The deprecated url.format formats ipv6 localhost address wrongly

[chenxsan]

• Version: I've tested it against Node.js 11, 12, 13, 14, 15
• Platform: I've tested it on macOS Big Sur.
• Subsystem:

What steps will reproduce the bug?

Create a new format.js file with content below:

const url = require('url')
console.log(url.format({
    protocol: 'http',
    hostname: '[::]'
}))

Then run it with node format.js.

How often does it reproduce? Is there a required condition?

Always.

What is the expected behavior?

Should output http://[::] instead.

What do you see instead?

It will output http://[[::]] which I believe is wrong.

Additional information

1. I know url.format(urlobject) was deprecated as per https://nodejs.org/api/url.html#url_url_format_urlobject, but there're still libraries using it.
2. url.format(new URL('http://[::]')) would output http://[::]/ as I expected.

[lpinca]

The problem as I see it is not with url.format() but with using '[::]' as hostname. You can use '[::]' as host and it works as expected.

console.log(url.format({ protocol: 'http', host: '[::]' }));

Also both

url.format(new URL('http://[::]'));

and

url.format(url.parse('http://[::]'));

work as expected.

When using '[::]' as hostname, url.format() assumes that it is a literal IPv6 address and encloses it in the [ and ] characters.

We can document this behavior or make url.format() smarter making it use the hostname as is if it is already enclosed in [ and ] characters.

[chenxsan]

If we recognize [::] as host, then we would fail to handle cases with port:

> const url = require('url');
> console.log(url.format({ protocol: 'http', host: '[::]', port: 3000 }));
http://[::]

It would be more reasonable to make url.format() smarter in my opinion according to https://tools.ietf.org/html/rfc2732:

To use a literal IPv6 address in a URL, the literal address should be
enclosed in "[" and "]" characters

[lpinca]

host is the concatenation of hostname and port so it should be [::]:3000 and url.format() would handle that correctly. Also if you use :: or any other literal IPv6 address as hostname, url.format() would handle that correctly.

I find it a bit surprising that the WHATWG URL hostname getter returns [::] instead of :: but that is consistent with the specification.