All files / path / posix / extname.ts

// Copyright 2018-2026 the Deno authors. MIT license.
// This module is browser compatible.

import { CHAR_DOT } from "../_common/constants.ts";
import { assertPath } from "../_common/assert_path.ts";
import { isPosixPathSeparator } from "./_util.ts";
import { fromFileUrl } from "./from_file_url.ts";

/**
 * Return the extension of the `path` with leading period.
 *
 * @example Usage
 * ```ts
 * import { extname } from "@std/path/posix/extname";
 * import { assertEquals } from "@std/assert";
 *
 * assertEquals(extname("/home/user/Documents/file.ts"), ".ts");
 * assertEquals(extname("/home/user/Documents/"), "");
 * assertEquals(extname("/home/user/Documents/image.png"), ".png");
 * assertEquals(extname(new URL("file:///home/user/Documents/file.ts")), ".ts");
 * assertEquals(extname(new URL("file:///home/user/Documents/file.ts?a=b")), ".ts");
 * assertEquals(extname(new URL("file:///home/user/Documents/file.ts#header")), ".ts");
 * ```
 *
 * @example Working with URLs
 *
 * Note: This function doesn't automatically strip hash and query parts from
 * URLs. If your URL contains a hash or query, remove them before passing the
 * URL to the function. This can be done by passing the URL to `new URL(url)`,
 * and setting the `hash` and `search` properties to empty strings.
 *
 * ```ts
 * import { extname } from "@std/path/posix/extname";
 * import { assertEquals } from "@std/assert";
 *
 * assertEquals(extname("https://deno.land/std/path/mod.ts"), ".ts");
 * assertEquals(extname("https://deno.land/std/path/mod.ts?a=b"), ".ts?a=b");
 * assertEquals(extname("https://deno.land/std/path/mod.ts#header"), ".ts#header");
 * ```
 *
 * @param path The path to get the extension from.
 * @returns The extension (ex. for `file.ts` returns `.ts`).
 */
export function extname(path: string | URL): string {
  if (path instanceof URL) {
    path = fromFileUrl(path);
  }
  assertPath(path);

  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;
  for (let i = path.length - 1; i >= 0; --i) {
    const code = path.charCodeAt(i);
    if (isPosixPathSeparator(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);
}