Fix length with default arguments

rhino/src/main/java/org/mozilla/javascript/CodeGenUtils.java

@@ -108,6 +108,9 @@ private static void fillInCommon(JSDescriptor.Builder builder, ScriptNode script
         builder.hasRestArg = scriptOrFn.hasRestParameter();
         builder.hasDefaultParameters = scriptOrFn.getDefaultParams() != null;
 
+        // Calculate arity (function.length) - count params before first default
+        builder.arity = FunctionNode.calculateFunctionArity(scriptOrFn);
+
         builder.rawSourceStart = scriptOrFn.getRawSourceStart();
         builder.rawSourceEnd = scriptOrFn.getRawSourceEnd();
     }

rhino/src/main/java/org/mozilla/javascript/JSDescriptor.java

@@ -45,6 +45,7 @@ public final class JSDescriptor<T extends ScriptOrFn<T>> implements Serializable
     private final int languageVersion;
     private final int paramAndVarCount;
     private final int paramCount;
+    private final int arity; // number of params before first default for reporting as length
     private final SecurityController securityController;
     private final Object securityDomain;
     private final int functionType;
@@ -72,6 +73,7 @@ public JSDescriptor(
             int languageVersion,
             int paramAndVarCount,
             int paramCount,
+            int arity,
             boolean hasDefaultParameters,
             boolean requiresActivationFrame,
             boolean requiresArgumentObject,
@@ -109,6 +111,7 @@ public JSDescriptor(
         this.languageVersion = languageVersion;
         this.paramAndVarCount = paramAndVarCount;
         this.paramCount = paramCount;
+        this.arity = arity;
         this.securityController = securityController;
         this.securityDomain = securityDomain;
         this.functionType = functionType;
@@ -195,6 +198,10 @@ public int getParamCount() {
         return paramCount;
     }
 
+    public int getArity() {
+        return arity;
+    }
+
     public boolean getParamOrVarConst(int index) {
         return paramIsConst[index];
     }
@@ -302,6 +309,7 @@ public static class Builder<T extends ScriptOrFn<T>> {
         public int languageVersion;
         public int paramAndVarCount;
         public int paramCount;
+        public int arity; // number of params before first default for reporting as length
         public boolean hasDefaultParameters;
         public boolean requiresActivationFrame;
         public boolean requiresArgumentObject;
@@ -371,6 +379,7 @@ private JSDescriptor<T> build(JSDescriptor<?> parent, Consumer<JSDescriptor<?>>
                             languageVersion,
                             paramAndVarCount,
                             paramCount,
+                            arity,
                             hasDefaultParameters,
                             requiresActivationFrame,
                             requiresArgumentObject,

rhino/src/main/java/org/mozilla/javascript/JSFunction.java

@@ -48,7 +48,7 @@ public boolean isStrict() {
 
     @Override
     public int getArity() {
-        return getParamCount();
+        return descriptor.getArity();
     }
 
     public DebuggableScript getDebuggableView() {
@@ -61,14 +61,14 @@ protected int getLanguageVersion() {
 
     @Override
     public int getLength() {
-        int paramCount = getParamCount();
+        int arity = descriptor.getArity();
         if (getLanguageVersion() != Context.VERSION_1_2) {
-            return paramCount;
+            return arity;
         }
         Context cx = Context.getContext();
         NativeCall activation = ScriptRuntime.findFunctionActivation(cx, this);
         if (activation == null) {
-            return paramCount;
+            return arity;
         }
         return activation.originalArgs.length;
     }

