上传图片时如何获取图片的宽高？？

Question

为什么这样写大部分时候获取到的都是是0呢 只有个别的图片能输出宽度 这是为什么？

var upload = document.getElementById('upload');
upload.onchange=function(){
        var file = this.files[0];
        if (file) {
                var reader = new FileReader();
                reader.onload = function (event) {
                    var txt = event.target.result;
                    var img = document.createElement("img");
                    img.src = txt;
                    console.log(img.width);
                    document.getElementById("result").appendChild(img);
                };
            }
            reader.readAsDataURL(file);
}

Answer 1

var upload = document.getElementById('upload');
upload.onchange=function(){
        var file = this.files[0];
        if (file) {
                var reader = new FileReader();
                reader.onload = function (event) {
                    var txt = event.target.result;
                    var img = document.createElement("img");
                    img.src = txt;
                    
                    img.onload = function () {
                        console.log(img.width);
                        document.getElementById("result").appendChild(img);
                    }
                    
                    
                };
            }
            reader.readAsDataURL(file);
}