You need lastIndexOf and substr…
var t = "\\some\\route\\here";
t = t.substr(0, t.lastIndexOf("\\"));
alert(t);
Also, you need to double up \ chars in strings as they are used for escaping special characters.
Update
Since this is regularly proving useful for others, here’s a snippet example…
// the original string
var t = "\\some\\route\\here";
// remove everything after the last backslash
var afterWith = t.substr(0, t.lastIndexOf("\\") + 1);
// remove everything after & including the last backslash
var afterWithout = t.substr(0, t.lastIndexOf("\\"));
// show the results
console.log("before : " + t);
console.log("after (with \\) : " + afterWith);
console.log("after (without \\) : " + afterWithout);