All files / path / windows / extname.ts

// Copyright 2018-2025 the Deno authors. MIT license.
// This module is browser compatible.

import { CHAR_COLON, CHAR_DOT } from "../_common/constants.ts";
import { assertPath } from "../_common/assert_path.ts";
import { isPathSeparator, isWindowsDeviceRoot } from "./_util.ts";

/**
 * Return the extension of the `path` with leading period.
 *
 * @example Usage
 * ```ts
 * import { extname } from "@std/path/windows/extname";
 * import { assertEquals } from "@std/assert";
 *
 * const ext = extname("file.ts");
 * assertEquals(ext, ".ts");
 * ```
 *
 * Note: If you are working with file URLs,
 * use the new version of `extname` from `@std/path/windows/unstable-extname`.
 *
 * @param path The path to get the extension from.
 * @returns The extension of the `path`.
 */
export function extname(path: string): string {
  assertPath(path);

  let start = 0;
  let startDot = -1;
  let startPart = 0;
  let end = -1;
  let matchedSlash = true;
  // Track the state of characters (if any) we see before our first dot and
  // after any path separator we find
  let preDotState = 0;

  // Check for a drive letter prefix so as not to mistake the following
  // path separator as an extra separator at the end of the path that can be
  // disregarded

  if (
    path.length >= 2 &&
    path.charCodeAt(1) === CHAR_COLON &&
    isWindowsDeviceRoot(path.charCodeAt(0))
  ) {
    start = startPart = 2;
  }

  for (let i = path.length - 1; i >= start; --i) {
    const code = path.charCodeAt(i);
    if (isPathSeparator(code)) {
      // If we reached a path separator that was not part of a set of path
      // separators at the end of the string, stop now
      if (!matchedSlash) {
        startPart = i + 1;
        break;
      }
      continue;
    }
    if (end === -1) {
      // We saw the first non-path separator, mark this as the end of our
      // extension
      matchedSlash = false;
      end = i + 1;
    }
    if (code === CHAR_DOT) {
      // If this is our first dot, mark it as the start of our extension
      if (startDot === -1) startDot = i;
      else if (preDotState !== 1) preDotState = 1;
    } else if (startDot !== -1) {
      // We saw a non-dot and non-path separator before our dot, so we should
      // have a good chance at having a non-empty extension
      preDotState = -1;
    }
  }

  if (
    startDot === -1 ||
    end === -1 ||
    // We saw a non-dot character immediately before the dot
    preDotState === 0 ||
    // The (right-most) trimmed path component is exactly '..'
    (preDotState === 1 && startDot === end - 1 && startDot === startPart + 1)
  ) {
    return "";
  }
  return path.slice(startDot, end);
}