fs.realpathSync(path, [cache])

var fs = require('fs');
 
// 点号表示当前文件所在路径
var str = fs.realpathSync('.');
console.log(str);

fs.realpathSync = function realpathSync(p, cache) {
  // make p is absolute
  p = pathModule.resolve(p);
  if (cache && Object.prototype.hasOwnProperty.call(cache, p)) {
    return cache[p];
  }
  var original = p,
      seenLinks = {},
      knownHard = {};
  // current character position in p
  var pos;
  // the partial path so far, including a trailing slash if any
  var current;
  // the partial path without a trailing slash (except when pointing at a root)
  var base;
  // the partial path scanned in the previous round, with slash
  var previous;
  start();
  function start() {
    // Skip over roots
    var m = splitRootRe.exec(p);
    pos = m[0].length;
    current = m[0];
    base = m[0];
    previous = '';
    // On windows, check that the root exists. On unix there is no need.
    if (isWindows && !knownHard[base]) {
      fs.lstatSync(base);
      knownHard[base] = true;
    }
  }
  // walk down the path, swapping out linked pathparts for their real
  // values
  // NB: p.length changes.
  while (pos < p.length) {
    // find the next part
    nextPartRe.lastIndex = pos;
    var result = nextPartRe.exec(p);
    previous = current;
    current += result[0];
    base = previous + result[1];
    pos = nextPartRe.lastIndex;
    // continue if not a symlink
    if (knownHard[base] || (cache && cache[base] === base)) {
      continue;
    }
    var resolvedLink;
    if (cache && Object.prototype.hasOwnProperty.call(cache, base)) {
      // some known symbolic link. no need to stat again.
      resolvedLink = cache[base];
    } else {
      var stat = fs.lstatSync(base);
      if (!stat.isSymbolicLink()) {
        knownHard[base] = true;
        if (cache) cache[base] = base;
        continue;
      }
      // read the link if it wasn't read before
      // dev/ino always return 0 on windows, so skip the check.
      var linkTarget = null;
      if (!isWindows) {
        var id = stat.dev.toString(32) + ':' + stat.ino.toString(32);
        if (seenLinks.hasOwnProperty(id)) {
          linkTarget = seenLinks[id];
        }
      }
      if (util.isNull(linkTarget)) {
        fs.statSync(base);
        linkTarget = fs.readlinkSync(base);
      }
      resolvedLink = pathModule.resolve(previous, linkTarget);
      // track this, if given a cache.
      if (cache) cache[base] = resolvedLink;
      if (!isWindows) seenLinks[id] = linkTarget;
    }
    // resolve the link, then start over
    p = pathModule.resolve(resolvedLink, p.slice(pos));
    start();
  }
  if (cache) cache[original] = p;
  return p;
};