rs b/crates/biome_js_formatter/src/js/auxiliary/case_clause.rs
@@ -70,12 +70,22 @@ impl FormatNodeRule<JsCaseClause> for FormatJsCaseClause {
         //   default:
         //     break;
         // }
+        // If the case clause has a trailing line comment after the colon
+        // (e.g. `case 1337: // ELITE`), we must not hug the block statement on
+        // the same line. Doing so produces `case 1337: { // ELITE`, and on the
+        // next format pass the comment is reparsed as a leading comment inside
+        // the block, causing it to move to its own line — an idempotence bug.
+        // The comment is stored as a trailing comment of the `test` node.
+        let has_trailing_comment = test
+            .as_ref()
+            .is_ok_and(|test| f.comments().has_trailing_comments(test.syntax()));
+
         if consequent.is_empty() {
             // Print nothing to ensure that trailing comments on the same line
             // are printed on the same line. The parent list formatter takes
             // care of inserting a hard line break between cases.
             Ok(())
-        } else if is_single_block_statement {
+        } else if is_single_block_statement && !has_trailing_comment {
             write![f, [space(), consequent.format()]]
         } else {
             // no line break needed after because it is added by the indent in the switch statement
diff --git a/crates/biome_js_formatter/tests/specs/js/module/statement/switch_comment.js b/crates/biome_js_formatter/tests/specs/js/module/statement/switch_comment.js
@@ -16,3 +16,21 @@ switch(x) {
     a(); // ab
     break;
 }
+
+// Trailing comment on a case clause followed by a single block statement.
+// The comment must stay on the `case` line and not migrate into the block
+// on subsequent format passes (idempotence, issue #2786).
+function cool(x) {
+  switch (x) {
+    case 4: // guaranteed to be random
+    case 42: // classic
+    case 1337: // ELITE
+    {
+      console.log("x is cool");
+      break;
+    }
+    default: {
+      console.error("x is not cool");
+    }
+  }
+}
diff --git a/crates/biome_js_formatter/tests/specs/js/module/statement/switch_comment.js.snap b/crates/biome_js_formatter/tests/specs/js/module/statement/switch_comment.js.snap
@@ -25,6 +25,24 @@ switch(x) {
     break;
 }
 
+// Trailing comment on a case clause followed by a single block statement.
+// The comment must stay on the `case` line and not migrate into the block
+// on subsequent format passes (idempotence, issue #2786).
+function cool(x) {
+  switch (x) {
+    case 4: // guaranteed to be random
+    case 42: // classic
+    case 1337: // ELITE
+    {
+      console.log("x is cool");
+      break;
+    }
+    default: {
+      console.error("x is not cool");
+    }
+  }
+}
+
 ```
 
 
@@ -75,4 +93,22 @@ switch (x) {
 		break;
 }
 
+// Trailing comment on a case clause followed by a single block statement.
+// The comment must stay on the `case` line and not migrate into the block
+// on subsequent format passes (idempotence, issue #2786).
+function cool(x) {
+	switch (x) {
+		case 4: // guaranteed to be random
+		case 42: // classic
+		case 1337: // ELITE
+			{
+				console.log("x is cool");
+				break;
+			}
+		default: {
+			console.error("x is not cool");
+		}
+	}
+}
+
 ```

-Original file line number
+Diff line change
@@ @@ -0,0 +1,5 @@ @@
 +---
 +"@biomejs/biome": patch
 +---
++
 +Fixed [#2786](https://github.com/biomejs/biome/issues/2786): The formatter no longer produces different output on subsequent runs when a `case` clause has a trailing line comment followed by a single block statement.