diff --git a/.travis.yml b/.travis.yml
index 73f7742..ebbbde9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,5 +1,8 @@
 language: node_js
 node_js:
+  - 12
+  - 10
+  - 8
   - 6
 cache:
   directories:
diff --git a/README.md b/README.md
index ee2f9d6..36259bb 100644
--- a/README.md
+++ b/README.md
@@ -24,8 +24,12 @@ npm install punycode --save
 
 In [Node.js](https://nodejs.org/):
 
+> ⚠️ Note that userland modules don't hide core modules.
+> For example, `require('punycode')` still imports the deprecated core module even if you executed `npm install punycode`.
+> Use `require('punycode/')` to import userland modules rather than core modules.
+
 ```js
-const punycode = require('punycode');
+const punycode = require('punycode/');
 ```
 
 ## API
diff --git a/package.json b/package.json
index 9202ccf..7cf866f 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "punycode",
-  "version": "2.1.1",
+  "version": "2.2.0",
   "description": "A robust Punycode converter that fully complies to RFC 3492 and RFC 5891, and works on nearly all JavaScript platforms.",
   "homepage": "https://mths.be/punycode",
   "main": "punycode.js",
diff --git a/punycode.js b/punycode.js
index ea61fd0..4fb262c 100644
--- a/punycode.js
+++ b/punycode.js
@@ -50,11 +50,11 @@ function error(type) {
  * item.
  * @returns {Array} A new array of values returned by the callback function.
  */
-function map(array, fn) {
+function map(array, callback) {
 	const result = [];
 	let length = array.length;
 	while (length--) {
-		result[length] = fn(array[length]);
+		result[length] = callback(array[length]);
 	}
 	return result;
 }
@@ -66,22 +66,22 @@ function map(array, fn) {
  * @param {String} domain The domain name or email address.
  * @param {Function} callback The function that gets called for every
  * character.
- * @returns {Array} A new string of characters returned by the callback
+ * @returns {String} A new string of characters returned by the callback
  * function.
  */
-function mapDomain(string, fn) {
-	const parts = string.split('@');
+function mapDomain(domain, callback) {
+	const parts = domain.split('@');
 	let result = '';
 	if (parts.length > 1) {
 		// In email addresses, only the domain name should be punycoded. Leave
 		// the local part (i.e. everything up to `@`) intact.
 		result = parts[0] + '@';
-		string = parts[1];
+		domain = parts[1];
 	}
 	// Avoid `split(regex)` for IE8 compatibility. See #17.
-	string = string.replace(regexSeparators, '\x2E');
-	const labels = string.split('.');
-	const encoded = map(labels, fn).join('.');
+	domain = domain.replace(regexSeparators, '\x2E');
+	const labels = domain.split('.');
+	const encoded = map(labels, callback).join('.');
 	return result + encoded;
 }
 
@@ -130,7 +130,7 @@ function ucs2decode(string) {
  * @param {Array} codePoints The array of numeric code points.
  * @returns {String} The new Unicode string (UCS-2).
  */
-const ucs2encode = array => String.fromCodePoint(...array);
+const ucs2encode = codePoints => String.fromCodePoint(...codePoints);
 
 /**
  * Converts a basic code point into a digit/integer.
@@ -342,7 +342,7 @@ const encode = function(input) {
 			if (currentValue < n && ++delta > maxInt) {
 				error('overflow');
 			}
-			if (currentValue == n) {
+			if (currentValue === n) {
 				// Represent delta as a generalized variable-length integer.
 				let q = delta;
 				for (let k = base; /* no condition */; k += base) {
@@ -359,7 +359,7 @@ const encode = function(input) {
 				}
 
 				output.push(stringFromCharCode(digitToBasic(q, 0)));
-				bias = adapt(delta, handledCPCountPlusOne, handledCPCount == basicLength);
+				bias = adapt(delta, handledCPCountPlusOne, handledCPCount === basicLength);
 				delta = 0;
 				++handledCPCount;
 			}