It's a bit more complicated to get a cover functionality, though here is one solution for this:

Updated 2016-04-03 to address special cases. Also see @Yousef's comment below.

/**

 * By Ken Fyrstenberg Nilsen

 *

 * drawImageProp(context, image [, x, y, width, height [,offsetX, offsetY]])

 *

 * If image and context are only arguments rectangle will equal canvas

*/

function drawImageProp(ctx, img, x, y, w, h, offsetX, offsetY) {



    if (arguments.length === 2) {

        x = y = 0;

        w = ctx.canvas.width;

        h = ctx.canvas.height;

    }



    // default offset is center

    offsetX = typeof offsetX === "number" ? offsetX : 0.5;

    offsetY = typeof offsetY === "number" ? offsetY : 0.5;



    // keep bounds [0.0, 1.0]

    if (offsetX < 0) offsetX = 0;

    if (offsetY < 0) offsetY = 0;

    if (offsetX > 1) offsetX = 1;

    if (offsetY > 1) offsetY = 1;



    var iw = img.width,

        ih = img.height,

        r = Math.min(w / iw, h / ih),

        nw = iw * r,   // new prop. width

        nh = ih * r,   // new prop. height

        cx, cy, cw, ch, ar = 1;



    // decide which gap to fill    

    if (nw < w) ar = w / nw;                             

    if (Math.abs(ar - 1) < 1e-14 && nh < h) ar = h / nh;  // updated

    nw *= ar;

    nh *= ar;



    // calc source rectangle

    cw = iw / (nw / w);

    ch = ih / (nh / h);



    cx = (iw - cw) * offsetX;

    cy = (ih - ch) * offsetY;



    // make sure source rectangle is valid

    if (cx < 0) cx = 0;

    if (cy < 0) cy = 0;

    if (cw > iw) cw = iw;

    if (ch > ih) ch = ih;



    // fill image in dest. rectangle

    ctx.drawImage(img, cx, cy, cw, ch,  x, y, w, h);

}

Now you can call it like this:

drawImageProp(ctx, image, 0, 0, width, height);

and it will scale the image proportionally to fit inside in that container.

Use the two last parameters to offset the image:

var offsetX = 0.5;   // center x

var offsetY = 0.5;   // center y

drawImageProp(ctx, image, 0, 0, width, height, offsetX, offsetY);