title: javaScript problems with NaN and += operator
body: <p>first off thanks for taking the time. I'm working on an IBU calculator for beer making. Please see the comment below. parseB is a text input converted to a number I've seen the typeof ibuFinal is a number and the inBU[i] is also a number. why is it returning Nan? </p>
  
  <pre><code>var AAU=[]; 
  var og=1.050;   
  for(var i=0;i&lt;6;i++){
      util[i]=(1.65*Math.pow(0.000125, og-1))*(1-Math.exp(-0.04*parseB[i]))/4.15;;                   
  }
  
              function aau(){
                  for(var i=0;i&lt;6;i++){
                      AAU[i]=weight[i]*alpha[i];
  
                  }
                  return AAU;
              }
  
              function ibu(){
  
                  var alphaAcid=aau();
                  var inBU=[];
                  var ibuFinal;
  
                  for(var i=0;i,alphaAcid.length;i++){
  
                      inBU[i]=alphaAcid[i]*util[i]*75/batchSize;
  
                      ibuFinal+=inBU[i];//returns NaN for some reason!!
                  }
  
                  alert(ibuFinal);
              }
              ibu();
  </code></pre>