[bes]

devote,

var url = "file:///d:/w.o.r.d.doc";

console.time("time1");
var ext1 = url.split('.').pop();
console.log(ext1);
console.timeEnd("time1");

console.time("time2");
var ext2 = url.slice(url.lastIndexOf(".") + 1);
console.log(ext2);
console.timeEnd("time2");

chrome 39
doc
time1: 3.577ms
doc
time2: 0.144ms

var url = "file:///d:/w.o.r.d.doc";

console.time("time2");
var ext2 = url.slice(url.lastIndexOf(".") + 1);
console.log(ext2);
console.timeEnd("time2");

console.time("time1");
var ext1 = url.split('.').pop();
console.log(ext1);
console.timeEnd("time1");

chrome 39
doc
time1: 3.499ms
doc
time2: 0.124ms

в ff34 всегда разница в 0.01-0.02 ms