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Create a file preview generator

Written by

Bernardo Palos

in

To create a file preview generator, I’ll assume you want a basic web-based preview tool that can display previews for common file types like images, PDFs, and text files in the browser. Here’s a simple example using HTML, JavaScript, and a bit of CSS to preview these files client-side without uploading them anywhere.

html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<title>File Preview Generator</title>
<style>
  body {
    font-family: Arial, sans-serif;
    padding: 20px;
  }
  #preview {
    margin-top: 20px;
    max-width: 100%;
    max-height: 500px;
    border: 1px solid #ccc;
    padding: 10px;
    overflow: auto;
  }
  img, iframe, pre {
    max-width: 100%;
    max-height: 500px;
  }
</style>
</head>
<body>

<h2>Select a file to preview</h2>
<input type="file" id="fileInput" />

<div id="preview"></div>

<script>
  const fileInput = document.getElementById('fileInput');
  const preview = document.getElementById('preview');

  fileInput.addEventListener('change', () => {
    const file = fileInput.files[0];
    if (!file) return;

    preview.innerHTML = ''; // Clear previous preview

    const fileType = file.type;

    if (fileType.startsWith('image/')) {
      // Preview image
      const img = document.createElement('img');
      img.src = URL.createObjectURL(file);
      img.onload = () => URL.revokeObjectURL(img.src); // Free memory
      preview.appendChild(img);

    } else if (fileType === 'application/pdf') {
      // Preview PDF
      const iframe = document.createElement('iframe');
      iframe.src = URL.createObjectURL(file);
      iframe.width = '100%';
      iframe.height = '500px';
      preview.appendChild(iframe);

    } else if (fileType.startsWith('text/') || fileType === '') {
      // Preview text file or files without mime type
      const reader = new FileReader();
      reader.onload = () => {
        const pre = document.createElement('pre');
        pre.textContent = reader.result;
        preview.appendChild(pre);
      };
      reader.readAsText(file);

    } else {
      preview.textContent = 'Preview not available for this file type.';
    }
  });
</script>

</body>
</html>

How it works:

  • User selects a file using the file input.

  • JavaScript detects the file type:

    • If it’s an image, shows it using an <img> tag.

    • If it’s a PDF, embeds it with an <iframe>.

    • If it’s a text file (or unknown mime but text-based), reads and displays the text content.

  • Other file types show a “Preview not available” message.

If you want a preview generator in a different environment (e.g., Python, Node.js backend, or desktop app), just let me know!

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