• Version: v11.12.0
• Platform: macOS Mojave 10.14.4 (Darwin brewster.local 18.5.0 Darwin Kernel Version 18.5.0: Mon Mar 11 20:40:32 PDT 2019; root:xnu-4903.251.3~3/RELEASE_X86_64 x86_64)
• Subsystem: worker_threads

In worker threads, process.stdout.isTTY and process.stderr.isTTY are undefined.

const { Worker, isMainThread } = require('worker_threads')
if (isMainThread) {
  console.log('mainThread, process.stdout.isTTY', process.stdout.isTTY)
  console.log('mainThread, process.stderr.isTTY', process.stderr.isTTY)
  new Worker(__filename)
} else {
  console.log('workerThread, process.stdout.isTTY', process.stdout.isTTY)
  console.log('workerThread, process.stderr.isTTY', process.stderr.isTTY)
}

$ node ./main.js 
mainThread, process.stdout.isTTY true
mainThread, process.stderr.isTTY true
workerThread, process.stdout.isTTY undefined
workerThread, process.stderr.isTTY undefined

I am not sure if this is intended behavior or not, but I found it while looking into why supports-color reports that color is not supported from worker threads (see chalk/supports-color#97). If I ignore the isTTY flag, then color output does seem to work correctly.