javascript: 用图片加载演示promise的应用(chrome 107.0.5304.110)

一，js代码：

<html>
<head>
    <meta charset="utf-8"/>
    <title>测试</title>
</head>
<body>
    <img id="img" src="" />
<script>
//记录开始时间
let a = new Date();
console.log('开始时间:'+getNowTime(a));

//加载图片
let url = "https://www.marukyu-koyamaen.net/resources/c_media/themes/theme_55/images/banner4_s.jpg";
const img = new Image();
img.src = url;

//用promise保存加载情况
const pImg = new Promise((resolve,reject)=>{
    img.onload = () =>{
        resolve(img);
    }
    img.onerror = () => {
        reject("图片加载失败");
    }
});

//加载的处理
pImg.then((data)=>{
    let b = new Date();
    console.log('加载结束时间:'+getNowTime(b));
    let t = b-a;
    console.log('共用时:'+t+"毫秒");
    console.log('运行成功:参数是:');
    console.log(data);
    document.getElementById('img').src=url;
}).catch((data)=>{
    console.log('运行失败:参数是:');
    console.log(data);
}).then(()=>{
    console.log('pImg执行结束');
});

//得到当前时间的指定格式
function getNowTime(date) {
    this.year = date.getFullYear();
    this.month = date.getMonth() + 1;
    this.date = date.getDate();
    this.hour = date.getHours() < 10 ? "0" + date.getHours() : date.getHours();
    this.minute = date.getMinutes() < 10 ? "0" + date.getMinutes() : date.getMinutes();
    this.second = date.getSeconds() < 10 ? "0" + date.getSeconds() : date.getSeconds();
    this.milliSeconds = date.getMilliseconds();
    var currentTime = this.year+'-'+this.month + '-' + this.date + ' ' + this.hour + ':' + this.minute + ':' + this.second + '.' + this.milliSeconds;
    return currentTime;
};

</script>
</body>
</html>