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js实现加载进度条

2026-01-30 16:26:01JavaScript

使用XMLHttpRequest监听进度事件

通过XMLHttpRequest对象的progress事件可以获取文件加载进度。创建一个进度条div，根据事件中的loaded和total属性更新进度条宽度：

const xhr = new XMLHttpRequest();
const progressBar = document.getElementById('progress-bar');

xhr.open('GET', 'large-file.url', true);
xhr.onprogress = (e) => {
  if (e.lengthComputable) {
    const percent = (e.loaded / e.total) * 100;
    progressBar.style.width = `${percent}%`;
  }
};
xhr.send();

使用Fetch API配合ReadableStream

Fetch API结合ReadableStream可以实现更现代的进度监控方案。通过读取响应体的流数据计算进度：

js实现加载进度条

fetch('large-file.url')
  .then(response => {
    const reader = response.body.getReader();
    const contentLength = +response.headers.get('Content-Length');
    let received = 0;

    return new ReadableStream({
      start(controller) {
        function push() {
          reader.read().then(({done, value}) => {
            if (done) {
              controller.close();
              return;
            }
            received += value.length;
            const percent = (received / contentLength) * 100;
            updateProgressBar(percent);
            controller.enqueue(value);
            push();
          });
        }
        push();
      }
    });
  });

页面加载进度模拟

对于页面整体加载进度，可以结合DOM事件模拟进度效果。监听DOMContentLoaded和load事件分阶段更新进度：

window.addEventListener('DOMContentLoaded', () => {
  updateProgress(70); // 模拟DOM加载完成70%
});

window.addEventListener('load', () => {
  updateProgress(100); // 页面完全加载
});

function updateProgress(percent) {
  document.querySelector('.progress-bar').style.width = `${percent}%`;
}

CSS动画作为后备方案

当无法获取真实加载进度时，可以使用CSS动画创建无限循环的进度条效果：

js实现加载进度条

.progress-bar {
  animation: progress 2s ease-in-out infinite;
  background: linear-gradient(to right, #4cd964, #5ac8fa);
}

@keyframes progress {
  0% { width: 0%; }
  100% { width: 100%; }
}

分段加载进度计算

对于多资源加载场景，需要跟踪每个资源的加载状态。维护一个加载队列，计算总体进度：

const resources = ['a.jpg', 'b.js', 'c.css'];
let loadedCount = 0;

resources.forEach(url => {
  fetch(url).then(() => {
    loadedCount++;
    const percent = (loadedCount / resources.length) * 100;
    updateProgressBar(percent);
  });
});

进度条UI组件实现

创建可复用的进度条组件，包含动画和状态管理功能：

class ProgressBar {
  constructor(selector) {
    this.element = document.querySelector(selector);
    this.value = 0;
  }

  setValue(percent) {
    this.value = Math.min(100, Math.max(0, percent));
    this.element.style.width = `${this.value}%`;
  }

  complete() {
    this.setValue(100);
    setTimeout(() => this.element.style.opacity = 0, 500);
  }
}