feat(minifier): compress x = x || 1 to x ||= 1

crates/oxc_minifier/src/ast_passes/peephole_substitute_alternate_syntax.rs

@@ -138,6 +138,7 @@ impl<'a> Traverse<'a> for PeepholeSubstituteAlternateSyntax {
             }
             Expression::AssignmentExpression(e) => {
                 self.try_compress_normal_assignment_to_combined_assignment(e, ctx);
+                self.try_compress_normal_assignment_to_combined_logical_assignment(e, ctx);
             }
             _ => {}
         }
@@ -631,6 +632,42 @@ impl<'a, 'b> PeepholeSubstituteAlternateSyntax {
         self.changed = true;
     }
 
+    /// Compress `a = a || b` to `a ||= b`
+    ///
+    /// This can only be done for resolved identifiers as this would avoid setting `a` when `a` is truthy.
+    fn try_compress_normal_assignment_to_combined_logical_assignment(
+        &mut self,
+        expr: &mut AssignmentExpression<'a>,
+        ctx: Ctx<'a, 'b>,
+    ) {
+        if self.target < ESTarget::ES2020 {
+            return;
+        }
+        if !matches!(expr.operator, AssignmentOperator::Assign) {
+            return;
+        }
+
+        let Expression::LogicalExpression(logical_expr) = &mut expr.right else { return };
+        let new_op = logical_expr.operator.to_assignment_operator();
+
+        let (
+            AssignmentTarget::AssignmentTargetIdentifier(write_id_ref),
+            Expression::Identifier(read_id_ref),
+        ) = (&expr.left, &logical_expr.left)
+        else {
+            return;
+        };
+        // It should also early return when the reference might refer to a reference value created by a with statement
+        // when the minifier supports with statements
+        if write_id_ref.name != read_id_ref.name || ctx.is_global_reference(write_id_ref) {
+            return;
+        }
+
+        expr.operator = new_op;
+        expr.right = ctx.ast.move_expression(&mut logical_expr.right);
+        self.changed = true;
+    }
+
     fn try_compress_assignment_to_update_expression(
         expr: &mut AssignmentExpression<'a>,
         ctx: Ctx<'a, 'b>,
@@ -1210,6 +1247,27 @@ mod test {
         // test_same("var x; with (z) { x.y || (x.y = 3) }");
     }
 
+    #[test]
+    fn test_compress_normal_assignment_to_combined_logical_assignment() {
+        test("var x; x = x || 1", "var x; x ||= 1");
+        test("var x; x = x && 1", "var x; x &&= 1");
+        test("var x; x = x ?? 1", "var x; x ??= 1");
+
+        // `x` is a global reference and might have a setter
+        // Example case: `Object.defineProperty(globalThis, 'x', { get: () => true, set: () => console.log('x') }); x = x || 1`
+        test_same("x = x || 1");
+        // setting x.y might have a side effect
+        test_same("var x; x.y = x.y || 1");
+        // This case is not supported, since the minifier does not support with statements
+        // test_same("var x; with (z) { x = x || 1 }");
+
+        let allocator = Allocator::default();
+        let target = ESTarget::ES2019;
+        let mut pass = super::PeepholeSubstituteAlternateSyntax::new(target, false);
+        let code = "var x; x = x || 1";
+        tester::test(&allocator, code, code, &mut pass);
+    }
+
     #[test]
     fn test_fold_subtraction_assignment() {
         test("x -= 1", "--x");

tasks/minsize/minsize.snap

@@ -3,25 +3,25 @@ Original   | minified   | minified   | gzip       | gzip       | Fixture
 -------------------------------------------------------------------------------------
 72.14 kB   | 23.70 kB   | 23.70 kB   | 8.61 kB    | 8.54 kB    | react.development.js
 
-173.90 kB  | 59.80 kB   | 59.82 kB   | 19.41 kB   | 19.33 kB   | moment.js 
+173.90 kB  | 59.79 kB   | 59.82 kB   | 19.40 kB   | 19.33 kB   | moment.js 
 
-287.63 kB  | 90.10 kB   | 90.07 kB   | 32.04 kB   | 31.95 kB   | jquery.js 
+287.63 kB  | 90.08 kB   | 90.07 kB   | 32.02 kB   | 31.95 kB   | jquery.js 
 
-342.15 kB  | 118.11 kB  | 118.14 kB  | 44.45 kB   | 44.37 kB   | vue.js    
+342.15 kB  | 118.09 kB  | 118.14 kB  | 44.43 kB   | 44.37 kB   | vue.js    
 
 544.10 kB  | 71.74 kB   | 72.48 kB   | 26.14 kB   | 26.20 kB   | lodash.js 
 
 555.77 kB  | 273.19 kB  | 270.13 kB  | 90.91 kB   | 90.80 kB   | d3.js     
 
-1.01 MB    | 460.18 kB  | 458.89 kB  | 126.76 kB  | 126.71 kB  | bundle.min.js
+1.01 MB    | 460.17 kB  | 458.89 kB  | 126.75 kB  | 126.71 kB  | bundle.min.js
 
-1.25 MB    | 652.82 kB  | 646.76 kB  | 163.51 kB  | 163.73 kB  | three.js  
+1.25 MB    | 652.81 kB  | 646.76 kB  | 163.50 kB  | 163.73 kB  | three.js  
 
-2.14 MB    | 726.27 kB  | 724.14 kB  | 180.14 kB  | 181.07 kB  | victory.js
+2.14 MB    | 726.23 kB  | 724.14 kB  | 180.12 kB  | 181.07 kB  | victory.js
 
-3.20 MB    | 1.01 MB    | 1.01 MB    | 331.80 kB  | 331.56 kB  | echarts.js
+3.20 MB    | 1.01 MB    | 1.01 MB    | 331.78 kB  | 331.56 kB  | echarts.js
 
-6.69 MB    | 2.32 MB    | 2.31 MB    | 492.65 kB  | 488.28 kB  | antd.js   
+6.69 MB    | 2.32 MB    | 2.31 MB    | 492.64 kB  | 488.28 kB  | antd.js   
 
-10.95 MB   | 3.49 MB    | 3.49 MB    | 907.49 kB  | 915.50 kB  | typescript.js
+10.95 MB   | 3.49 MB    | 3.49 MB    | 907.46 kB  | 915.50 kB  | typescript.js