Avoid HTTP redirects (no-http-redirects)

no-http-redirects checks if there are any HTTP redirects in the page
webhint is analyzing.

Why is this important?

Consider the following simplified description of what happens when
a user requests a URL within a browser:

1. DNS Lookup: Translate the server domain to an IP. If the browser
doesn’t know it, it asks a DNS server which in some cases involves
multiple queries until the final IP is obtained.
1. Open a [TCP connection][wikipedia-tcp-establishment] to the server
IP address requesting the URL.
1. The server responds to that request by sending some content over
the TCP connection.
If the resource uses SSL, then [TLS negotiation(s)][wikipedia-tls-handshake]
happens as well.

When a redirect occurs, 3. contains the new URL the browser needs to
request, repeating the whole sequence of steps. DNS Lookup isn’t cheap,
neither is [creating a TCP connection][tcp-connection-diagram]. The
impact of redirects is felt even more by mobile users, where the [network
latency is usually higher][pagespeed-insights].
As a rule of thumb, the more you can avoid redirects the better.

What does the hint check?

This hint checks:

* If the target URL passed to webhint has any redirect. E.g.:
http://www.example.com --> http://example.com
* If any resource in the page has any redirect. E.g.:
http://example.com/script.js --> https://example.com/script.js

and alerts if at least one is found.

$3

* Any URL passed to webhint that redirects to another one
* Any page with a resource (script, css, image) behind a redirect

$3

* No redirect for resources nor the target URL.

Can the hint be configured?

By default, no redirects are allowed but you can change this behavior.

The following hint configuration used in the [.hintrc][hintrc]
file will allow 3 redirects for resources and 1 for the main URL:

``json
{
"connector": {...},
"formatters": [...],
"hints": {
"no-http-redirects": ["error", {
"max-resource-redirects": 3,
"max-html-redirects": 1
}],
...
},
...
}
`

How to use this hint?

This package is installed automatically by webhint:

`bash
npm install hint --save-dev
`

To use it, activate it via the [.hintrc][hintrc] configuration file:

`json
{
"connector": {...},
"formatters": [...],
"hints": {
"no-http-redirects": "error",
...
},
"parsers": [...],
...
}
`

Note: The recommended way of running webhint is as a devDependency` of
your project.

Further Reading

* [Mobile Analysis in PageSpeed Insights][pagespeed-insights]
* [50 performance tricks to make your HTML5 apps and sites faster][50-tricks]
(skip to 19:35)
* [Domain Names - Implementation and Specification][rfc1035]
* [How DNS Works][how-dns-works]
* [Redirections in HTTP][MDN-Redirections]
* [Transmission Control Protocol][wikipedia-tcp]

[50-tricks]: https://channel9.msdn.com/events/Build/2012/3-132#time=19m35s
[how-dns-works]: https://www.verisign.com/en_US/website-presence/online/how-dns-works/index.xhtml
[MDN-Redirections]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Redirections
[pagespeed-insights]: https://developers.google.com/speed/docs/insights/mobile#adapting-to-high-latency-mobile-networks
[rfc1035]: https://tools.ietf.org/html/rfc1035
[hintrc]: https://webhint.io/docs/user-guide/configuring-webhint/summary/
[tcp-connection-diagram]: https://www.eventhelix.com/RealtimeMantra/Networking/tcp/#.WgOQBkxFy2c
[wikipedia-tcp-establishment]: https://en.wikipedia.org/wiki/Transmission_Control_Protocol#Connection_establishment
[wikipedia-tcp]: https://en.wikipedia.org/wiki/Transmission_Control_Protocol
[wikipedia-tls-handshake]: https://en.wikipedia.org/wiki/Transport_Layer_Security#TLS_handshake