Determines if filename is a relative path.

is-relative-path.js

const {sep: platformSep} = require('path')
const {inspect} = require('util')

const POSIX_SEP = '/'
const WIN32_SEP = '\\'

const PATH_SEPARATORS = {
  'aix': POSIX_SEP,
  'darwin': POSIX_SEP,
  'freebsd': POSIX_SEP,
  'linux': POSIX_SEP,
  'openbsd': POSIX_SEP,
  'posix': POSIX_SEP,
  'sunos': POSIX_SEP,
  'unix': POSIX_SEP,
  'win': WIN32_SEP,
  'win32': WIN32_SEP,
}

function isRelative (filename, sep) {
  if (typeof filename !== 'string') {
    throw new TypeError('Path must be a string. Received ' + inspect(filename))
  }

  sep = sep ? PATH_SEPARATORS[sep] || POSIX_SEP : platformSep
  filename = filename[0] === ' ' ? filename.trim() : filename

  return filename[0] === sep ? false : sep !== POSIX_SEP ? filename[1] !== ':' : true
}

module.exports = isRelative

This function can't be used to detect a relative module ID.

To do that, the test typeof moduleId === 'string' && moduleId[0] === '.' is enough.