JS: Most optimized way to remove a filename from a path in a string?

Question

I have strings formatted as follows:
path/to/a/filename.txt

Now I'd like to do some string manipulation which allows me to very efficiently remove the "filename.txt" part from this code. In other words, I want my string to become this:
path/to/a/

What's the most efficient way to do this? Currently I'm splitting the string and reconnecting the seperate elements except for the last one, but I get the feeling this is a really, REALLY inefficient way to do it. Here's my current, inefficient code:

res.getPath = function(file) {   var elem = file.split("/");   var str = "";   for (var i = 0; i < elem.length-1; i++)     str += elem[i] + "/";   return str; } 

Answer 1

Use lastIndexOf() to find the position of the last slash and get the part before the slash with substring().

str.substring(0, str.lastIndexOf("/")); 

Answer 2

If you're using Node.js:

const path = require("path") const removeFilePart = dirname => path.parse(dirname).dir  removeFilePart("/a/b/c/d.txt") // Returns "/a/b/c"