title: trying to understand node.js callback scheme
body: <pre><code>function load_file_contents(path, callback) {
      fs.open(path, 'r', function (err, f) {
          if (err) {
              callback(err);
              return;
          } else if (!f) {
              callback(make_error("invalid_handle",
                  "bad file handle from fs.open"));
              return;
          }
          fs.fstat(f, function (err, stats) {
              if (err) {
                  callback(err);
                  return;
              }
              if (stats.isFile()) {
                  var b = new Buffer(10000);
                  fs.read(f, b, 0, 10000, null, function (err, br, buf) {
                      if (err) {
                          callback(err);
                          return;
                      }
  
                      fs.close(f, function (err) {
                          if (err) {
                              callback(err);
                              return;
                          }
                          callback(null, b.toString('utf8', 0, br));
                      });
                  });
              } else {
                  calback(make_error("not_file", "Can't load directory"));
                  return;
              }
          });
      });
  }
  
  
  load_file_contents(
      "test.txt",
      function (err, contents) {
          if (err)
              console.log(err);
          else
              console.log(contents);
      }
  );
  </code></pre>
  
  <p>In this code, I don't quite understand where does this "f" come from?
  after "fs.open()", there is a line </p>
  
  <pre><code>" } else if (!f) {"
  </code></pre>
  
  <p>what does this mean, where does this f comefrom?</p>