以下是通过javaScript的方法实现文件预览的方法汇总：
1.使用HTML5的File API和Canvas来预览图片文件：
<!DOCTYPE html>
<html>
<head><title>Image Preview</title>
</head>
<body><input type="file" id="fileInput"><img id="previewImage"><script>document.getElementById('fileInput').addEventListener('change', function() {var file = this.files[0];var reader = new FileReader();reader.onload = function(e) {document.getElementById('previewImage').src = e.target.result;};reader.readAsDataURL(file);});</script>
</body>
</html>
2.使用HTML5的File API和Video标签来预览视频文件：
<!DOCTYPE html>
<html>
<head><title>Video Preview</title>
</head>
<body><input type="file" id="fileInput"><video id="previewVideo" controls></video><script>document.getElementById('fileInput').addEventListener('change', function() {var file = this.files[0];var reader = new FileReader();reader.onload = function(e) {document.getElementById('previewVideo').src = e.target.result;};reader.readAsDataURL(file);});</script>
</body>
</html>
3.使用第三方库如PDF.js来预览PDF文件：
<!DOCTYPE html>
<html>
<head><title>PDF Preview</title><script src="https://cdnjs.cloudflare.com/ajax/libs/pdf.js/2.8.335/pdf.min.js"></script>
</head>
<body><input type="file" id="fileInput"><iframe id="previewPDF" width="100%" height="500px"></iframe><script>document.getElementById('fileInput').addEventListener('change', function() {var file = this.files[0];var reader = new FileReader();reader.onload = function(e) {var fileData = new Uint8Array(e.target.result);pdfjsLib.getDocument({data: fileData}).promise.then(function(pdf) {pdf.getPage(1).then(function(page) {var scale = 1.5;var viewport = page.getViewport({ scale: scale });var canvas = document.createElement('canvas');var context = canvas.getContext('2d');canvas.height = viewport.height;canvas.width = viewport.width;var renderContext = {canvasContext: context,viewport: viewport};page.render(renderContext).promise.then(function() {document.getElementById('previewPDF').src = canvas.toDataURL();});});});};reader.readAsArrayBuffer(file);});</script>
</body>
</html>
)
(Leetcode) C++递归实现（超详细）)
