HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>File validation</title>
  <style>
    body { font-family: system-ui, sans-serif; max-width: 32rem; margin: 2rem auto; padding: 0 1rem; }
    #status { margin-top: 1rem; padding: 0.75rem 1rem; border-radius: 8px; background: #f1f5f9; }
    #status.error { background: #fef2f2; color: #b91c1c; }
    #status.ok { background: #ecfdf5; color: #047857; }
  </style>
</head>
<body>
  <h1>Validate file</h1>
  <p>Max size: 1 MB. Allowed: plain text (.txt).</p>
  <input type="file" id="fileInput" accept=".txt,text/plain">
  <div id="status">Pick a file to validate.</div>
  <script>
    var MAX_BYTES = 1024 * 1024;

    function validateFile(file) {
      if (file.size > MAX_BYTES) {
        return "File is too large. Max size is 1 MB.";
      }
      if (!file.type.startsWith("text/") && !file.name.endsWith(".txt")) {
        return "Only plain text files are allowed.";
      }
      return null;
    }

    document.getElementById("fileInput").addEventListener("change", function (event) {
      var file = event.target.files[0];
      var status = document.getElementById("status");
      status.className = "";

      if (!file) {
        status.textContent = "No file selected.";
        return;
      }

      var error = validateFile(file);
      if (error) {
        status.textContent = error;
        status.className = "error";
        event.target.value = "";
        return;
      }

      status.textContent = "Valid file: " + file.name + " (" + file.size + " bytes)";
      status.className = "ok";
    });
  </script>
</body>
</html>

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JavaScript File Handling — validate size and type (Try It)