How to run canvas animation without effecting other javascript code
One solution to allow multiple functions called when the page loads is to add a callback using EventTarget.addEventListener() for the DOMContentLoaded event. 

window.addEventListener('DOMContentLoaded', function() {
    //canvas init
    //..
});


One could also simply use the load event. Many believe that modern browsers typically won't have DOM elements not query-able via JavaScript- especially if the script tag is loaded in the &lt;body&gt; tag instead of the &lt;head&gt; tag, and thus one wouldn't need to wait for the DOMContentLoaded event.



window.addEventListener('DOMContentLoaded', function() {
  //canvas init
  var canvas = document.getElementById("snow");
  var ctx = canvas.getContext("2d");

  //canvas dimensions
  var W = window.innerWidth;
  var H = window.innerHeight;
  canvas.width = W;
  canvas.height = H;

  //snowflake particles
  var mp = 250; //max particles
  var particles = [];
  for (var i = 0; i &lt; mp; i++) {
    particles.push({
      x: Math.random() * W, //x-coordinate
      y: Math.random() * H, //y-coordinate
      r: Math.random() * 4 + 1, //radius
      d: Math.random() * mp //density
    });
  }

  //Lets draw the flakes
  function draw() {
    ctx.clearRect(0, 0, W, H);

    ctx.fillStyle = "rgba(255, 255, 255, 0.8)";
    ctx.beginPath();
    for (var i = 0; i &lt; mp; i++) {
      var p = particles[i];
      ctx.moveTo(p.x, p.y);
      ctx.arc(p.x, p.y, p.r, 0, Math.PI * 2, true);
    }
    ctx.fill();
    update();
  }

  //Function to move the snowflakes
  //angle will be an ongoing incremental flag. Sin and Cos functions will be applied to it to create vertical and horizontal movements of the flakes
  var angle = 0;

  function update() {
    angle += 0.01;
    for (var i = 0; i &lt; mp; i++) {
      var p = particles[i];
      //Updating X and Y coordinates
      //We will add 1 to the cos function to prevent negative values which will lead flakes to move upwards
      //Every particle has its own density which can be used to make the downward movement different for each flake
      //Lets make it more random by adding in the radius
      p.y += Math.cos(angle + p.d) + 1 + p.r / 2;
      p.x += Math.sin(angle) * 2;

      //Sending flakes back from the top when it exits
      //Lets make it a bit more organic and let flakes enter from the left and right also.
      if (p.x &gt; W + 5 || p.x &lt; -5 || p.y &gt; H) {
        if (i % 3 &gt; 0) {
          //66.67% of the flakes
          particles[i] = {
            x: Math.random() * W,
            y: -10,
            r: p.r,
            d: p.d
          };
        } else {
          //If the flake is exitting from the right
          if (Math.sin(angle) &gt; 0) {
            //Enter from the left
            particles[i] = {
              x: -5,
              y: Math.random() * H,
              r: p.r,
              d: p.d
            };
          } else {
            //Enter from the right
            particles[i] = {
              x: W + 5,
              y: Math.random() * H,
              r: p.r,
              d: p.d
            };
          }
        }
      }
    }
  }

  //animation loop
  setInterval(draw, 33);
});
window.addEventListener('DOMContentLoaded', function() {
  console.log('another onload callback');
});
body {
  background: tomato;
}
&lt;script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"&gt;&lt;/script&gt;
&lt;canvas id="snow" style="display:block;position:fixed;z-index:999;pointer-events:none;"&gt;&lt;/canvas&gt;