Prepending "http://" to a URL that doesn't already contain "http://"

If you also want to allow "https://", I would use a regular expression like this:

if (!/^https?:\/\//i.test(url)) {
    url = 'http://' + url;
}

If you're not familiar with regular expressions, here's what each part means.

• ^ - Only match at the beginning of the string
• http - Match the literal string "http"
• s? - Optionally match an "s"
• : - Match a colon
• \/\/ - Escape the "/" characters since they mark the beginning/end of the regular expression
• The "i" after the regular expression makes it case-insensitive so it will match "HTTP://", etc.

A simple solution for what you want is the following:

var prefix = 'http://';
if (s.substr(0, prefix.length) !== prefix)
{
    s = prefix + s;
}

However there are a few things you should be aware of...

The test here is case-sensitive. This means that if the string is initially Http://example.com this will change it to http://Http://example.com which is probably not what you want. You probably should also not modify any string starting with foo:// otherwise you could end up with something like http://https://example.com.

On the other hand if you receive an input such as example.com?redirect=http://othersite.com then you probably do want to prepend http:// so just searching for :// might not be good enough for a general solution.

Alternative approaches

• Using a regular expression:

  if (!s.match(/^[a-zA-Z]+:\/\//))
  {
      s = 'http://' + s;
  }
  

• Using a URI parsing library such as JS-URI.

  if (new URI(s).scheme === null)
  {
      s = 'http://' + s;
  }
  

Related questions

• Javascript equalsIgnoreCase: case insensitive string comparation
• javascript startswith
• How do I parse a URL into hostname and path in javascript?

Lifted from the Linkenizer (Null won't mind)

link = (link.indexOf('://') === -1) ? 'http://' + link : link;

This will prepend 'http://' to link if it can't find the :// indicating protocol. This won't work well if :// occurs elsewhere in the string, but it's good enough.

Examples:

http://www.google.com -> http://www.google.com
ftp://google.com      -> ftp://google.com
www.google.com        -> http://www.google.com
google.com            -> http://google.com

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Since you said you are saving this URL, it would be a better idea to do this on the server-side, so clients who have js disabled won't mess up the links.