spring.ts

export interface IStepOptions {
  secondsPerFrame?: number;
  v: number;
  x: number;
  destX: number;
  k: number;
  b: number;
  precision?: number;
}
export default function stepper({
  secondsPerFrame = 1 / 60,
  v,
  x,
  destX,
  k,
  b,
  precision = 0.01
}: IStepOptions): Required<IStepOptions> {
  const fSpring = -k * (x - destX);
  const fDamper = -b * v;
  const a = fSpring + fDamper;
  const newV = v + a * secondsPerFrame;
  const newX = x + newV * secondsPerFrame;

  if (Math.abs(newV) < precision && Math.abs(newX - destX) < precision) {
    return {
      secondsPerFrame,
      v: 0,
      x: destX,
      destX,
      k,
      b,
      precision
    };
  }
  return {
    secondsPerFrame,
    v: newV,
    x: newX,
    destX,
    k,
    b,
    precision
  };
}