If you wish to only use the Processing API (not the language) you can interact with it like so:
var p = Processing(CanvasElement);
p.ellipse(50, 50, 50, 50);
To use the full power of the language, you would pass in your Processing code as the second argument.
Processing(CanvasElement, "size(100, 100); background(0);" +
"fill(255); ellipse(50, 50, 50, 50);");