feat(es/minifier): Drop redundant function parameters

crates/swc_ecma_transforms_optimization/src/simplify/dce/mod.rs

@@ -211,6 +211,7 @@ struct Analyzer<'a> {
     #[allow(dead_code)]
     config: &'a Config,
     in_var_decl: bool,
+    in_param: bool,
     scope: Scope<'a>,
     data: &'a mut Data,
     cur_class_id: Option<Id>,
@@ -518,10 +519,17 @@ impl Visit for Analyzer<'_> {
         }
     }
 
+    fn visit_param(&mut self, param: &Param) {
+        let old_is_in_param = self.in_param;
+        self.in_param = true;
+        param.visit_children_with(self);
+        self.in_param = old_is_in_param;
+    }
+
     fn visit_pat(&mut self, p: &Pat) {
         p.visit_children_with(self);
 
-        if !self.in_var_decl {
+        if !self.in_var_decl && !self.in_param {
             if let Pat::Ident(i) = p {
                 self.add(i.to_id(), true);
             }
@@ -844,6 +852,35 @@ impl VisitMut for TreeShaker {
         self.in_fn = old_in_fn;
     }
 
+    fn visit_mut_params(&mut self, params: &mut Vec<Param>) {
+        self.visit_mut_par(cpu_count() * 8, params);
+
+        // We iterate through trailing parameters until we either find a
+        // non-identifier parameter, or a parameter that can't be dropped.
+        let invalid_param_index = params.iter().rev().position(|param| {
+            let Some(id) = param.pat.as_ident() else {
+                // Non-identifier parameter
+                return true;
+            };
+
+            !self.can_drop_binding(id.to_id(), false)
+        });
+
+        match invalid_param_index {
+            None => {
+                // No invalid parameter found, we can remove all parameters.
+                params.clear();
+            }
+
+            Some(idx) if idx < params.len() - 1 => {
+                // Remove every parameter up to the invalid parameter.
+                params.truncate(params.len() - idx);
+            }
+
+            _ => {} // No change
+        }
+    }
+
     fn visit_mut_import_specifiers(&mut self, ss: &mut Vec<ImportSpecifier>) {
         ss.retain(|s| {
             let local = match s {
@@ -880,6 +917,7 @@ impl VisitMut for TreeShaker {
             let mut analyzer = Analyzer {
                 config: &self.config,
                 in_var_decl: false,
+                in_param: false,
                 scope: Default::default(),
                 data: &mut data,
                 cur_class_id: Default::default(),
@@ -943,6 +981,7 @@ impl VisitMut for TreeShaker {
             let mut analyzer = Analyzer {
                 config: &self.config,
                 in_var_decl: false,
+                in_param: false,
                 scope: Default::default(),
                 data: &mut data,
                 cur_class_id: Default::default(),

crates/swc_ecma_transforms_optimization/tests/__swc_snapshots__/tests/simplify_dce.rs/issue_763_4.js

@@ -13,7 +13,7 @@ const resources = [
         label: 'Instagram'
     }
 ];
-export function foo(websites) {
+export function foo() {
     const a = resources.map((resource)=>({
             value: resource.value
         }));

crates/swc_ecma_transforms_optimization/tests/simplify.rs

@@ -489,6 +489,24 @@ fn test_template_strings_known_methods() {
     test("x = parseFloat(`1.23`)", "x = 1.23");
 }
 
