Add canvas to a page with javascript

Use something like Node.appendChild( child ) for adding it to the DOM:

var canv = document.createElement('canvas');
canv.id = 'someId';

document.body.appendChild(canv); // adds the canvas to the body element
document.getElementById('someBox').appendChild(canv); // adds the canvas to #someBox

Or you can use element.innerHTML:

document.body.innerHTML += '<canvas id="someId"></canvas>'; // the += means we add this to the inner HTML of body
document.getElementById('someBox').innerHTML = '<canvas id="someId"></canvas>'; // replaces the inner HTML of #someBox to a canvas