cdf.js

function cdf(x, mean, variance) {
  return 0.5 * (1 + erf((x - mean) / (Math.sqrt(2 * variance))));
}

function erf(x) {
  // save the sign of x
  var sign = (x >= 0) ? 1 : -1;
  x = Math.abs(x);

  // constants
  var a1 =  0.254829592;
  var a2 = -0.284496736;
  var a3 =  1.421413741;
  var a4 = -1.453152027;
  var a5 =  1.061405429;
  var p  =  0.3275911;

  // A&S formula 7.1.26
  var t = 1.0/(1.0 + p*x);
  var y = 1.0 - (((((a5 * t + a4) * t) + a3) * t + a2) * t + a1) * t * Math.exp(-x * x);
  return sign * y; // erf(-x) = -erf(x);
}