在此页

• 范例
• 规范
• 浏览器兼容性
• 另请参阅
• 相关话题

loadstart event is fired when a file read operation has begun.

冒泡	No
可取消	No
接口	ProgressEvent
事件处理程序特性	FileReader.onloadstart

范例

实时范例

HTML

<div class="example">
    <div class="file-select">
        <label for="avatar">Choose a profile picture:</label>
        <input type="file"
               id="avatar" name="avatar"
               accept="image/png, image/jpeg">
    </div>
    <img src="" class="preview" height="200" alt="Image preview...">
    <div class="event-log">
        <label>Event log:</label>
        <textarea readonly class="event-log-contents"></textarea>
    </div>
  </div>
					

CSS

img.preview {
  margin: 1rem 0;
}
.event-log-contents {
  width: 18rem;
  height: 5rem;
  border: 1px solid black;
  margin: .2rem;
  padding: .2rem;
}
.example {
  display: grid;
  grid-template-areas:
              "select  log"
              "preview log";
}
.file-select {
  grid-area: select;
}
.preview {
  grid-area: preview;
}
.event-log {
  grid-area: log;
}
.event-log>label {
  display: block;
}
.event-log-contents {
  resize: none;
}
					

JS

const fileInput = document.querySelector('input[type="file"]');
const preview = document.querySelector('img.preview');
const eventLog = document.querySelector('.event-log-contents');
const reader = new FileReader();
function handleEvent(event) {
    eventLog.textContent = eventLog.textContent + `${event.type}: ${event.loaded} bytes transferred\n`;
    if (event.type === "load") {
        preview.src = reader.result;
    }
}
function addListeners(reader) {
    reader.addEventListener('loadstart', handleEvent);
    reader.addEventListener('load', handleEvent);
    reader.addEventListener('loadend', handleEvent);
    reader.addEventListener('progress', handleEvent);
    reader.addEventListener('error', handleEvent);
    reader.addEventListener('abort', handleEvent);
}
function handleSelected(e) {
    eventLog.textContent = '';
    const selectedFile = fileInput.files[0];
    if (selectedFile) {
        addListeners(reader);
        reader.readAsDataURL(selectedFile);
    }
}
fileInput.addEventListener('change', handleSelected);
					

结果

规范

规范	状态
文件 API	工作草案

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	桌面						移动
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
loadstart event	Chrome Yes	Edge 12	Firefox 79 79 不支持 ? — 79 loadstart event dispatches synchronously (should be asynchronously as per spec).	IE Yes	Opera Yes	Safari Yes	WebView Android Yes	Chrome Android Yes	Firefox Android Yes	Opera Android Yes	Safari iOS Yes	Samsung Internet Android Yes

图例

完整支持

完整支持

见实现注意事项。

另请参阅

• 相关事件： FileReader.load_event , FileReader.loadend_event , FileReader.progress_event , FileReader.error_event , FileReader.abort_event

元数据

• 最后修改： Mar 18, 2019

相关话题

1. FileReader
2. 构造函数

   1. FileReader()
3. 特性

   1. error
   2. onabort
   3. onload
   4. readyState
   5. result
4. 方法

   1. abort()
   2. readAsArrayBuffer()
   3. readAsBinaryString()
   4. readAsDataURL()
   5. readAsText()
5. 事件

   1. abort
   2. error
   3. load
   4. loadend
   5. loadstart
   6. progress
6. 继承：

   1. EventTarget