How to extract extension from filename string in Javascript?

A variant that works with all of the following inputs:

• "file.name.with.dots.txt"
• "file.txt"
• "file"
• ""
• null
• undefined

would be:

var re = /(?:\.([^.]+))?$/;

var ext = re.exec("file.name.with.dots.txt")[1];   // "txt"
var ext = re.exec("file.txt")[1];                  // "txt"
var ext = re.exec("file")[1];                      // undefined
var ext = re.exec("")[1];                          // undefined
var ext = re.exec(null)[1];                        // undefined
var ext = re.exec(undefined)[1];                   // undefined

Explanation

(?:         # begin non-capturing group
  \.        #   a dot
  (         #   begin capturing group (captures the actual extension)
    [^.]+   #     anything except a dot, multiple times
  )         #   end capturing group
)?          # end non-capturing group, make it optional
$           # anchor to the end of the string

I personally prefer to split the string by . and just return the last array element :)

var fileExt = filename.split('.').pop();

If there is no . in filename you get the entire string back.

Examples:

'some_value'                                   => 'some_value'
'.htaccess'                                    => 'htaccess'
'../images/something.cool.jpg'                 => 'jpg'
'http://www.w3schools.com/jsref/jsref_pop.asp' => 'asp'
'http://stackoverflow.com/questions/680929'    => 'com/questions/680929'

Use the lastIndexOf method to find the last period in the string, and get the part of the string after that:

var ext = fileName.substr(fileName.lastIndexOf('.') + 1);

I would recommend using lastIndexOf() as opposed to indexOf()

var myString = "this.is.my.file.txt"
alert(myString.substring(myString.lastIndexOf(".")+1))

var x = "1.txt";
alert (x.substring(x.indexOf(".")+1));

note 1: this will not work if the filename is of the form file.example.txt
note 2: this will fail if the filename is of the form file

I use code below:

var fileSplit = filename.split('.');
var fileExt = '';
if (fileSplit.length > 1) {
fileExt = fileSplit[fileSplit.length - 1];
} 
return fileExt;

This is the solution if your file has more . (dots) in the name.

<script type="text/javascript">var x = "file1.asdf.txt";
var y = x.split(".");
alert(y[(y.length)-1]);</script>