[Fix] preserve original function’s length when possible

.eslintrc

@@ -5,11 +5,13 @@
 
 	"rules": {
 		"func-name-matching": 0,
+		"id-length": 0,
 		"new-cap": [2, {
 			"capIsNewExceptions": [
 				"GetIntrinsic",
 			],
 		}],
 		"no-magic-numbers": 0,
+		"operator-linebreak": [2, "before"],
 	},
 }

index.js

@@ -7,7 +7,9 @@ var $apply = GetIntrinsic('%Function.prototype.apply%');
 var $call = GetIntrinsic('%Function.prototype.call%');
 var $reflectApply = GetIntrinsic('%Reflect.apply%', true) || bind.call($call, $apply);
 
+var $gOPD = GetIntrinsic('%Object.getOwnPropertyDescriptor%', true);
 var $defineProperty = GetIntrinsic('%Object.defineProperty%', true);
+var $max = GetIntrinsic('%Math.max%');
 
 if ($defineProperty) {
 	try {
@@ -18,8 +20,19 @@ if ($defineProperty) {
 	}
 }
 
-module.exports = function callBind() {
-	return $reflectApply(bind, $call, arguments);
+module.exports = function callBind(originalFunction) {
+	var func = $reflectApply(bind, $call, arguments);
+	if ($gOPD && $defineProperty) {
+		var desc = $gOPD(func, 'length');
+		if (desc.configurable) {
+			$defineProperty(
+				func,
+				'length',
+				{ value: $max(0, originalFunction.length - (arguments.length - 1)) }
+			);
+		}
+	}
+	return func;
 };
 
 var applyBind = function applyBind() {

test/index.js

@@ -4,27 +4,36 @@ var callBind = require('../');
 
 var test = require('tape');
 
+var functionsHaveConfigurableLengths = !!(
+	Object.getOwnPropertyDescriptor
+	&& Object.getOwnPropertyDescriptor(function () {}, 'length').configurable
+);
+
 test('callBind', function (t) {
 	var sentinel = { sentinel: true };
 	var func = function (a, b) {
 		// eslint-disable-next-line no-invalid-this
 		return [this, a, b];
 	};
+	t.equal(func.length, 2, 'original function length is 2');
 	t.deepEqual(func(), [undefined, undefined, undefined], 'unbound func with too few args');
 	t.deepEqual(func(1, 2), [undefined, 1, 2], 'unbound func with right args');
 	t.deepEqual(func(1, 2, 3), [undefined, 1, 2], 'unbound func with too many args');
 
 	var bound = callBind(func);
+	t.equal(bound.length, func.length, 'function length is preserved', { skip: !functionsHaveConfigurableLengths });
 	t.deepEqual(bound(), [undefined, undefined, undefined], 'bound func with too few args');
 	t.deepEqual(bound(1, 2), [1, 2, undefined], 'bound func with right args');
 	t.deepEqual(bound(1, 2, 3), [1, 2, 3], 'bound func with too many args');
 
 	var boundR = callBind(func, sentinel);
+	t.equal(boundR.length, func.length - 1, 'function length is preserved', { skip: !functionsHaveConfigurableLengths });
 	t.deepEqual(boundR(), [sentinel, undefined, undefined], 'bound func with receiver, with too few args');
 	t.deepEqual(boundR(1, 2), [sentinel, 1, 2], 'bound func with receiver, with right args');
 	t.deepEqual(boundR(1, 2, 3), [sentinel, 1, 2], 'bound func with receiver, with too many args');
 
 	var boundArg = callBind(func, sentinel, 1);
+	t.equal(boundArg.length, func.length - 2, 'function length is preserved', { skip: !functionsHaveConfigurableLengths });
 	t.deepEqual(boundArg(), [sentinel, 1, undefined], 'bound func with receiver and arg, with too few args');
 	t.deepEqual(boundArg(2), [sentinel, 1, 2], 'bound func with receiver and arg, with right arg');
 	t.deepEqual(boundArg(2, 3), [sentinel, 1, 2], 'bound func with receiver and arg, with too many args');