`styleText()` doesn't work within a worker thread

[slorber]

Version

26.8.1

Platform

Darwin MacBook-Pro-de-Sebastien.local 24.6.0 Darwin Kernel Version 24.6.0: Tue Apr 21 20:16:56 PDT 2026; root:xnu-11417.140.69.710.16~1/RELEASE_ARM64_T6030 arm64

Subsystem

No response

What steps will reproduce the bug?

const {Worker} = require('node:worker_threads');
const {styleText} = require('node:util');
const tty = require('node:tty');

console.log('MAIN', {
  isTTY: process.stdout.isTTY,
  isatty: tty.isatty(1),
  styled: JSON.stringify(styleText('red', 'hello')),
});

await new Promise((resolve, reject) => {
  const worker = new Worker(`
    const {parentPort} = require('node:worker_threads');
    const {styleText} = require('node:util');
    const tty = require('node:tty');

    parentPort.postMessage({
      isTTY: process.stdout.isTTY,
      isatty: tty.isatty(1),
      styled: JSON.stringify(styleText('red', 'hello')),
    });
  `, {eval: true});

  worker.on('message', msg => {
    console.log('WORKER', msg);
    resolve();
  });

  worker.on('error', reject);
});

Output:

MAIN {
  isTTY: true,
  isatty: true,
  styled: '"\\u001b[31mhello\\u001b[39m"'
}

WORKER {
  isTTY: undefined,
  isatty: true,
  styled: '"hello"'
}

---

Even simpler repro:

const { Worker } = require('node:worker_threads');
const { styleText } = require('node:util');

console.log('main:', styleText('red', 'hello'));

new Worker(`
  const { styleText } = require('node:util');

  console.log('worker:', styleText('red', 'hello'));
`, { eval: true });

How often does it reproduce? Is there a required condition?

constantly

What is the expected behavior? Why is that the expected behavior?

I think it's reasonable to expect styleText() to also work in worker threads by default if the parent has colors enabled. I'd rather have an opt-out than an opt-in through worker thread config/env.

What do you see instead?

The worker thread

Additional information

From what I understand, the heuristic used in styleText() is too limited and could be improved to support colors in worker threads better

• https://github.com/nodejs/node/blob/main/lib/util.js
• https://github.com/nodejs/node/blob/main/lib/internal/util/colors.js

In chalk, this works out of the box, based on https://github.com/chalk/supports-color

[kiliczsh]

I investigated this on the current main branch at
a1bfce980c386d22f7d3a77a2ab218c16b6e1c0b.

The behavior appears to come from how worker stdio is represented rather than from ANSI generation in styleText() itself.

util.styleText() is implemented in lib/util.js#L246.
It uses process.stdout as the default stream at lib/util.js#L279
and calls shouldColorize(stream) at lib/util.js#L287.

shouldColorize() is implemented at lib/internal/util/colors.js#L18-L25.
When FORCE_COLOR is not set, it relies on stream.isTTY and, when available, stream.getColorDepth().

Inside a worker, process.stdout is a message-port-backed WritableWorkerStdio, implemented at lib/internal/worker/io.js#L279-L326.

Worker stdout and stderr are instantiated at lib/internal/worker/io.js#L337-L338.
WritableWorkerStdio does not expose isTTY, getColorDepth(), or hasColors().

On Windows, using the locally built node.exe, I observed the following inside a worker:

{
  isTTY: process.stdout.isTTY, // undefined
  isatty: tty.isatty(1),       // true
}

Consequently, shouldColorize(process.stdout) evaluates to a falsy value and styleText() returns plain text.

Disabling stream validation confirms that ANSI generation itself works:

styleText('red', 'hello', { validateStream: false });
// "\u001b[31mhello\u001b[39m"

The existing tests cover the two relevant areas separately:

• test/parallel/test-util-styletext.js covers styleText() with TTY and non-TTY streams and the color-related environment variables.
• test/parallel/test-worker-stdio.js covers worker stdio transport.

Both tests pass on this commit, but I could not find coverage combining styleText() with worker stdio.

@daeyeon the remaining design question seems to be which color capability a worker should report. With the default stdio handling, inheriting the parent destination's capability appears reasonable. However, worker.stdout can be captured or redirected with stdout: true, so treating worker stdout unconditionally as the parent's TTY could add ANSI sequences when the output is not ultimately written to a terminal.

Potential implementation directions include propagating the parent's color capability when creating the worker or adding appropriate TTY/color metadata to worker stdio. The expected behavior for redirected worker stdout likely needs to be decided before choosing the fix and regression-test structure.