a function to generate a one-time use passcode otp in typescript

index.ts
import * as crypto from 'crypto';

function generateOTP(secret: string, window: number = 30): string {
  const epoch = Math.round(Date.now() / 1000);
  const timeSteps = Math.floor(epoch / window);
  const timeBuffer = Buffer.alloc(8);
  for (let i = 7; i >= 0; i--) {
    timeBuffer.writeUInt8(timeSteps & 0xff, i);
    timeSteps >>= 8;
  }

  const hmac = crypto.createHmac('sha1', secret).update(timeBuffer).digest();
  const start = hmac[hmac.length - 1] & 0x0f;
  const codeData = hmac.slice(start, start + 4);

  const fullcode = codeData.readUInt32BE(0) & 0x7fffffff;
  const code = fullcode % 1000000;

  return code.toString().padStart(6, '0');
}
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