Fix #91

Author: MichaelXF

src/templates/functionLength.ts

@@ -0,0 +1,14 @@
+import Template from "./template";
+
+/**
+ * Helper function to set `function.length` property.
+ */
+export const FunctionLengthTemplate = Template(`
+function {name}(functionObject, functionLength){
+  Object["defineProperty"](functionObject, "length", {
+    "value": functionLength,
+    "configurable": true
+  });
+  return functionObject;
+}
+`);

src/transforms/minify.ts

@@ -22,6 +22,7 @@ import {
   isForInitialize,
   append,
   isVarContext,
+  computeFunctionLength,
 } from "../util/insert";
 import { isValidIdentifier, isEquivalent } from "../util/compare";
 import { walk, isBlock } from "../traverse";
@@ -258,8 +259,15 @@ export default class Minify extends Transform {
             append(
               parents[parents.length - 1] || object,
               Template(`
-            function ${this.arrowFunctionName}(arrowFn){
-              return function(){ return arrowFn(...arguments) }
+            function ${this.arrowFunctionName}(arrowFn, functionLength){
+              var functionObject = function(){ return arrowFn(...arguments) };
+
+              Object["defineProperty"](functionObject, "length", {
+                "value": functionLength,
+                "configurable": true
+              });
+
+              return functionObject;
             }
             `).single()
             );
@@ -268,6 +276,7 @@ export default class Minify extends Transform {
           const wrap = (object: Node) => {
             return CallExpression(Identifier(this.arrowFunctionName), [
               clone(object),
+              Literal(computeFunctionLength(object.params)),
             ]);
           };
 

test/transforms/minify.test.ts

@@ -517,3 +517,22 @@ test("Variant #26: Don't break nested redefined function declaration", async ()
 
   expect(TEST_OUTPUT).toStrictEqual(1);
 });
+
+// https://github.com/MichaelXF/js-confuser/issues/91
+test("Variant #27: Preserve function.length property", async () => {
+  var output = await JsConfuser(
+    `
+    function oneParameter(a){};
+    var twoParameters = function({a},{b,c},...d){};
+    function threeParameters(a,b,c,d = 1,{e},...f){};
+
+    TEST_OUTPUT = oneParameter.length + twoParameters.length + threeParameters.length;
+  `,
+    { target: "node", minify: true }
+  );
+
+  var TEST_OUTPUT;
+  eval(output);
+
+  expect(TEST_OUTPUT).toStrictEqual(6);
+});