fix: babel-plugin-minify-mangle-names default parameter error (#41)

packages/webcrack/src/transforms/mangle.ts

@@ -1,8 +1,37 @@
-import traverse, { NodePath } from '@babel/traverse';
+import { statement } from '@babel/template';
+import traverse, { NodePath, Visitor, visitors } from '@babel/traverse';
 import * as t from '@babel/types';
 import mangle from 'babel-plugin-minify-mangle-names';
 import { Transform } from '../ast-utils';
 
+// See https://github.com/j4k0xb/webcrack/issues/41 and https://github.com/babel/minify/issues/1023
+const fixDefaultParamError: Visitor = {
+  Function(path) {
+    const { params } = path.node;
+    for (let i = params.length - 1; i >= 0; i--) {
+      const param = params[i];
+      if (
+        !t.isAssignmentPattern(param) ||
+        !t.isIdentifier(param.left) ||
+        t.isLiteral(param.right)
+      )
+        continue;
+
+      if (
+        path.isArrowFunctionExpression() &&
+        !t.isBlockStatement(path.node.body)
+      ) {
+        path.node.body = t.blockStatement([t.returnStatement(path.node.body)]);
+      }
+      (path.get('body') as NodePath<t.BlockStatement>).unshiftContainer(
+        'body',
+        statement`if (${param.left} !== undefined) ${param.left} = ${param.right}`(),
+      );
+      param.right = t.identifier('undefined');
+    }
+  },
+};
+
 export default {
   name: 'mangle',
   tags: ['safe'],
@@ -12,8 +41,12 @@ export default {
     // eslint-disable-next-line @typescript-eslint/unbound-method
     const { getSource } = NodePath.prototype;
     NodePath.prototype.getSource = () => '';
+    const visitor = visitors.merge([
+      fixDefaultParamError,
+      mangle({ types: t, traverse }).visitor,
+    ]);
 
-    traverse(ast, mangle({ types: t, traverse }).visitor, undefined, {
+    traverse(ast, visitor, undefined, {
       opts: {
         eval: true,
         topLevel: true,

packages/webcrack/test/mangle.test.ts

@@ -0,0 +1,28 @@
+import { test } from 'vitest';
+import { testTransform } from '.';
+import mangle from '../src/transforms/mangle';
+
+const expectJS = testTransform(mangle);
+
+// https://github.com/j4k0xb/webcrack/issues/41
+test('rename and extract non-literal default parameters', () => {
+  expectJS(`
+    function func(arg1, arg2 = 0, arg3 = arg1, arg4 = 30) {
+      return arg1;
+    }
+  `).toMatchInlineSnapshot(`
+    function a(a, b = 0, c = undefined, d = 30) {
+      if (c !== undefined) c = a;
+      return a;
+    }
+  `);
+
+  expectJS(`
+    const func = (arg1, arg2 = 0, arg3 = arg1, arg4 = 30) => arg1;
+  `).toMatchInlineSnapshot(`
+    const a = (a, b = 0, c = undefined, d = 30) => {
+      if (c !== undefined) c = a;
+      return a;
+    };
+  `);
+});