url: fix url.format with ipv6 hostname

Fixes: nodejs#36654

lib/url.js

@@ -26,6 +26,7 @@ const {
   ObjectCreate,
   ObjectKeys,
   SafeSet,
+  StringPrototypeCharCodeAt,
 } = primordials;
 
 const { toASCII } = require('internal/idna');
@@ -156,6 +157,14 @@ function urlParse(url, parseQueryString, slashesDenoteHost) {
   return urlObject;
 }
 
+function isIpv6Hostname(hostname) {
+  return (
+    StringPrototypeCharCodeAt(hostname, 0) === CHAR_LEFT_SQUARE_BRACKET &&
+    StringPrototypeCharCodeAt(hostname, hostname.length - 1) ===
+    CHAR_RIGHT_SQUARE_BRACKET
+  );
+}
+
 Url.prototype.parse = function parse(url, parseQueryString, slashesDenoteHost) {
   validateString(url, 'url');
 
@@ -364,8 +373,7 @@ Url.prototype.parse = function parse(url, parseQueryString, slashesDenoteHost) {
 
     // If hostname begins with [ and ends with ]
     // assume that it's an IPv6 address.
-    const ipv6Hostname = hostname.charCodeAt(0) === CHAR_LEFT_SQUARE_BRACKET &&
-      hostname.charCodeAt(hostname.length - 1) === CHAR_RIGHT_SQUARE_BRACKET;
+    const ipv6Hostname = isIpv6Hostname(hostname);
 
     // validate a little.
     if (!ipv6Hostname) {
@@ -590,7 +598,7 @@ Url.prototype.format = function format() {
     host = auth + this.host;
   } else if (this.hostname) {
     host = auth + (
-      this.hostname.includes(':') ?
+      this.hostname.includes(':') && !isIpv6Hostname(this.hostname) ?
         '[' + this.hostname + ']' :
         this.hostname
     );

test/parallel/test-url-format.js

@@ -148,6 +148,12 @@ const formatTests = {
     host: '[fedc:ba98:7654:3210:fedc:ba98:7654:3210]:61616',
     pathname: '/s/stopButton'
   },
+  'http://[::]/': {
+    href: 'http://[::]/',
+    protocol: 'http:',
+    hostname: '[::]',
+    pathname: '/'
+  },
 
   // Encode context-specific delimiters in path and query, but do not touch
   // other non-delimiter chars like `%`.