Contain script within div tag using p5 parent element.

this-is-a-test.js

//RGB rainbow colors
var rainbowR = [248, 255, 255, 254, 208, 105, 18, 68, 59];
var rainbowG = [12, 51, 102, 174, 195, 208, 189, 68, 12];
var rainbowB = [18, 17, 68, 45, 16, 37, 185, 221, 189];

var i;
var canvas;
var t = rainbowR.length;

function setup() {
  canvas = createCanvas(640, 100);
  canvas.parent('script-holder');
  i = 0;
}

function draw(){
  //background
  background(255);
  
  //borders
  fill(255);
  stroke(0);
  rect(0, 0, 639, 99);
  
  //ellipse click
  stroke(255);
  fill(rainbowR[i], rainbowG[i], rainbowB[i]);
  ellipse(mouseX, mouseY, 50, 50);
  
  //ellipse static
  stroke(255);
  fill(0, 100);
  ellipse(320, 50, 50, 50);
}

function mousePressed(){
  if(i < t - 1){
    i += 1;
  }
  else{
    i = 0;
  }
}