let lineHeights = [];

let lineSpeeds = [];

​

function setup() {

  createCanvas(400, 400);

  for(let x = 0; x < lines; x++){

    const lineHeight = random(height);

    lineHeights[x] = lineHeight;

    lineSpeeds[x] = random(-10, 10);

  }

}

​

function draw() {

  background(32);

  for(let i = 0; i < lines; i++){

    lineHeights[i] += lineSpeeds[i];

    if(lineHeights[i] < 0 || lineHeights[i] > height) {

      lineSpeeds[i] *= -1;

    }

    const x = width * (i / lines);

    const lineWidth = width / lines;

    fill(0, 100, 255);

    stroke(0, 100, 255);

    rect(x, height / 2 - lineHeights[i] / 2,

         lineWidth, lineHeights[i] / 2);

    fill(100, 0, 255);

    stroke(100, 0, 255);

    rect(x, height / 2,

         lineWidth, lineHeights[i] / 2);

  }

}