code: function erf(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;
  
    // Save the sign of x
    var sign = 1;
    if (x < 0) {
        sign = -1;
    }
    x = Math.abs(x);
  
    // 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;
  }
docs: http://picomath.org/javascript/erf.js.html