You are using drawImage() the wrong way. Instead of drawing the image at (0,0) you are drawing it just outside the canvas area as width and height is where position normally goes.
The valid signatures are:
context.drawImage(image, dx, dy)
context.drawImage(image, dx, dy, dw, dh)
context.drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)
Where dx and dy are delta position (relative to origin, normally (0,0) when untranslated). Without width and height specified drawImage() will by default use the image's natural width and height.
The second version allows to override the default size, and the third will allow you to draw from one region to another.
Source
Corrected example:
window.onload = function() {
var canvas = document.createElement('canvas');
var context = canvas.getContext("2d");
canvas.id = "canvas_id";
canvas.className = "canvas"; // should be className
canvas.height = 400; // should be numbers
canvas.width = 800;
var image = new Image();
image.onload = function() {
// or set canvas size = image, here: (this = currently loaded image)
// canvas.width = this.width;
// canvas.height = this.height;
context.drawImage(this, 0, 0); // draw at (0,0), size = image size
// or, if you want to fill the canvas independent on image size:
// context.drawImage(this, 0, 0, canvas.width, canvas.height);
}
// set src last (recommend to use relative paths where possible)
image.src = "http://lorempixel.com/output/fashion-q-c-800-400-7.jpg";
document.body.appendChild(canvas);
}
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