JS parabolic spend

parabola.js

function defaultCurve( length ) {
  // first point
  let x1 = 1;
  let y1 = 1.9;

  // mid point
  let x2 = length / 2;
  let y2 = 0.5;

  // end point
  let x3 = length;
  let y3 = y1;

  // equation vars
  let A1 = -Math.pow( x1, 2 ) + Math.pow( x2, 2 );
  let B1 = -x1 + x2;
  let B2 = -x2 + x3;
  let D1 = -y1 + y2;
  let A2 = -Math.pow( x2, 2 ) + Math.pow( x3, 2 );
  let D2 = -y2 + y3;
  let B  = -( B2 / B1 );
  let A3 = B * A1 + A2;
  let D3 = B * D1 + D2;

  // final equation vars
  let a  = D3 / A3;
  let b  = ( D1 - A1 * a ) / B1;
  let c  = y1 - a * Math.pow( x1, 2 ) - b * x1;

  return function plot( x ) {
    return a * Math.pow( x, 2 ) + b * x + c;
  }
}