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

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"

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

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