[New] add functionsHaveConfigurableNames function on main export

.eslintrc

@@ -4,5 +4,7 @@
 	"extends": "@ljharb",
 
 	"rules": {
-	}
+		"func-name-matching": [2, "always"],
+		"id-length": 1,
+	},
 }

index.js

@@ -1,5 +1,13 @@
 'use strict';
 
-module.exports = function functionsHaveNames() {
+var functionsHaveNames = function functionsHaveNames() {
 	return function f() {}.name === 'f';
 };
+
+var gOPD = Object.getOwnPropertyDescriptor;
+
+functionsHaveNames.functionsHaveConfigurableNames = function functionsHaveConfigurableNames() {
+	return functionsHaveNames() && gOPD && !!gOPD(function () {}, 'name').configurable;
+};
+
+module.exports = functionsHaveNames;

test/index.js

@@ -8,7 +8,32 @@ test('named functions', function (t) {
 	function f() {} // eslint-disable-line func-style
 	var g = function h() {};
 
+	t.equal(typeof hasNames, 'function', 'is a function');
 	t.equal(hasNames(), f.name === 'f' && g.name === 'h', 'functions have names or not as expected');
 
 	t.end();
 });
+
+test('functionsHaveConfigurableNames', function (t) {
+	t.equal(typeof hasNames.functionsHaveConfigurableNames, 'function', 'is a function');
+
+	if (hasNames()) {
+		var fn = function f() {};
+		if (Object.defineProperty) {
+			Object.defineProperty(fn, 'name', { configurable: true, value: 'foo' });
+			if (fn.name === 'f') {
+				t.equal(hasNames.functionsHaveConfigurableNames(), false, 'function names are not configurable');
+			} else if (fn.name === 'foo') {
+				t.equal(hasNames.functionsHaveConfigurableNames(), true, 'function names are not configurable');
+			} else {
+				t.fail('functions have names, but something surprising has happened. Please report this!');
+			}
+		} else {
+			t.equal(hasNames.functionsHaveConfigurableNames(), false, 'function names are not configurable');
+		}
+	} else {
+		t.equal(hasNames.functionsHaveConfigurableNames(), false, 'functions do not have names');
+	}
+
+	t.end();
+});