From e35990390b271fffca7e4e7a89971d82df0dc2e6 Mon Sep 17 00:00:00 2001
From: Austaras <austaras@outlook.com>
Date: Wed, 10 Aug 2022 16:26:33 +0800
Subject: [PATCH] fix(es/fixer): Remove needless parens for nested `??` (#5432)

---
 crates/swc_ecma_ast/src/operators.rs          |  4 ++++
 crates/swc_ecma_transforms_base/src/fixer.rs  | 23 ++++++++++---------
 .../src/simplify/branch/mod.rs                |  2 +-
 3 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/crates/swc_ecma_ast/src/operators.rs b/crates/swc_ecma_ast/src/operators.rs
index ea26b88c569..8d17a1ec587 100644
--- a/crates/swc_ecma_ast/src/operators.rs
+++ b/crates/swc_ecma_ast/src/operators.rs
@@ -104,6 +104,10 @@ impl BinaryOp {
             BinaryOp::NullishCoalescing => 1,
         }
     }
+
+    pub fn may_short_circuit(&self) -> bool {
+        matches!(self, op!("??") | op!("||") | op!("&&"))
+    }
 }
 
 #[derive(StringEnum, Clone, Copy, Eq, PartialEq, PartialOrd, Ord, Hash, EqIgnoreSpan)]
diff --git a/crates/swc_ecma_transforms_base/src/fixer.rs b/crates/swc_ecma_transforms_base/src/fixer.rs
index 2cf7f9ff4fe..c76dcc20c09 100644
--- a/crates/swc_ecma_transforms_base/src/fixer.rs
+++ b/crates/swc_ecma_transforms_base/src/fixer.rs
@@ -255,14 +255,10 @@ impl VisitMut for Fixer<'_> {
             }
             Expr::Bin(BinExpr { op: op_of_rhs, .. }) => {
                 if *op_of_rhs == expr.op {
-                    match expr.op {
-                        // `a && (b && c)` == `a && b && c`
-                        // `a || (b || c)` == `a || b || c`
-                        // `a ** (b ** c)` == `a ** b ** c`
-                        op!("&&") | op!("||") | op!("**") => {}
-                        _ => {
-                            self.wrap(&mut expr.right);
-                        }
+                    // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence#precedence_and_associativity
+                    // `**` is the only right associative operator in js
+                    if !(expr.op.may_short_circuit() || expr.op == op!("**")) {
+                        self.wrap(&mut expr.right);
                     }
                 } else if op_of_rhs.precedence() <= expr.op.precedence()
                     || (*op_of_rhs == op!("&&") && expr.op == op!("??"))
@@ -274,7 +270,7 @@ impl VisitMut for Fixer<'_> {
         };
 
         match &mut *expr.left {
-            Expr::Bin(BinExpr { op: op!("??"), .. }) => {
+            Expr::Bin(BinExpr { op: op!("??"), .. }) if expr.op != op!("??") => {
                 self.wrap(&mut expr.left);
             }
 
@@ -321,8 +317,11 @@ impl VisitMut for Fixer<'_> {
         }
 
         if let op!("??") = expr.op {
-            if let Expr::Bin(..) = &mut *expr.left {
-                self.wrap(&mut expr.left);
+            match &*expr.left {
+                Expr::Bin(BinExpr { op, .. }) if *op != op!("??") => {
+                    self.wrap(&mut expr.left);
+                }
+                _ => (),
             }
         }
     }
@@ -1641,5 +1640,7 @@ var store = global[SHARED] || (global[SHARED] = {});
         "
     );
 
+    identical!(issue_5417, "console.log(a ?? b ?? c)");
+
     identical!(bin_and_unary, "console.log(a++ && b--)");
 }
diff --git a/crates/swc_ecma_transforms_optimization/src/simplify/branch/mod.rs b/crates/swc_ecma_transforms_optimization/src/simplify/branch/mod.rs
index fd9538fa952..d21a567cfd0 100644
--- a/crates/swc_ecma_transforms_optimization/src/simplify/branch/mod.rs
+++ b/crates/swc_ecma_transforms_optimization/src/simplify/branch/mod.rs
@@ -1430,7 +1430,7 @@ fn ignore_result(e: Expr, drop_str_lit: bool, ctx: &ExprCtx) -> Option<Expr> {
             left,
             op,
             right,
-        }) if op != op!("&&") && op != op!("||") && op != op!("??") => {
+        }) if !op.may_short_circuit() => {
             let left = ignore_result(*left, true, ctx);
             let right = ignore_result(*right, true, ctx);