Simple path join and dirname functions for generic javascript

path.js

// Joins path segments.  Preserves initial "/" and resolves ".." and "."
// Does not support using ".." to go above/outside the root.
// This means that join("foo", "../../bar") will not resolve to "../bar"
function join(/* path segments */) {
  // Split the inputs into a list of path commands.
  var parts = [];
  for (var i = 0, l = arguments.length; i < l; i++) {
    parts = parts.concat(arguments[i].split("/"));
  }
  // Interpret the path commands to get the new resolved path.
  var newParts = [];
  for (i = 0, l = parts.length; i < l; i++) {
    var part = parts[i];
    // Remove leading and trailing slashes
    // Also remove "." segments
    if (!part || part === ".") continue;
    // Interpret ".." to pop the last segment
    if (part === "..") newParts.pop();
    // Push new path segments.
    else newParts.push(part);
  }
  // Preserve the initial slash if there was one.
  if (parts[0] === "") newParts.unshift("");
  // Turn back into a single string path.
  return newParts.join("/") || (newParts.length ? "/" : ".");
}

// A simple function to get the dirname of a path
// Trailing slashes are ignored. Leading slash is preserved.
function dirname(path) {
  return join(path, "..");
}

My variant:

/**
 * A simple analog of Node.js's `path.join(...)`.
 * https://gist.github.com/creationix/7435851#gistcomment-3698888
 * @param  {...string} segments
 * @return {string}
 */
export default function joinPath(...segments) {
  const parts = segments.reduce((parts, segment) => {
    // Remove leading slashes from non-first part.
    if (parts.length > 0) {
      segment = segment.replace(/^\//, '')
    }
    // Remove trailing slashes.
    segment = segment.replace(/\/$/, '')
    return parts.concat(segment.split('/'))
  }, [])
  const resultParts = []
  for (const part of parts) {
    if (part === '.') {
      continue
    }
    if (part === '..') {
      resultParts.pop()
      continue
    }
    resultParts.push(part)
  }
  return resultParts.join('/')
}