在图像完全加载之前使用Javascript获取图像尺寸

var img = new Image();
img.onload = function() {
  var width = this.width;
  var height = this.height;
  console.log("Image dimensions: " + width + " x " + height);
};
img.src = "image_url";

var img = document.createElement("img");
img.onload = function() {
  var width = this.width;
  var height = this.height;
  console.log("Image dimensions: " + width + " x " + height);
};
img.src = "image_url";

var fileInput = document.createElement("input");
fileInput.type = "file";
fileInput.addEventListener("change", function(event) {
  var file = event.target.files[0];
  var reader = new FileReader();
  reader.onload = function(event) {
    var img = new Image();
    img.onload = function() {
      var width = this.width;
      var height = this.height;
      console.log("Image dimensions: " + width + " x " + height);
    };
    img.src = event.target.result;
  };
  reader.readAsDataURL(file);
});