Simple promise based timeout implementation.

timeout.js

function timeout (ms, x) {
  return new Promise((resolve, reject) => {
    let timeoutId = setTimeout(() => {
      timeoutId = null
      reject('timeout')
    }, ms)

    const onSettle = () => {
      if (timeoutId != null) {
        clearTimeout(timeoutId)
        resolve(x) // propagate fulfilment/rejection
      }
    }

    Promise.resolve(x).then(onSettle, onSettle)
  })
}