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

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)

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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('git+ssh://example.con/item'); // true - absolute URL with '+' in scheme
r.test('/myfolder/test.txt'); // false - relative URL
r.test('test'); // false - also relative URL

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Explain the RegExp

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

^ - beginning of the string
(?: - beginning of a non-captured group
[a-z+]+ - any character of 'a' to 'z' or "+" 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