rhino/src/main/java/org/mozilla/javascript/ast/FunctionNode.java

@@ -503,4 +503,56 @@ public void visit(NodeVisitor v) {
             }
         }
     }
+
+    /**
+     * Calculate the arity (function.length) for a function. According to ECMAScript spec, the
+     * length property should only count parameters up to (but not including) the first one with a
+     * default value.
+     *
+     * <p>Ref: ECMA 2026, 15.1.5 Static Semantics: ExpectedArgumentCount
+     *
+     * @param scriptOrFn the function or script node
+     * @return the arity (number of parameters before first default parameter)
+     */
+    public static int calculateFunctionArity(ScriptNode scriptOrFn) {
+        int paramCount = scriptOrFn.getParamCount();
+        int arity = paramCount;
+
+        if (scriptOrFn instanceof FunctionNode) {
+            FunctionNode fnNode = (FunctionNode) scriptOrFn;
+            java.util.List<Object> defaultParams = fnNode.getDefaultParams();
+
+            if (defaultParams != null && !defaultParams.isEmpty()) {
+                // defaultParams stores pairs: [paramName (String), defaultValue (AstNode), ...]
+                // count up to the first parameter that has a default value
+                java.util.List<AstNode> params = fnNode.getParams();
+                if (params != null && !params.isEmpty()) {
+                    for (int i = 0; i < defaultParams.size(); i += 2) {
+                        if (defaultParams.get(i) instanceof String) {
+                            String paramWithDefault = (String) defaultParams.get(i);
+
+                            for (int paramIndex = 0; paramIndex < params.size(); paramIndex++) {
+                                AstNode param = params.get(paramIndex);
+                                if (param instanceof Name) {
+                                    String paramName = ((Name) param).getIdentifier();
+                                    if (paramName.equals(paramWithDefault)) {
+                                        arity = paramIndex;
+                                        break;
+                                    }
+                                }
+                            }
+                            if (arity != paramCount) break;
+                        }
+                    }
+                }
+            }
+        }
+
+        // Rest parameters don't count toward length
+        if (scriptOrFn.hasRestParameter()) {
+            arity = Math.max(0, arity - 1);
+        }
+
+        return arity;
+    }
 }

rhino/src/main/java/org/mozilla/javascript/optimizer/ClassCompiler.java

@@ -318,6 +318,7 @@ private void buildDescriptor(
         cfw.addLoadConstant(builder.languageVersion);
         cfw.addLoadConstant(builder.paramAndVarCount);
         cfw.addLoadConstant(builder.paramCount);
+        cfw.addLoadConstant(builder.arity);
         cfw.add(builder.hasDefaultParameters ? ByteCode.ICONST_1 : ByteCode.ICONST_0);
         cfw.add(builder.requiresActivationFrame ? ByteCode.ICONST_1 : ByteCode.ICONST_0);
         cfw.add(builder.requiresArgumentObject ? ByteCode.ICONST_1 : ByteCode.ICONST_0);

tests/src/test/java/org/mozilla/javascript/tests/DefaultParametersTest.java

@@ -463,4 +463,28 @@ public void parserThrowsOnLowerLanguageLevelArrowBasic() throws Exception {
         Utils.assertEvaluatorException_1_8(
                 "Default values are only supported in version >= 200", script + "()");
     }
+
+    // Test that length property reflects the number of parameters without defaults
+    @Test
+    public void arrowFunctionLengthSimple() throws Exception {
+        Utils.assertWithAllModes_ES6(1, "var f = (a, b = 39) => a; f.length");
+    }
+
+    @Test
+    public void arrowFunctionLengthWithDefaultParameters() throws Exception {
+        final String script =
+                "var callCount = 0;\n"
+                        + "var ref;\n"
+                        + "ref = (a, b = 39,) => {\n"
+                        + "    if (a !== 42) throw 'a should be 42';\n"
+                        + "    if (b !== 39) throw 'b should be 39';\n"
+                        + "    callCount = callCount + 1;\n"
+                        + "};\n"
+                        + "\n"
+                        + "ref(42, undefined, 1);\n"
+                        + "if (callCount !== 1) throw 'arrow function not invoked exactly once';\n"
+                        + "ref.length;";
+
+        Utils.assertWithAllModes_ES6(1, script);
+    }
 }

tests/testsrc/test262.properties

@@ -3572,7 +3572,7 @@ language/expressions/array 22/52 (42.31%)
     spread-sngl-iter.js
     spread-sngl-literal.js
 
