fix(js_formatter): array printing and grouping in arrows

crates/biome_formatter/src/printer/mod.rs

@@ -162,6 +162,12 @@ impl<'a> Printer<'a> {
                         _ => {
                             self.state.measured_group_fits = true;
 
+                            if let Some(id) = group.id() {
+                                self.state
+                                    .group_modes
+                                    .insert_print_mode(id, PrintMode::Flat);
+                            }
+
                             // Measure to see if the group fits up on a single line. If that's the case,
                             // print the group in "flat" mode, otherwise continue in expanded mode
                             stack.push(TagKind::Group, args.with_print_mode(PrintMode::Flat));

crates/biome_js_formatter/src/js/expressions/arrow_function_expression.rs

@@ -286,6 +286,15 @@ fn format_signature(
     })
 }
 
+/// Returns a `true` result if the arrow function contains any elements which
+/// should force the chain to break onto multiple lines. This includes any kind
+/// of return type annotation if the function also takes parameters (e.g.,
+/// `(a, b): bool => ...`), or any kind of rest/object/array binding parameter
+/// (e.g., `({a, b: foo}) => ...`).
+///
+/// The complexity of these expressions limits their legibility when printed
+/// inline, so they force the chain to break to preserve clarity. Any other
+/// cases are considered simple enough to print in a single line.
 fn should_break_chain(arrow: &JsArrowFunctionExpression) -> SyntaxResult<bool> {
     if arrow.type_parameters().is_some() {
         return Ok(true);
@@ -438,31 +447,58 @@ impl Format<JsFormatContext> for ArrowChain {
         } = self;
 
         let head_parent = head.syntax().parent();
+        let tail_body = tail.body()?;
+        let is_assignment_rhs = self.options.assignment_layout.is_some();
         let ancestor_call_expr_or_logical_expr = head.syntax().ancestors().any(|ancestor| {
             matches!(
                 ancestor.kind(),
                 JsSyntaxKind::JS_CALL_EXPRESSION | JsSyntaxKind::JS_LOGICAL_EXPRESSION
             )
         });
 
-        let tail_body = tail.body()?;
-
-        let is_assignment_rhs = self.options.assignment_layout.is_some();
-
+        // If this chain is the callee in a parent call expression, then we
+        // want it to break onto a new line to clearly show that the arrow
+        // chain is distinct and the _result_ is what's being called.
+        // Example:
+        //      (() => () => a)()
+        // becomes
+        //      (
+        //        () => () =>
+        //          a
+        //      )();
         let is_callee = head_parent
             .as_ref()
             .map_or(false, |parent| is_callee(head.syntax(), parent));
 
+        // With arrays, objects, sequence expressions, and block function bodies,
+        // the opening brace gives a convenient boundary to insert a line break,
+        // allowing that token to live immediately after the last arrow token
+        // and save a line from being printed with just the punctuation.
+        //
+        // (foo) => (bar) => [a, b]
+        //
+        // (foo) => (bar) => [
+        //   a,
+        //   b
+        // ]
+        //
+        // If the body is _not_ one of those kinds, then we'll want to insert a
+        // soft line break before the body so that it prints on a separate line
+        // in its entirety.
         let body_on_separate_line = !matches!(
             tail_body,
             AnyJsFunctionBody::JsFunctionBody(_)
                 | AnyJsFunctionBody::AnyJsExpression(
                     AnyJsExpression::JsObjectExpression(_)
+                        | AnyJsExpression::JsArrayExpression(_)
                         | AnyJsExpression::JsSequenceExpression(_)
                 )
         );
 
