var file1 = "50.xsl";

var file2 = "30.doc";

​

function getFileExtension1(filename) {

  return (/[.]/.exec(filename)) ? /[^.]+$/.exec(filename)[0] : undefined;

}

​

//console.log(getFileExtension1(file1));

//console.log(getFileExtension1(file2));

​

function getFileExtension2(filename) {

  return filename.split('.').pop();

}

​

//console.log(getFileExtension2(file1));

//console.log(getFileExtension2(file2));

​

function getFileExtension3(filename) {

  return filename.slice((filename.lastIndexOf(".") - 1 >>> 0) + 2);

}

​

//console.log(getFileExtension3(file1));

//console.log(getFileExtension3(file2));

​

var file3 = '';

var file4 = 'name';

var file5 = 'name.txt';

var file6 = '.htpasswd';

var file7 = 'name.with.many.dots.myext';

​

console.log(getFileExtension2(file3));

console.log(getFileExtension2(file4));

console.log(getFileExtension2(file5));

console.log(getFileExtension2(file6));

console.log(getFileExtension2(file7));

​