feat(minifier): compress a == null && b to a ?? b when return value is ignored

crates/oxc_minifier/src/peephole/mod.rs

@@ -151,6 +151,7 @@ impl<'a> Traverse<'a> for PeepholeOptimizations {
         let ctx = Ctx(ctx);
         self.minimize_conditions_exit_statement(stmt, ctx);
         self.remove_dead_code_exit_statement(stmt, ctx);
+        self.substitute_exit_statement(stmt, ctx);
     }
 
     fn exit_return_statement(&mut self, stmt: &mut ReturnStatement<'a>, ctx: &mut TraverseCtx<'a>) {

crates/oxc_minifier/src/peephole/substitute_alternate_syntax.rs

@@ -844,6 +844,62 @@ impl<'a> PeepholeOptimizations {
             self.mark_current_function_as_changed();
         }
     }
+
+    pub fn substitute_exit_statement(&mut self, stmt: &mut Statement<'a>, ctx: Ctx<'a, '_>) {
+        if let Statement::ExpressionStatement(expr_stmt) = stmt {
+            if let Some(folded_expr) = match &mut expr_stmt.expression {
+                Expression::LogicalExpression(expr) => {
+                    self.try_compress_is_null_and_to_nullish_coalescing(expr, ctx)
+                }
+                _ => None,
+            } {
+                expr_stmt.expression = folded_expr;
+                self.mark_current_function_as_changed();
+            }
+        }
+    }
+
+    /// Compress `a == null && b` to `a ?? b`
+    ///
+    /// - `a == null && b` -> `a ?? b`
+    /// - `a != null || b` -> `a ?? b`
+    ///
+    /// This can be only done when the return value is not used.
+    /// For example when a = 1, `a == null && b` returns `false` while `a ?? b` returns `1`.
+    fn try_compress_is_null_and_to_nullish_coalescing(
+        &self,
+        expr: &mut LogicalExpression<'a>,
+        ctx: Ctx<'a, '_>,
+    ) -> Option<Expression<'a>> {
+        if self.target < ESTarget::ES2020 {
+            return None;
+        }
+        let target_op = match expr.operator {
+            LogicalOperator::And => BinaryOperator::Equality,
+            LogicalOperator::Or => BinaryOperator::Inequality,
+            LogicalOperator::Coalesce => return None,
+        };
+        let Expression::BinaryExpression(binary_expr) = &mut expr.left else {
+            return None;
+        };
+        if binary_expr.operator != target_op {
+            return None;
+        }
+        let new_left_hand_expr = if binary_expr.left.is_null() {
+            ctx.ast.move_expression(&mut binary_expr.right)
+        } else if binary_expr.right.is_null() {
+            ctx.ast.move_expression(&mut binary_expr.left)
+        } else {
+            return None;
+        };
+
+        Some(ctx.ast.expression_logical(
+            expr.span,
+            new_left_hand_expr,
+            LogicalOperator::Coalesce,
+            ctx.ast.move_expression(&mut expr.right),
+        ))
+    }
 }
 
 impl<'a> LatePeepholeOptimizations {
@@ -1645,4 +1701,13 @@ mod test {
             run(code, None)
         );
     }
+
+    #[test]
+    fn test_compress_is_null_and_to_nullish_coalescing() {
+        test("x == null && y", "x ?? y");
+        test("x != null || y", "x ?? y");
+        test_same("v = x == null && y");
+        test_same("v = x != null || y");
+        test("void (x == null && y)", "x ?? y");
+    }
 }

tasks/minsize/minsize.snap

@@ -1,27 +1,27 @@
            | Oxc        | ESBuild    | Oxc        | ESBuild    |
 Original   | minified   | minified   | gzip       | gzip       | Fixture
 -------------------------------------------------------------------------------------
-72.14 kB   | 23.61 kB   | 23.70 kB   | 8.55 kB    | 8.54 kB    | react.development.js
+72.14 kB   | 23.57 kB   | 23.70 kB   | 8.55 kB    | 8.54 kB    | react.development.js
 
-173.90 kB  | 59.70 kB   | 59.82 kB   | 19.26 kB   | 19.33 kB   | moment.js 
+173.90 kB  | 59.68 kB   | 59.82 kB   | 19.25 kB   | 19.33 kB   | moment.js 
 
-287.63 kB  | 89.58 kB   | 90.07 kB   | 31.08 kB   | 31.95 kB   | jquery.js 
+287.63 kB  | 89.54 kB   | 90.07 kB   | 31.07 kB   | 31.95 kB   | jquery.js 
 
-342.15 kB  | 117.72 kB  | 118.14 kB  | 43.66 kB   | 44.37 kB   | vue.js    
+342.15 kB  | 117.69 kB  | 118.14 kB  | 43.66 kB   | 44.37 kB   | vue.js    
 
-544.10 kB  | 71.50 kB   | 72.48 kB   | 25.92 kB   | 26.20 kB   | lodash.js 
+544.10 kB  | 71.49 kB   | 72.48 kB   | 25.92 kB   | 26.20 kB   | lodash.js 
 
-555.77 kB  | 271.68 kB  | 270.13 kB  | 88.48 kB   | 90.80 kB   | d3.js     
+555.77 kB  | 271.48 kB  | 270.13 kB  | 88.45 kB   | 90.80 kB   | d3.js     
 
-1.01 MB    | 457.66 kB  | 458.89 kB  | 123.79 kB  | 126.71 kB  | bundle.min.js
+1.01 MB    | 457.63 kB  | 458.89 kB  | 123.79 kB  | 126.71 kB  | bundle.min.js
 
-1.25 MB    | 650.64 kB  | 646.76 kB  | 161.49 kB  | 163.73 kB  | three.js  
+1.25 MB    | 650.59 kB  | 646.76 kB  | 161.49 kB  | 163.73 kB  | three.js  
 
-2.14 MB    | 718.99 kB  | 724.14 kB  | 162.41 kB  | 181.07 kB  | victory.js
+2.14 MB    | 718.82 kB  | 724.14 kB  | 162.40 kB  | 181.07 kB  | victory.js
 
-3.20 MB    | 1.01 MB    | 1.01 MB    | 325.29 kB  | 331.56 kB  | echarts.js
+3.20 MB    | 1.01 MB    | 1.01 MB    | 325.18 kB  | 331.56 kB  | echarts.js
 
 6.69 MB    | 2.30 MB    | 2.31 MB    | 469.99 kB  | 488.28 kB  | antd.js   
 
-10.95 MB   | 3.37 MB    | 3.49 MB    | 866.60 kB  | 915.50 kB  | typescript.js
+10.95 MB   | 3.37 MB    | 3.49 MB    | 866.63 kB  | 915.50 kB  | typescript.js