-language/expressions/arrow-function 150/343 (43.73%)
+language/expressions/arrow-function 148/343 (43.15%)
     dstr/ary-init-iter-close.js
     dstr/ary-init-iter-get-err.js
     dstr/ary-init-iter-get-err-array-prototype.js
@@ -3704,9 +3704,7 @@ language/expressions/arrow-function 150/343 (43.73%)
     dflt-params-duplicates.js non-strict
     dflt-params-ref-later.js
     dflt-params-ref-self.js
-    dflt-params-trailing-comma.js
     eval-var-scope-syntax-err.js non-strict
-    length-dflt.js
     lexical-new.target.js {unsupported: [new.target]}
     lexical-new.target-closure-returned.js {unsupported: [new.target]}
     lexical-super-call-from-within-constructor.js
@@ -4187,7 +4185,7 @@ language/expressions/equals 0/47 (0.0%)
 
 language/expressions/exponentiation 0/44 (0.0%)
 
-language/expressions/function 149/264 (56.44%)
+language/expressions/function 147/264 (55.68%)
     dstr/ary-init-iter-close.js
     dstr/ary-init-iter-get-err.js
     dstr/ary-init-iter-get-err-array-prototype.js
@@ -4305,9 +4303,7 @@ language/expressions/function 149/264 (56.44%)
     dflt-params-ref-later.js
     dflt-params-ref-self.js
     dflt-params-rest.js
-    dflt-params-trailing-comma.js
     eval-var-scope-syntax-err.js non-strict
-    length-dflt.js
     name-arguments-strict-body.js non-strict
     named-no-strict-reassign-fn-name-in-body.js non-strict
     named-no-strict-reassign-fn-name-in-body-in-arrow.js non-strict
@@ -4335,7 +4331,7 @@ language/expressions/function 149/264 (56.44%)
     unscopables-with.js non-strict
     unscopables-with-in-nested-fn.js non-strict
 
-language/expressions/generators 179/290 (61.72%)
+language/expressions/generators 177/290 (61.03%)
     dstr/ary-init-iter-close.js
     dstr/ary-init-iter-get-err.js
     dstr/ary-init-iter-get-err-array-prototype.js
@@ -4476,11 +4472,9 @@ language/expressions/generators 179/290 (61.72%)
     dflt-params-ref-later.js
     dflt-params-ref-self.js
     dflt-params-rest.js
-    dflt-params-trailing-comma.js
     eval-body-proto-realm.js
     eval-var-scope-syntax-err.js non-strict
     has-instance.js
-    length-dflt.js
     named-no-strict-reassign-fn-name-in-body.js non-strict
     named-no-strict-reassign-fn-name-in-body-in-arrow.js non-strict
     named-no-strict-reassign-fn-name-in-body-in-eval.js non-strict
@@ -4641,7 +4635,7 @@ language/expressions/new 22/59 (37.29%)
 
