chore: Add some comments to correctProtocol

Author: markovejnovic

lib/parse-url.js

@@ -21,20 +21,26 @@ const correctProtocol = (arg, protocols) => {
     return arg
   }
 
-  const doubleSlash = arg.indexOf('//')
-  if (doubleSlash === firstColon + 1) {
+  if (arg.substr(firstColon, 3) === '://') {
+    // If arg is given as <foo>://<bar>, then this is already a valid URL and
+    // we assume the user provided a protocol.
     return arg
   }
 
   const firstAt = arg.indexOf('@')
   if (firstAt > -1) {
     if (firstAt > firstColon) {
+      // The URL has the form of <foo>:<bar>@<baz>. We assume this is a git+ssh
+      // URL.
       return `git+ssh://${arg}`
     } else {
+      // The URL has the form of <foo>@<bar> which is likely a shortcut URL
+      // such as @npm/cli
       return arg
     }
   }
 
+  // Correct <foo>:<bar> to <foo>://<bar>
   return `${arg.slice(0, firstColon + 1)}//${arg.slice(firstColon + 1)}`
 }