-        let break_before_chain = (is_callee && body_on_separate_line)
+        // If the arrow chain will break onto multiple lines, either because
+        // it's a callee or because the body is printed on its own line, then
+        // the signatures should be expanded first.
+        let break_signatures = (is_callee && body_on_separate_line)
             || matches!(
                 self.options.assignment_layout,
                 Some(AssignmentLikeLayout::ChainTailArrowFunction)
@@ -602,7 +638,7 @@ impl Format<JsFormatContext> for ArrowChain {
                 [
                     group(&indent(&format_arrow_signatures))
                         .with_group_id(Some(group_id))
-                        .should_expand(break_before_chain),
+                        .should_expand(break_signatures),
                     space(),
                     tail.fat_arrow_token().format(),
                     indent_if_group_breaks(&format_tail_body, group_id),
@@ -710,7 +746,7 @@ fn template_literal_contains_new_line(template: &JsTemplateExpression) -> bool {
 ///
 ///
 /// # Examples
-//
+///
 /// ```javascript
 /// "test" + `
 ///   some content

crates/biome_js_formatter/tests/specs/prettier/js/arrows/currying-4.js.snap

@@ -75,77 +75,7 @@ a(
 ```diff
 --- Prettier
 +++ Biome
-@@ -41,47 +41,49 @@
-   "sky.",
- ];
-
--const x2 = () => () => [
--  "The",
--  "green",
--  "dragon",
--  "liked",
--  "to",
--  "knit",
--  "sweaters",
--  "for",
--  "the",
--  "fluffy",
--  "clouds",
--  "in",
--  "the",
--  "sky.",
--];
-+const x2 = () => () =>
-+  [
-+    "The",
-+    "green",
-+    "dragon",
-+    "liked",
-+    "to",
-+    "knit",
-+    "sweaters",
-+    "for",
-+    "the",
-+    "fluffy",
-+    "clouds",
-+    "in",
-+    "the",
-+    "sky.",
-+  ];
-
--const x3 = () => () => () => [
--  "The",
--  "green",
--  "dragon",
--  "liked",
--  "to",
--  "knit",
--  "sweaters",
--  "for",
--  "the",
--  "fluffy",
--  "clouds",
--  "in",
--  "the",
--  "sky.",
--];
-+const x3 = () => () => () =>
-+  [
-+    "The",
-+    "green",
-+    "dragon",
-+    "liked",
-+    "to",
-+    "knit",
-+    "sweaters",
-+    "for",
-+    "the",
-+    "fluffy",
-+    "clouds",
-+    "in",
-+    "the",
-+    "sky.",
-+  ];
+@@ -77,11 +77,11 @@
 
  f((a) => (1, 2, 3) /* a */);
  f((a) => (b) => (1, 2, 3) /* b */ /* a */);
@@ -207,41 +137,39 @@ const x1 = () => [
   "sky.",
 ];
 
-const x2 = () => () =>
-  [
-    "The",
-    "green",
-    "dragon",
-    "liked",
-    "to",
-    "knit",
-    "sweaters",
-    "for",
-    "the",
-    "fluffy",
-    "clouds",
-    "in",
-    "the",
-    "sky.",
-  ];
-
-const x3 = () => () => () =>
-  [
-    "The",
-    "green",
-    "dragon",
-    "liked",
-    "to",
-    "knit",
-    "sweaters",
-    "for",
-    "the",
-    "fluffy",
-    "clouds",
-    "in",
-    "the",
-    "sky.",
-  ];
+const x2 = () => () => [
+  "The",
+  "green",
+  "dragon",
+  "liked",
+  "to",
+  "knit",
+  "sweaters",
+  "for",
+  "the",
+  "fluffy",
+  "clouds",
+  "in",
+  "the",
+  "sky.",
+];
+
+const x3 = () => () => () => [
+  "The",
+  "green",
+  "dragon",
+  "liked",
+  "to",
+  "knit",
+  "sweaters",
+  "for",
+  "the",
+  "fluffy",
+  "clouds",
+  "in",
+  "the",
+  "sky.",
+];
 
 f((a) => (1, 2, 3) /* a */);
 f((a) => (b) => (1, 2, 3) /* b */ /* a */);