 ~language/expressions/new.target
 
-language/expressions/object 688/1170 (58.8%)
+language/expressions/object 683/1170 (58.38%)
     dstr/async-gen-meth-ary-init-iter-close.js {unsupported: [async-iteration, async]}
     dstr/async-gen-meth-ary-init-iter-get-err.js {unsupported: [async-iteration]}
     dstr/async-gen-meth-ary-init-iter-get-err-array-prototype.js {unsupported: [async-iteration]}
@@ -5233,7 +5227,6 @@ language/expressions/object 688/1170 (58.8%)
     method-definition/gen-meth-dflt-params-ref-later.js
     method-definition/gen-meth-dflt-params-ref-self.js
     method-definition/gen-meth-dflt-params-rest.js
-    method-definition/gen-meth-dflt-params-trailing-comma.js
     method-definition/gen-meth-eval-var-scope-syntax-err.js non-strict
     method-definition/gen-meth-object-destructuring-param-strict-body.js
     method-definition/gen-meth-rest-param-strict-body.js
@@ -5243,7 +5236,6 @@ language/expressions/object 688/1170 (58.8%)
     method-definition/gen-yield-spread-arr-single.js
     method-definition/gen-yield-spread-obj.js
     method-definition/generator-invoke-fn-strict.js non-strict
-    method-definition/generator-length-dflt.js
     method-definition/generator-param-init-yield.js non-strict
     method-definition/generator-param-redecl-let.js
     method-definition/generator-prop-name-yield-expr.js non-strict
@@ -5254,12 +5246,10 @@ language/expressions/object 688/1170 (58.8%)
     method-definition/meth-dflt-params-ref-later.js
     method-definition/meth-dflt-params-ref-self.js
     method-definition/meth-dflt-params-rest.js
-    method-definition/meth-dflt-params-trailing-comma.js
     method-definition/meth-eval-var-scope-syntax-err.js non-strict
     method-definition/meth-object-destructuring-param-strict-body.js
     method-definition/meth-rest-param-strict-body.js
     method-definition/name-invoke-fn-strict.js non-strict
-    method-definition/name-length-dflt.js
     method-definition/name-param-id-yield.js non-strict
     method-definition/name-param-init-yield.js non-strict
     method-definition/name-param-redecl.js
@@ -5325,7 +5315,6 @@ language/expressions/object 688/1170 (58.8%)
     scope-setter-body-lex-distinc.js non-strict
     scope-setter-paramsbody-var-open.js
     setter-body-strict-inside.js non-strict
-    setter-length-dflt.js
     setter-param-arguments-strict-inside.js non-strict
     setter-param-eval-strict-inside.js non-strict
     yield-non-strict-access.js non-strict
@@ -6680,7 +6669,7 @@ language/statements/for-of 438/751 (58.32%)
     typedarray-backed-by-resizable-buffer-shrink-mid-iteration.js {unsupported: [resizable-arraybuffer]}
     typedarray-backed-by-resizable-buffer-shrink-to-zero-mid-iteration.js {unsupported: [resizable-arraybuffer]}
 
-language/statements/function 161/451 (35.7%)
+language/statements/function 159/451 (35.25%)
     dstr/ary-init-iter-close.js
     dstr/ary-init-iter-get-err.js
     dstr/ary-init-iter-get-err-array-prototype.js
@@ -6817,9 +6806,7 @@ language/statements/function 161/451 (35.7%)
     dflt-params-ref-later.js
     dflt-params-ref-self.js
     dflt-params-rest.js
-    dflt-params-trailing-comma.js
     eval-var-scope-syntax-err.js non-strict
-    length-dflt.js
     name-arguments-strict-body.js non-strict
     name-eval-strict-body.js non-strict
     object-destructuring-param-strict-body.js
@@ -6840,7 +6827,7 @@ language/statements/function 161/451 (35.7%)
     unscopables-with.js non-strict
     unscopables-with-in-nested-fn.js non-strict
 
-language/statements/generators 165/266 (62.03%)
+language/statements/generators 163/266 (61.28%)
     dstr/ary-init-iter-close.js
     dstr/ary-init-iter-get-err.js
     dstr/ary-init-iter-get-err-array-prototype.js
@@ -6981,10 +6968,8 @@ language/statements/generators 165/266 (62.03%)
     dflt-params-ref-later.js
     dflt-params-ref-self.js
     dflt-params-rest.js
-    dflt-params-trailing-comma.js
     eval-var-scope-syntax-err.js non-strict
     has-instance.js
-    length-dflt.js
     object-destructuring-param-strict-body.js
     prototype-own-properties.js
     prototype-value.js