const file = event.target.files[0];
const img = new Image();
img.onload = function () {
  width = img.width;
  height = img.height;
};
img.src = URL.createObjectURL(file);
alert(width + "x" + height);

这个答案正是我所寻找的(在jQuery):

var imageNaturalWidth = $('image-selector').prop('naturalWidth');
var imageNaturalHeight = $('image-selector').prop('naturalHeight');

此外(除了Rex和Ian的回答)还有:

imageElement.naturalHeight

and

imageElement.naturalWidth

它们提供了图像文件本身的高度和宽度(而不仅仅是图像元素)。

使用jQuery，你可以这样做:

var imgWidth = $("#imgIDWhatever").width();

在获取元素的属性之前，文档页面应该是onload的:

window.onload = function(){
    console.log(img.offsetWidth,img.offsetHeight);
}

这是我的方法:

var imgSrc, imgW, imgH;

function myFunction(image) {
    var img = new Image();
    img.src = image;
    img.onload = function() {
        return {
            src: image,
            width: this.width,
            height: this.height};
        }
    return img;
}

var x = myFunction('http://www.google.com/intl/en_ALL/images/logo.gif');
    // Waiting for the image loaded. Otherwise, system returned 0 as both width and height.
x.addEventListener('load', function() {
    imgSrc = x.src;
    imgW = x.width;
    imgH = x.height;
});

x.addEventListener('load', function() {
    console.log(imgW + 'x' + imgH); // 276x110
});
console.log(imgW); // undefined.
console.log(imgH); // undefined.
console.log(imgSrc); // undefined.