+#[test]
+fn test_redundant_params() {
+    // One or more parameters can be dropped
+    test("x = function(a, b) {}", "x = function() {}");
+    test(
+        "x = function(a, b, c, d) {f(b);}",
+        "x = function(a, b) {f(b);}",
+    );
+    test(
+        "x = function(a, b, c, d) {f(c);}",
+        "x = function(a, b, c) {f(c);}",
+    );
+
+    // No parameters can be dropped
+    test_same("x = function(a, b) {f(b);}");
+    test_same("x = function(a) {f(a);}");
+}
+
 test!(
     Syntax::Typescript(TsSyntax {
         decorators: true,

crates/swc_node_bundler/tests/pass/cjs/conditional/output/entry.js

@@ -31,7 +31,7 @@ function __swcpack_require__(mod) {
     cache = interop(module.exports);
     return cache;
 }
-var load = __swcpack_require__.bind(void 0, function(module, exports) {
+var load = __swcpack_require__.bind(void 0, function() {
     console.log("foo");
     console.log("bar");
 });

crates/swc_node_bundler/tests/pass/cjs/issue-967-no-recursive-require/output/entry.js

@@ -31,13 +31,13 @@ function __swcpack_require__(mod) {
     cache = interop(module.exports);
     return cache;
 }
-var load = __swcpack_require__.bind(void 0, function(module, exports) {
+var load = __swcpack_require__.bind(void 0, function() {
     console.log("a");
 });
-var load1 = __swcpack_require__.bind(void 0, function(module, exports) {
+var load1 = __swcpack_require__.bind(void 0, function() {
     console.log("b");
 });
-var load2 = __swcpack_require__.bind(void 0, function(module, exports) {
+var load2 = __swcpack_require__.bind(void 0, function() {
     console.log("c");
 });
 load();

crates/swc_node_bundler/tests/pass/cjs/issue-967-recursive-require/output/entry.js

@@ -56,7 +56,7 @@ var load4 = __swcpack_require__.bind(void 0, function(module, exports) {
         bb: bb
     };
 });
-var load5 = __swcpack_require__.bind(void 0, function(module, exports) {
+var load5 = __swcpack_require__.bind(void 0, function() {
     console.log("c");
 });
 load4();

crates/swc_node_bundler/tests/pass/deno-001/full/output/entry.js

@@ -421,7 +421,7 @@ class MuxAsyncIterator {
 }
 function emptyReader() {
     return {
-        read (_) {
+        read () {
             return Promise.resolve(null);
         }
     };

crates/swc_node_bundler/tests/pass/deno-001/simple-1/output/entry.js

@@ -52,7 +52,7 @@ class MuxAsyncIterator {
 }
 function emptyReader() {
     return {
-        read (_) {
+        read () {
             return Promise.resolve(null);
         }
     };

crates/swc_node_bundler/tests/pass/deno-001/simple-2/output/entry.js

@@ -23,10 +23,10 @@ class ServerRequest {
     }
 }
 console.log(ServerRequest);
-async function writeResponse(w, r) {}
-async function readRequest(conn, bufr) {}
+async function writeResponse() {}
+async function readRequest() {}
 console.log(deferred, writeResponse, readRequest, MuxAsyncIterator);
-async function listenAndServe(addr, handler) {}
+async function listenAndServe() {}
 listenAndServe({
     port: 8080
 }, async (req)=>{});

crates/swc_node_bundler/tests/pass/issue-1328/case1/output/entry.js

@@ -127,7 +127,7 @@ function foo() {
 }
 function _foo() {
     _foo = _async_to_generator(function() {
-        return _ts_generator(this, function(_state) {
+        return _ts_generator(this, function() {
             return [
                 2
             ];

crates/swc_node_bundler/tests/pass/issue-2124/named-export/output/entry.js

@@ -32,7 +32,7 @@ function __swcpack_require__(mod) {
     return cache;
 }
 var load = __swcpack_require__.bind(void 0, function(module, exports) {
-    function lodash(value) {
+    function lodash() {
         console.log("lodash");
     }
     function memoize() {

crates/swc_node_bundler/tests/pass/regenerator/1/output/entry.js

@@ -127,7 +127,7 @@ function foo() {
 }
 function _foo() {
     _foo = _async_to_generator(function() {
-        return _ts_generator(this, function(_state) {
+        return _ts_generator(this, function() {
             return [
                 2
             ];