This function ensures the correct decodings of percent-encoded characters as well as ensuring a cross-platform valid absolute path string.
import { fileURLToPath } from 'node:url';
const __filename = fileURLToPath(import.meta.url);
new URL('file:http:///C:/path/').pathname; // Incorrect: /C:/path/
fileURLToPath('file:http:///C:/path/'); // Correct: C:\path\ (Windows)
new URL('file:http://nas/foo.txt').pathname; // Incorrect: /foo.txt
fileURLToPath('file:http://nas/foo.txt'); // Correct: \\nas\foo.txt (Windows)
new URL('file:http:///你好.txt').pathname; // Incorrect: /%E4%BD%A0%E5%A5%BD.txt
fileURLToPath('file:http:///你好.txt'); // Correct: /你好.txt (POSIX)
new URL('file:http:///hello world').pathname; // Incorrect: /hello%20world
fileURLToPath('file:http:///hello world'); // Correct: /hello world (POSIX)
Security Considerations:
This function decodes percent-encoded characters, including encoded dot-segments (%2e as . and %2e%2e as ..), and then normalizes the resulting path. This means that encoded directory traversal sequences (such as %2e%2e) are decoded and processed as actual path traversal, even though encoded slashes (%2F, %5C) are correctly rejected.
Applications must not rely on fileURLToPath() alone to prevent directory traversal attacks. Always perform explicit path validation and security checks on the returned path value to ensure it remains within expected boundaries before using it for file system operations.