feat(minifier): remove unnecessary "use strict"

crates/oxc_minifier/src/ast_passes/peephole_fold_constants.rs

@@ -527,7 +527,14 @@ mod test {
     fn test_nospace(source_text: &str, expected: &str) {
         let allocator = Allocator::default();
         let mut pass = super::PeepholeFoldConstants::new();
-        tester::test_impl(&allocator, source_text, expected, &mut pass, true);
+        tester::test_impl(
+            &allocator,
+            source_text,
+            expected,
+            &mut pass,
+            oxc_span::SourceType::mjs(),
+            true,
+        );
     }
 
     fn test_same(source_text: &str) {

crates/oxc_minifier/src/ast_passes/remove_syntax.rs

@@ -21,6 +21,18 @@ impl<'a> CompressorPass<'a> for RemoveSyntax {
 }
 
 impl<'a> Traverse<'a> for RemoveSyntax {
+    fn enter_program(&mut self, program: &mut Program<'a>, ctx: &mut TraverseCtx<'a>) {
+        Self::drop_use_strict_directives_in_program(program, ctx);
+    }
+
+    fn enter_function_body(&mut self, body: &mut FunctionBody<'a>, ctx: &mut TraverseCtx<'a>) {
+        Self::drop_use_strict_directives_in_function_body(body, ctx);
+    }
+
+    fn exit_function_body(&mut self, body: &mut FunctionBody<'a>, ctx: &mut TraverseCtx<'a>) {
+        Self::drop_use_strict_directives_if_function_is_empty(body, ctx);
+    }
+
     fn exit_statements(&mut self, stmts: &mut Vec<'a, Statement<'a>>, _ctx: &mut TraverseCtx<'a>) {
         stmts.retain(|stmt| {
             !(matches!(stmt, Statement::EmptyStatement(_))
@@ -89,6 +101,38 @@ impl<'a> RemoveSyntax {
         let Some(ident) = obj.get_identifier_reference() else { return false };
         ident.name == "console"
     }
+
+    /// Drop `"use strict";` directives if the input is strict mode (e.g. written in ESM).
+    fn drop_use_strict_directives_in_program(
+        program: &mut Program<'a>,
+        _ctx: &mut TraverseCtx<'a>,
+    ) {
+        if program.source_type.is_strict() {
+            program.directives.retain(|directive| !directive.is_use_strict());
+        }
+    }
+
+    /// Drop `"use strict";` directives if the parent scope is already strict mode.
+    fn drop_use_strict_directives_in_function_body(
+        body: &mut FunctionBody<'a>,
+        ctx: &mut TraverseCtx<'a>,
+    ) {
+        let current_scope_id = ctx.current_scope_id();
+        let Some(parent_scope_id) = ctx.scopes().get_parent_id(current_scope_id) else { return };
+        if ctx.scopes().get_flags(parent_scope_id).is_strict_mode() {
+            body.directives.retain(|directive| !directive.is_use_strict());
+        }
+    }
+
+    /// Drop `"use strict";` directives if the function is empty.
+    fn drop_use_strict_directives_if_function_is_empty(
+        body: &mut FunctionBody<'a>,
+        _ctx: &mut TraverseCtx<'a>,
+    ) {
+        if body.statements.is_empty() {
+            body.directives.retain(|directive| !directive.is_use_strict());
+        }
+    }
 }
 
 #[cfg(test)]
@@ -103,6 +147,23 @@ mod test {
         tester::test(&allocator, source_text, expected, &mut pass);
     }
 
+    fn test_script(source_text: &str, expected: &str) {
+        let allocator = Allocator::default();
+        let mut pass = super::RemoveSyntax::new(CompressOptions::all_true());
+        tester::test_impl(
+            &allocator,
+            source_text,
+            expected,
+            &mut pass,
+            oxc_span::SourceType::cjs(),
+            true,
+        );
+    }
+
+    fn test_script_same(source_text: &str) {
+        test_script(source_text, source_text);
+    }
+
     #[test]
     fn parens() {
         test("(((x)))", "x");
@@ -118,4 +179,33 @@ mod test {
     fn drop_debugger() {
         test("debugger", "");
     }
+
+    #[test]
+    fn use_strict() {
+        test("'use strict';", "");
+
+        test_script(
+            "'use strict'; function foo() { 'use strict'; alert(1); }",
+            "'use strict'; function foo() { alert(1); }",
+        );
+        test_script(
+            "'use strict'; const foo = () => { 'use strict'; alert(1); }",
+            "'use strict'; const foo = () => { alert(1); }",
+        );
+        test_script_same("function foo() { 'use strict'; alert(1); }");
+        test_script(
+            "function foo() { 'use strict'; return function foo() { 'use strict'; alert(1); }; } ",
+            "function foo() { 'use strict'; return function foo() { alert(1); }; } ",
+        );
+        test_script(
+            "class Foo { foo() { 'use strict'; alert(1); } } ",
+            "class Foo { foo() { alert(1); } } ",
+        );
+        test_script(
+            "const Foo = class { foo() { 'use strict'; alert(1); } } ",
+            "const Foo = class { foo() { alert(1); } } ",
+        );
+
+        test_script("function foo() { 'use strict';}", "function foo() {}");
+    }
 }

