How to use the tweetnacl/nacl-fast-light.js.scalarMult function in tweetnacl

async function genPrivateEphemeralKey(curve, V, Q, d) {
  if (d.length !== curve.payloadSize) {
    const privateKey = new Uint8Array(curve.payloadSize);
    privateKey.set(d, curve.payloadSize - d.length);
    d = privateKey;
  }
  switch (curve.type) {
    case 'curve25519': {
      const secretKey = d.slice().reverse();
      const sharedKey = nacl.scalarMult(secretKey, V.subarray(1));
      return { secretKey, sharedKey }; // Note: sharedKey is little-endian here, unlike below
    }
    case 'web':
      if (curve.web && util.getWebCrypto()) {
        try {
          return await webPrivateEphemeralKey(curve, V, Q, d);
        } catch (err) {
          util.print_debug_error(err);
        }
      }
      break;
    case 'node':
      return nodePrivateEphemeralKey(curve, V, d);
  }
  return ellipticPrivateEphemeralKey(curve, V, d);
}