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the is-main guard built by concatenating file:// with argv[1] can never be true on Windows, so your CLI exits 0 doing nothing

env paths · case

bothsilentscriptciagentfirst-partyrepro: historicalwin32-path-normalization
Affectsnode, bun, windows
Fails assilent
Mechanismwin32 path normalization
Safe fixrealpath both sides

Symptom

Section titled “Symptom”

A dual-purpose ESM module — importable as a library, runnable as a CLI — runs on Windows, prints nothing, and exits 0. No error, no stack, no usage text. It works on macOS and Linux.

The guard looks correct and is the shape half the ecosystem copied out of CommonJS require.main === module:

const isDirect = process.argv[1] !== undefined
  && import.meta.url === `file://${process.argv[1]}`;
if (isDirect) main(process.argv.slice(2));

On Windows that comparison is false for every possible invocation. main() is never called, so the process falls off the end of the module and exits cleanly. Exit code 0 is the worst part: every wrapper, CI step, and installer treats it as success.

Repro

Section titled “Repro”
whoami.mjs
console.log("meta:", import.meta.url);
console.log("argv:", process.argv[1]);
console.log("equal:", import.meta.url === `file://${process.argv[1]}`);
PS> node whoami.mjs
meta: file:///D:/work/whoami.mjs
argv: D:\work\whoami.mjs
equal: False
$ node whoami.mjs        # macOS / Linux
meta: file:///home/u/whoami.mjs
argv: /home/u/whoami.mjs
equal: true

The POSIX case works by coincidence: an absolute POSIX path starts with /, so "file://" + "/home/u/x.mjs" accidentally produces the correct three-slash URL.

Cause

Section titled “Cause”

import.meta.url is a real file: URL. A Windows one is file:///D:/work/whoami.mjs: three slashes, a drive letter, forward slashes, and percent-encoding for anything non-ASCII. process.argv[1] is a plain Win32 path, D:\work\whoami.mjs. Concatenation produces file://D:\work\whoami.mjs, which differs in the slash count, the separator direction, and the encoding — three independent reasons the strings cannot match.

A space in the path breaks it a fourth way (%20 in the URL, a literal space in argv), and on a case-differing drive letter a fifth.

The adjacent trap, which bites on every OS: even a correctly built URL compares unequal when the script is reached through a symlink, because import.meta.url reports the realpath while argv[1] preserves the link. That is how npm global bins and plugin caches invoke things, so a guard can pass your local test and skip main() for every installed user.

Workaround

Section titled “Workaround”

Compare resolved paths, not strings, and resolve both sides:

import { realpathSync } from "node:fs";
import { fileURLToPath } from "node:url";


const isDirect = (() => {
  try {
    if (process.argv[1] === undefined) return false;
    const self = realpathSync(fileURLToPath(import.meta.url));
    let invoked = process.argv[1];
    try { invoked = realpathSync(invoked); } catch { /* keep unresolved */ }
    return self === invoked;
  } catch { return false; }
})();

fileURLToPath handles the drive letter, the slashes, and the percent-decoding. The realpathSync pair handles the symlink. On Node 20.11+ and Bun, import.meta.main does all of this for you and is the right answer when you can require that version.

Never reach for import.meta.url.endsWith(basename) as the quick fix: it fires for any file with the same name anywhere on the machine.

Worth its own entry because the failure is silent and exit-0. The corpus’s other path traps are about finding things — node-path-host-delimiter is the PATH-list separator, pathext-bare-name-enoent is extension resolution. This one is about identity: two spellings of the same file that Windows makes unequal.

Refs

Section titled “Refs”