title: node js callback() function is not defined?
body: <p>Hello I am new to node js and I am trying to write a simple callback function however get "ReferenceError: sum1 is not defined", can anyone help me?
  code:</p>
  
  <pre><code>sum1(1,2, function(sum){
    console.log(3 + sum);
  });
  sum1 = function (a,b, callback){
    callback(a + b);
  };
  </code></pre>
  
  <p>However, I tried to use 
      <code>function sum1(a,b,callback){...}</code>
  and it works. Is this a naming problem? Can anyone explain a little bit?</p>