resolve.js

let url = require('url')

function resolve(baseUrl, extraUrl) {
  function urlParse(url) {
    let m = url.match(/^(http[s]?:\/\/[^/]+)?(.*)?$/)
    if(! m) throw new Error(`Can't parse "${url}"`)
    let host = m[1] || ''
    let path = m[2] || ''
    if(host && ! path) path = '/'
    return {host, path}
  }

  let base = urlParse(baseUrl)
  let extra = urlParse(extraUrl)

  if(extra.path.slice(0, 2) == './') extra.path = extra.path.slice(2)

  if(extra.path) {
    let m = base.path.match(/^(.*\/)[^/]*$/)
    base.path = m ? m[1] : ''
    if(extra.path[0] == '/') base.path = ''
    if(base.path.slice(0, 2) == './') base.path = base.path.slice(2)
  }

  let host = extra.host || base.host
  let path = base.path + extra.path

  return host + path
}

function testsuite() {
  const TESTS = [
    '',
    'a.txt',
    '/a.txt',
    './a.txt',
    '/a/b/c.txt',
    'http://grep.ro',
    'http://grep.ro/x',
    'http://grep.ro/x/',
    'http://grep.ro/x/y/',
  ]

  for(let base of TESTS) {
    for(let extra of TESTS) {
      let result = resolve(base, extra)
      let expected = url.resolve(base, extra)
      console.log(`"${base}", "${extra}", "${result}"`)
      if(result != expected) {
        console.error('error!', [result, expected])
        return
      }
    }
  }
  console.log('all ok')
}

testsuite()