Implement the [es-shim API](es-shims/api).

implementation.js

@@ -0,0 +1,7 @@
+'use strict';
+
+/* http://www.ecma-international.org/ecma-262/6.0/#sec-number.isnan */
+
+module.exports = function isNaN(value) {
+	return value !== value;
+};

index.js

@@ -2,19 +2,16 @@
 
 var define = require('define-properties');
 
-/* http://www.ecma-international.org/ecma-262/6.0/#sec-number.isnan */
+var implementation = require('./implementation');
+var getPolyfill = require('./polyfill');
+var shim = require('./shim');
 
-var numberIsNaN = function isNaN(value) {
-	return value !== value;
-};
+/* http://www.ecma-international.org/ecma-262/6.0/#sec-number.isnan */
 
-define(numberIsNaN, {
-	shim: function shimNumberIsNaN() {
-		if (!Number.isNaN) {
-			define(Number, { isNaN: numberIsNaN });
-		}
-		return Number.isNaN || numberIsNaN;
-	}
+define(implementation, {
+	getPolyfill: getPolyfill,
+	implementation: implementation,
+	shim: shim
 });
 
-module.exports = numberIsNaN;
+module.exports = implementation;

package.json

@@ -6,7 +6,8 @@
 	"license": "MIT",
 	"main": "index.js",
 	"scripts": {
-		"test": "npm run lint && npm run test:function && npm run test:shimmed",
+		"test": "npm run lint && es-shim-api && npm run tests-only",
+		"tests-only": "npm run test:function && npm run test:shimmed",
 		"test:function": "node test/index.js",
 		"test:shimmed": "node test/shimmed.js",
 		"coverage": "covert test/*.js",
@@ -42,7 +43,8 @@
 		"jscs": "^2.1.0",
 		"eslint": "^1.1.0",
 		"@ljharb/eslint-config": "^1.0.4",
-		"es5-shim": "^4.1.10"
+		"es5-shim": "^4.1.10",
+		"@es-shims/api": "^1.0.0"
 	},
 	"testling": {
 		"files": "test.js",

polyfill.js

@@ -0,0 +1,10 @@
+'use strict';
+
+var implementation = require('./implementation');
+
+module.exports = function getPolyfill() {
+	if (Number.isNaN && Number.isNaN(NaN) && !Number.isNaN('a')) {
+		return Number.isNaN;
+	}
+	return implementation;
+};

shim.js

@@ -0,0 +1,12 @@
+'use strict';
+
+var define = require('define-properties');
+var getPolyfill = require('./polyfill');
+
+/* http://www.ecma-international.org/ecma-262/6.0/#sec-number.isnan */
+
+module.exports = function shimNumberIsNaN() {
+	var polyfill = getPolyfill();
+	define(Number, { isNaN: polyfill }, { isNaN: function () { return Number.isNaN !== polyfill; } });
+	return polyfill;
+};