Remove everything after last backslash

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);

As stated in @Archer's answer, you need to double up on the backslashes. I suggest using regex replace to get the string you want:

var t = "\\some\\route\\here";
t = t.replace(/\\[^\\]+$/,"");
alert(t);