# Pairing with custom views and using events to verify syntax of a cron string
**URL:** https://community.homey.app/t/pairing-with-custom-views-and-using-events-to-verify-syntax-of-a-cron-string/114264
**Category:** Developers
**Created:** [August 12, 2024, 9:58am UTC](https://community.homey.app/t/pairing-with-custom-views-and-using-events-to-verify-syntax-of-a-cron-string/114264 "2024-08-12T09:58:00Z")
**Posts on this page:** 14
**Page:** 1
I’m in the process of improving syntax checking in my app and eas planning to call the backend to wih Homey.emit() to verify crontab syntax of the input from user while adding new device.
main-driver has:
>
which is called perfectly from the custom pairing view:
>
The problem is, that the result is not returned. There is nothing output to the browser developer console.
There is propably something obvious I’m not seeing anymore.
On an side note. Homey.done at
>
doesn’t seem to get called as it should. At last how Homey SDK documentations says it should. What I get, is that the “configure” view is just loaded again and doesn’t close the pairing session.
---
Works fine for me:
I assume that the log line is executed?
```auto
this.log(`testCronTime: ${cronTime}`);
```
---
I would advise rewriting your code so that it uses `try/catch` and `async/await` instead of `.then().catch()`, or at least move [this entire block of code](https://github.com/oh2th/fi.oh2th.alarmutils/blob/c2e8bbdabf8782f993aaf43c62c382ea9bc072af/drivers/crontime/pair/configure.html#L55-L73) to _inside_ the `then()` handler, right [after this line](https://github.com/oh2th/fi.oh2th.alarmutils/blob/c2e8bbdabf8782f993aaf43c62c382ea9bc072af/drivers/crontime/pair/configure.html#L52).
---
The issue isn’t necessarily with `try/catch`, the issue is with the wrong use of promises. Using `async/await` would make the code easier to understand, but you’d then need to use `try/catch` instead of `.catch()`.
---
Ah, right. Just following the SDK sample in this one. Though the samples sometimes are also wrong.
So the try/catch in main-driver.js is fine, just need to adjust the pairing configure.html.
---
Changed to try/catch and moved createDevice into try-block.
Now I’m seeing random success when crontime passes the test afd the device is created. However Homey.done() is not closing the session / window as everyhting should be ready now.
```auto
2024-08-13T07:19:55.706Z [log] [ManagerDrivers] [Driver:crontime] testCronTime: 0 13 * * *
2024-08-13T07:19:55.707Z [log] [ManagerDrivers] [Driver:crontime] onPair - testCronTime - pass: 0 13 * * *
2024-08-13T07:19:56.257Z [log] [ManagerDrivers] [Driver:crontime] View: configure
2024-08-13T07:19:57.138Z [log] [ManagerDrivers] [Driver:crontime] testCronTime: 0 13 * * *
2024-08-13T07:19:57.138Z [log] [ManagerDrivers] [Driver:crontime] onPair - testCronTime - pass: 0 13 * * *
2024-08-13T07:19:57.732Z [log] [ManagerDrivers] [Driver:crontime] View: configure
2024-08-13T07:19:58.581Z [log] [ManagerDrivers] [Driver:crontime] testCronTime: 0 13 * * *
2024-08-13T07:19:58.584Z [log] [ManagerDrivers] [Driver:crontime] onPair - testCronTime - pass: 0 13 * * *
2024-08-13T07:19:58.774Z [log] [ManagerDrivers] [Driver:crontime] [Device:b3ddad9d-3dc9-4a82-b860-a69532f935e7] Crontime Ye914RyB - onInit
```
Several times testCronTime pass log entries, and then finally also create the device. only then is also console.log for Crontime is valid printed to Chrome devtools console.
When there is failed testCronTime, no error is reported either in dev console, nor the Homey.alert given. But Console is reporting:
---
I’m not sure if exceptions thrown inside the event handler in the backend are actually propagated to the frontend.
I would suggest something like this:
```auto
// backend
try {
cronParser.parseExpression(cronTime);
this.log(`onPair - testCronTime - pass: ${cronTime}`);
return { success : true, time : cronTime };
} catch (err) {
this.error(`onPair - testCronTime - error: ${err.message}`);
return { success : false };
}
// frontend
const response = await Homey.emit('testCronTime', time);
if (! response.success) { throw Error("...") }
```
Then when creating the device, you need to `await` the call to `createDevice` to wait until it’s finished before continuing:
```auto
await Homey.createDevice(...);
Homey.done();
```
---
When I start the app I immediately get an error:
```auto
2024-08-14T07:12:57.819Z [log] [ManagerDrivers] [Driver:scheduler] onInit - Driver initialized
unhandledRejection:
TypeError: source.toLowerCase is not a function
at CronTime._parse (/app/node_modules/cron/dist/time.js:360:25)
at new CronTime (/app/node_modules/cron/dist/time.js:37:18)
at new CronJob (/app/node_modules/cron/dist/job.js:25:29)
at AlarmUtils.initAppCronJob (/app/app.js:37:23)
at AlarmUtils.onInit (/app/app.js:21:10)
at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
at async SDK._initApp (/node_modules/@athombv/homey-apps-sdk-v3/lib/SDK.js:250:7)
at async SDK.createClient (/node_modules/@athombv/homey-apps-sdk-v3/lib/SDK.js:132:7)
```
But the main issue is with the HTML of the pairing page: if you don’t set the type of a `BUTTON` element, it defaults to being a submit button, and pressing it reloads the page (like submitting a form).
If you explicitly set the type to `button` it will not do this:
```auto