Single argument arrow functions (#1186)

swc_ecma_codegen:
 - Remove parens from arrow functions when possible.
 - Remove space after prop keys when minifying.
 - Remove line break after expressions when minifying.
 - Remove space after generator `*` when minifying.

ecmascript/codegen/src/decl.rs

@@ -59,9 +59,11 @@ impl<'a> Emitter<'a> {
         keyword!("function");
         if node.function.is_generator {
             punct!("*");
+            formatting_space!();
+        } else {
+            space!();
         }
 
-        space!();
         emit!(node.ident);
 
         self.emit_fn_trailing(&node.function)?;
@@ -100,3 +102,27 @@ impl<'a> Emitter<'a> {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::assert_min;
+
+    #[test]
+    fn issue_275() {
+        assert_min(
+            "function* foo(){
+            yield getServiceHosts()
+        }",
+            "function*foo(){yield getServiceHosts();}",
+        );
+    }
+
+    #[test]
+    fn single_argument_arrow_expression() {
+        assert_min("function* f(){ yield x => x}", "function*f(){yield x=>x;}");
+        assert_min(
+            "function* f(){ yield ({x}) => x}",
+            "function*f(){yield({x})=>x;}",
+        );
+    }
+}

ecmascript/codegen/src/expr.rs

@@ -188,14 +188,4 @@ mod tests {
     fn regression_increments() {
         assert_min("x++ + ++y", "x++ + ++y;");
     }
-
-    #[test]
-    fn issue_275() {
-        assert_min(
-            "function* foo(){
-            yield getServiceHosts()
-        }",
-            "function* foo(){yield getServiceHosts();}",
-        );
-    }
 }

ecmascript/codegen/src/lib.rs

@@ -644,9 +644,20 @@ impl<'a> Emitter<'a> {
         if node.is_generator {
             punct!("*")
         }
-        punct!("(");
+
+        let parens = !self.cfg.minify
+            || match node.params.as_slice() {
+                [Pat::Ident(_)] => false,
+                _ => true,
+            };
+
+        if parens {
+            punct!("(");
+        }
         self.emit_list(node.span, Some(&node.params), ListFormat::CommaListElements)?;
-        punct!(")");
+        if parens {
+            punct!(")");
+        }
 
         punct!("=>");
         emit!(node.body);
@@ -1114,7 +1125,9 @@ impl<'a> Emitter<'a> {
                 self.wr.increase_indent()?;
                 emit!(expr);
                 self.wr.decrease_indent()?;
-                self.wr.write_line()?;
+                if !self.cfg.minify {
+                    self.wr.write_line()?;
+                }
             }
         }
     }
@@ -1808,7 +1821,7 @@ impl<'a> Emitter<'a> {
         self.emit_leading_comments_of_pos(node.span().lo())?;
 
         emit!(node.key);
-        space!();
+        formatting_space!();
         if let Some(ref value) = node.value {
             punct!("=");
             emit!(node.value);

ecmascript/codegen/src/util.rs

@@ -197,8 +197,10 @@ impl StartsWithAlphaNum for Expr {
                 _ => false,
             },
 
-            // TODO(kdy1): Support `v => {}`
-            Expr::Arrow(ArrowExpr { .. }) => false,
+            Expr::Arrow(ref expr) => match expr.params.as_slice() {
+                [p] => p.starts_with_alpha_num(),
+                _ => false,
+            },
 
             Expr::Update(ref expr) => {
                 if expr.prefix {