crates/oxc_minifier/src/tester.rs

@@ -16,28 +16,29 @@ pub fn test<'a, P: CompressorPass<'a>>(
     expected: &'a str,
     pass: &mut P,
 ) {
-    test_impl(allocator, source_text, expected, pass, false);
+    test_impl(allocator, source_text, expected, pass, SourceType::mjs(), false);
 }
 
 pub fn test_impl<'a, P: CompressorPass<'a>>(
     allocator: &'a Allocator,
     source_text: &'a str,
     expected: &'a str,
     pass: &mut P,
+    source_type: SourceType,
     remove_whitespace: bool,
 ) {
-    let result = run(allocator, source_text, Some(pass), remove_whitespace);
-    let expected = run::<P>(allocator, expected, None, remove_whitespace);
+    let result = run(allocator, source_text, Some(pass), source_type, remove_whitespace);
+    let expected = run::<P>(allocator, expected, None, source_type, remove_whitespace);
     assert_eq!(result, expected, "\nfor source\n{source_text}\nexpect\n{expected}\ngot\n{result}");
 }
 
 fn run<'a, P: CompressorPass<'a>>(
     allocator: &'a Allocator,
     source_text: &'a str,
     pass: Option<&mut P>,
+    source_type: SourceType,
     remove_whitespace: bool,
 ) -> String {
-    let source_type = SourceType::mjs();
     let mut program = Parser::new(allocator, source_text, source_type).parse().program;
 
     if let Some(pass) = pass {

tasks/minsize/minsize.snap

@@ -1,7 +1,7 @@
            | Oxc        | ESBuild    | Oxc        | ESBuild    |
 Original   | minified   | minified   | gzip       | gzip       | Fixture
 -------------------------------------------------------------------------------------
-72.14 kB   | 23.74 kB   | 23.70 kB   | 8.61 kB    | 8.54 kB    | react.development.js
+72.14 kB   | 23.72 kB   | 23.70 kB   | 8.61 kB    | 8.54 kB    | react.development.js
 
 173.90 kB  | 60.22 kB   | 59.82 kB   | 19.49 kB   | 19.33 kB   | moment.js 
 
@@ -17,11 +17,11 @@ Original   | minified   | minified   | gzip       | gzip       | Fixture
 
 1.25 MB    | 656.81 kB  | 646.76 kB  | 164.16 kB  | 163.73 kB  | three.js  
 
-2.14 MB    | 735.33 kB  | 724.14 kB  | 180.99 kB  | 181.07 kB  | victory.js
+2.14 MB    | 735.31 kB  | 724.14 kB  | 180.98 kB  | 181.07 kB  | victory.js
 
 3.20 MB    | 1.01 MB    | 1.01 MB    | 332.27 kB  | 331.56 kB  | echarts.js
 
-6.69 MB    | 2.36 MB    | 2.31 MB    | 495.04 kB  | 488.28 kB  | antd.js   
+6.69 MB    | 2.36 MB    | 2.31 MB    | 494.94 kB  | 488.28 kB  | antd.js   
 
 10.95 MB   | 3.51 MB    | 3.49 MB    | 910.93 kB  | 915.50 kB  | typescript.js
 

tasks/minsize/src/lib.rs

@@ -124,7 +124,7 @@ pub fn run() -> Result<(), io::Error> {
 }
 
 fn minify_twice(file: &TestFile) -> String {
-    let source_type = SourceType::from_path(&file.file_name).unwrap();
+    let source_type = SourceType::from_path(&file.file_name).unwrap().with_script(true);
     let options = MinifierOptions {
         mangle: Some(MangleOptions::default()),
         compress: CompressOptions::default(),