最近有一个小需求：上传文件的时候显示上传进度（上传了百分之多少）。这个需求不难，比如以下代码就能实现：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<meta http-equiv="X-UA-Compatible" content="ie=edge" />
<title>Upload file</title>
<script>
function uploadFile() {
var fd = new FormData(document.forms[0]);
var xhr = new XMLHttpRequest();
xhr.upload.onprogress = function (e) {
console.log('e.loaded:' + e.loaded + ', e.total:' + e.total)
console.log(Math.round(e.loaded / e.total * 100) + '%') // 这里是上传进度
}
xhr.onreadystatechange = function(){
if ( xhr.readyState === 4 ) {
if ( xhr.status >= 200 && xhr.status < 300 || xhr.status === 304 ) {
console.log(xhr.responseText)
}
}
}
xhr.open("POST", "/");
xhr.send(fd);
}
</script>
</head>
<body>
<form action="/" method="POST" enctype="multipart/form-data">
<input type="file" name="file" id="file" required />
<input type="hidden" value="1" name="hidden">
<input type="text" name="text">
<input type="range" name="range">
<button onclick="uploadFile();" type="button" id="upload">Upload</button>
</form>
</body>
</html>