Standard deviation, in JavaScript

1.js

var sum = values.reduce(function(sum, value){
  return sum + value;
}, 0);

var avg = sum / data.length;

2.js

var diffs = values.map(function(value){
  var diff = value - avg;
  return diff;
});

3.js

var squareDiffs = values.map(function(value){
  var diff = value - avg;
  var sqr = diff * diff;
  return sqr;
});

4.js

function average(data){
  var sum = data.reduce(function(sum, value){
    return sum + value;
  }, 0);

  var avg = sum / data.length;
  return avg;
}

5.js

var avgSquareDiff = average(squareDiffs);

6.js

var stdDev = Math.sqrt(avgSquareDiff);

7.js

function standardDeviation(values){
  var avg = average(values);
  
  var squareDiffs = values.map(function(value){
    var diff = value - avg;
    var sqrDiff = diff * diff;
    return sqrDiff;
  });
  
  var avgSquareDiff = average(squareDiffs);

  var stdDev = Math.sqrt(avgSquareDiff);
  return stdDev;
}

function average(data){
  var sum = data.reduce(function(sum, value){
    return sum + value;
  }, 0);

  var avg = sum / data.length;
  return avg;
}

Array.prototype.stDev = function stDev() {
   const average = data => data.reduce((sum, value) => sum + value) / data.length
   return Math.sqrt(average(this.map(value => (value - average(this)) ** 2)))
};
[4, 8, 2, 4, 5].stDev()

This doesn't compute standard deviation, it computes the uncorrected standard deviation, because it uses 1/N. The denominator for stdev should be N-1, not N. The squared differences should not be averaged.