fix: close #22

src/standard/es5.ts

@@ -928,7 +928,7 @@ export const es5: ES5Map = {
       for (let i = 0; i < node.params.length; i++) {
         const param = node.params[i];
         if (isIdentifier(param)) {
-          funcScope.const(param.name, args[i]);
+          funcScope.let(param.name, args[i]);
         } else if (isAssignmentPattern(param)) {
           // @es2015 default parameters
           path.evaluate(path.createChild(param, funcScope, { value: args[i] }));

test/ecma5/function/FunctionExpression.test.ts

@@ -103,3 +103,21 @@ obj["a"]();
     );
   }, ErrIsNotFunction("obj.a").message);
 });
+
+test("function params should can be overwrite", t => {
+  const sandbox: any = vm.createContext({});
+
+  const test = vm.runInContext(
+    `
+function test (a) {
+  a = a || 'hello'
+  return a
+}
+
+module.exports = test
+    `,
+    sandbox
+  );
+
+  t.deepEqual(test(), "hello");
+});