Fix empty var declarations

packages/babel-plugin-transform-parameters/src/params.js

@@ -59,26 +59,34 @@ export default function convertFunctionParams(path, loose) {
   const body = [];
   const params = path.get("params");
 
-  const bindings = scope.bindings;
-
-  function isRedeclaredVarOrFunction(name) {
-    const kind = bindings[name]?.kind;
-    return kind === "var" || kind === "hoisted";
-  }
-
   let firstOptionalIndex = null;
 
   for (let i = 0; i < params.length; i++) {
     const param = params[i];
 
-    // If a parameter's binding shows that its kind is var or hoisted,
-    // this name was legally redeclared in the function body,
-    // and we need to wrap the function body in an IIFE.
-    if (
-      !state.iife &&
-      Object.keys(param.getBindingIdentifiers()).some(isRedeclaredVarOrFunction)
-    ) {
-      state.iife = true;
+    for (const name of Object.keys(param.getBindingIdentifiers())) {
+      const constantViolations = scope.bindings[name]?.constantViolations;
+      if (constantViolations) {
+        for (const redeclarator of constantViolations) {
+          const node = redeclarator.node;
+          // If a constant violation is a var or a function declaration,
+          // we first check to see if it's a var without an init.
+          // If so, we remove that declarator.
+          // Otherwise, we have to wrap it in an IIFE.
+          switch (node.type) {
+            case "VariableDeclarator":
+              if (node.init === null) {
+                redeclarator.remove();
+              } else {
+                state.iife = true;
+              }
+              break;
+            case "FunctionDeclaration":
+              state.iife = true;
+              break;
+          }
+        }
+      }
     }
 
     const paramIsAssignmentPattern = param.isAssignmentPattern();

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-no-init-same-as-param/input.js

@@ -0,0 +1,6 @@
+function f(a = 2, b = 3) {
+  var a, b = 4;
+  var a;
+  var b;
+  return a + b;
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-no-init-same-as-param/output.js

@@ -0,0 +1,8 @@
+function f() {
+  var a = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 2;
+  var b = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 3;
+  return function () {
+    var b = 4;
+    return a + b;
+  }();
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-no-init-then-var-with-init-same-as-param/input.js

@@ -0,0 +1,5 @@
+function f(a = 2, b = 3) {
+  var a;
+  var { a } = { a: 4 };
+  return a + b;
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-no-init-then-var-with-init-same-as-param/output.js

@@ -0,0 +1,11 @@
+function f() {
+  var a = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 2;
+  var b = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 3;
+  return function () {
+    var _a = {
+      a: 4
+    },
+        a = _a.a;
+    return a + b;
+  }();
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-with-init-then-var-no-init-same-as-param/input.js

@@ -0,0 +1,6 @@
+function f(a = 2, b = 3) {
+  var { a } = { a: 4 };
+  var a;
+  var b;
+  return a + b;
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-with-init-then-var-no-init-same-as-param/output.js

@@ -0,0 +1,11 @@
+function f() {
+  var a = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 2;
+  var b = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 3;
+  return function () {
+    var _a = {
+      a: 4
+    },
+        a = _a.a;
+    return a + b;
+  }();
+}

packages/babel-traverse/src/scope/index.js

@@ -531,14 +531,8 @@ export default class Scope {
 
         parent.references[name] = true;
 
-        // A redeclaration of an existing variable is a modification.
-        // However, a var or function declaration
-        // with the same name as a function parameter
-        // is valid, and should take precedence.
-        if (
-          local &&
-          !(local.kind === "param" && (kind === "var" || kind === "hoisted"))
-        ) {
+        // A redeclaration of an existing variable is a modification
+        if (local) {
           this.registerConstantViolation(bindingPath);
         } else {
           this.bindings[name] = new Binding({

packages/babel-traverse/test/fixtures/rename/parameter-redeclaration/output.js

@@ -1,9 +1,9 @@
-function f(a, b) {
+function f(z, b) {
   var z = "redeclared";
   return b;
 }
 
-function g(a) {
+function g(z) {
   function z() {
     return "function, redeclared";
   }