This is how I'd do it:

First rename your `errno.js` to `errno.json`:

```json
{
  "ERRNO_BADJSON": 1,
  "ERRNO_BADKEY": 2,
  "ERRNO_DURATION": 3,
  "ERRNO_BACKEND": 4,
  "ERRNO_EXPIRED": 5,
  "ERRNO_MISSING": 6,
  "ERRNO_INVALID_MSISDN": 7,
  "ERRNO_INVALID_CODE": 8
}
```

Then in `msisdn-gateway/index.js` keep using the very same code:

```js
var errors = require("./errno");
…
  sendError(res, 400, errors.ERRNO_MISSING, "Missing " + missingParams[0]);
```