I want to fill the background as big as fabric.js canvas, but I can't do math

Question

In the case of a square image, it is a function that centralizes it, but there is a problem. As I set the maximum width and maximum height, if I upload a picture of a smartphone that exceeds the maximum width, the ratio decreases, so I want the picture of a smartphone to fit the size of the canvas. Thank you in advance, are you a good math person? I'm depressed because I'm not good at math and I can't even implement this simple function.

const setBackground = (url, canvas) => {
  fabric.Image.fromURL(url, (img) => {
    canvas.setBackgroundImage(img, canvas.renderAll.bind(canvas), {
      setBackgroundFromDataUrl: setBackgroundFromDataUrl(url, canvas),
    });
  });
};
// 캔버스 배경 중앙정렬
const setBackgroundFromDataUrl = (dataUrl, canvas, options = {}) => {
  if (!dataUrl) {
    return true;
  }

  let img = new Image();
  img.setAttribute("crossOrigin", "anonymous");
  img.onload = () => {
    let maxWidth = 1080;
    let maxHeight = 1920;
    // maxWidth // Max width of the canvas
    // maxHeight //Max height of canvas

    let imgWidth = img.width; // Current image width
    let imgHeight = img.height; // Current image height

    let widthAspectRatio = maxWidth / imgWidth;
    let heightAspectRatio = maxHeight / imgHeight;

    let finalAspectRatio = Math.min(widthAspectRatio, heightAspectRatio);

    let finalHeight = imgHeight * finalAspectRatio;
    let finalWidth = imgWidth * finalAspectRatio;

    let imgTop = 0;
    if (maxHeight > finalHeight) {
      imgTop = (Math.round(maxHeight) - Math.round(finalHeight)) / 2;
    }

    let imgLeft = 0;
    if (maxWidth > finalWidth) {
      imgLeft = (Math.round(maxWidth) - Math.round(finalWidth)) / 2;
    }

    let nsgImage = new fabric.Image(img).scale(finalAspectRatio);
    nsgImage.set({
      left: imgLeft,
      top: imgTop
    });

    canvas.setBackgroundImage(nsgImage, () => canvas.renderAll());
  };

  img.src = dataUrl;
};