p5.js sketch that fills parent element

fill-container.js

/**
 * p5.js sketch that fills its parent element.
 * On my phone on a train so I haven’t actually tested…
 */

var container;

function setup() {
  var canvas;

  // E.g. for a `<div id="container-id"></div>`.
  container = document.getElementById('container-id');

  canvas = createCanvas(container.offsetWidth, container.offsetHeight);
  canvas.parent('container-id');

  // Other setup…
}

/**
 * Called automatically when the window is resized.
 */
function windowResized() {
  resizeCanvas(container.offsetWidth, container.offsetHeight);
}

function draw() {
  // Other code…

  // May have to call `resizeCanvas()` periodically here if you have other
  // things on your page which cause the container to change shape.
}