How to test if a URL string is absolute or relative?

How can I test a URL if it is a relative or absolute path in Javascript or jQuery? I want to handle accordingly depending if the passed in URL is a local or external path.

if (urlString starts with http:// or https://)
 //do this

FAST

If you only need to test for http:// or https:// then the most efficient way is:

if (urlString.indexOf('http://') === 0 || urlString.indexOf('https://') === 0)

UNIVERSAL

However, I would suggest a more universal, non case-sensitive, protocol-agnostic approach:

var r = new RegExp('^(?:[a-z]+:)?//', 'i');
r.test('http://example.com'); // true - regular http absolute URL
r.test('HTTP://EXAMPLE.COM'); // true - HTTP upper-case absolute URL
r.test('https://www.exmaple.com'); // true - secure http absolute URL
r.test('ftp://example.com/file.txt'); // true - file transfer absolute URL
r.test('//cdn.example.com/lib.js'); // true - protocol-relative absolute URL
r.test('/myfolder/test.txt'); // false - relative URL
r.test('test'); // false - also relative URL

Explain the RegExp

^(?:[a-z]+:)?//

^ - beginning of the string
(?: - beginning of a non-captured group
[a-z]+ - any character of 'a' to 'z' 1 or more times
: - string (colon character)
)? - end of the non-captured group. Group appearing 0 or 1 times
// - string (two forward slash characters)
'i' - non case-sensitive flag

var pat = /^https?:\/\//i;
if (pat.test(urlString))
{
    //do stuff
}

For protocol relative urls, use this regex:

/^https?:\/\/|^\/\//i