How to get the width and height of a picture when uploading a picture?

Why do you get 0 most of the time when you write like this? only individual pictures can output width. why? 

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);
}
Javascript html5
Nov.15,2021 
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);
}

Previous: How to add and edit on a page?

Next: Ask for help: how do I get CentOS7 to find the block file that stores the mapping?

css mysql arrays josn react html typescript webpack npm sass R objective-c .net sql-server jquery python-3.x angularjs django angular excel regex iphone ajax linux xml pandas vba spring database wordpress string wpf xcode windows bash postgresql oracle multithreading eclipse list firebase algorithm macos forms image scala visual-studio azure bootstrap spring-boot react-native python-2.7 docker performance function winforms matlab powershell apache dataframe api sqlite numpy rest shell selenium flutter dart maven loops qt swing android-studio csv express file class tensorflow sorting codeigniter perl
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b36b6e-2b39c.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b36b6e-2b39c.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
Need Help?