获得封面功能有点复杂，尽管这里有一个解决方案：

更新了 以解决特殊情况。另请参阅下面的@Yousef评论。

/**
 * 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);
}

现在您可以这样称呼它：

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

它将按比例缩放图像以适合该容器的内部。

使用最后两个参数偏移图像：

var offsetX = 0.5;   // center x
var offsetY = 0.5;   // center y
drawImageProp(ctx, image, 0, 0, width, height, offsetX, offsetY);

希望这可以帮助！