Variance for streams

variance.js

var stat = {};

stat.variance = function(x) {
  var counter = 0; 
  var runningMean = 0; 
  var newMean = 0; 
  var runningVariance = 0; 
  var newVariance = 0;
  var builder;
  
  builder = function(a) {
    // Check that input is a number
    if ( typeof a !== 'number' || isNaN(a) || !isFinite(a) ) { 
      throw 'Input Error :: ' + x + '()'; 
    
    } else { 
      
      counter++;
      if ( counter === 1 ) {
        runningMean = newMean = a;
      
      } else {
     
        newMean = runningMean + (a - runningMean)/counter;
        newVariance = runningVariance + (a - runningMean)*(a - newMean);
      
        runningMean = newMean;
        runningVariance = newVariance;
      
      }
        
    }
	

    // Calculate variance
    return counter > 1 ? newVariance/(counter-1) : 0 ;

	
  };
  return builder;
};

var s = stat.variance();

function out(b) {
  $('#var').text(s(b));
}

var sample = Stream.range ( 10, 99 );
sample.walk( out );

Live example:
http://jsbin.com/iwiwuz/latest

Translated to idiomatic javascript from here: http://www.johndcook.com/standard_deviation.html

Credit for bringing streams to my attention (and providing the implementation used here): http://streamjs.org/