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fix(formatter): switch case comments #9303

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ematipico merged 1 commit into from
Mar 2, 2026
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fix(formatter): switch case comments #9303

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5 changes: 5 additions & 0 deletions .changeset/fix-switch-case-comment-idempotence.md
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Original file line number Diff 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.
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⚠️ Potential issue | 🟡 Minor

Add a tiny before/after formatter diff in the changeset body.

Nice summary, but this formatter fix should include a concrete formatting diff so users can see the visible behaviour change at a glance.

Suggested changeset snippet 
 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
+-case 1337: {
+-  // ELITE
++case 1337: // ELITE
++{
+   console.log("x is cool");
+ }
+```

As per coding guidelines: for formatter changes in changesets, show formatting changes using diff code blocks.

📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
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.
---
package: `@biomejs/biome`
type: 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.
🤖 Prompt for AI Agents 
Verify each finding against the current code and only fix it if needed.

In @.changeset/fix-switch-case-comment-idempotence.md at line 5, Update the
changeset body in .changeset/fix-switch-case-comment-idempotence.md to include a
before/after formatter diff code block showing the concrete visible change (use
a unified diff-style snippet); insert a fenced code block (```diff) containing
the original lines (prefixed with -) and the formatted lines (prefixed with +) —
e.g., show the "-case 1337: {", "-  // ELITE", "-  console.log(\"x is cool\");",
"-}" and the corresponding "+case 1337: // ELITE", "+{", "+  console.log(\"x is
cool\");", "+}" lines — so users can see the exact formatting change.

12 changes: 11 additions & 1 deletion crates/biome_js_formatter/src/js/auxiliary/case_clause.rs
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Expand Up @@ -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
Expand Down
18 changes: 18 additions & 0 deletions crates/biome_js_formatter/tests/specs/js/module/statement/switch_comment.js
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Original file line number Diff line number Diff line change
Expand Up @@ -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");
}
}
}
36 changes: 36 additions & 0 deletions crates/biome_js_formatter/tests/specs/js/module/statement/switch_comment.js.snap
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Original file line number Diff line number Diff line change
Expand Up @@ -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");
}
}
}

```


Expand Down Expand Up @@ -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");
}
}
}

```
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