In order to be able to send transactional, or user-specific notifications to your users, you will need a way to identify which device belongs to each which user.
The nativeFunctions.onesignalSetExternalUserId() function allows you to assign a custom ID to the device in OneSignal, which can be the same ID used on your website to identify users, making it very easy for you to know to whom a specific device belongs, and allowing you to send user-specific notifications to that user.
We recommend triggering this function whenever you perform a check to see if the user is logged-in, so you can assign his user ID to OneSignal accordingly, here is an example of how to implement the Javascript code:
<script type="text/javascript">
// Listen to events
window.addEventListener("message", (event) => {
runNativeFunctions(event);
}, false);
// Get the user ID
var userId = "1";
// Function that runs when the event is triggered
function runNativeFunctions(event) {
try {
// Check if the native functions are ready to be used
if (event.data && event.data == 'nativeFunctionsLoaded') {
// Native function can be called here
nativeFunctions.onesignalSetExternalUserId($userId);
}
} catch (ex) {
// Log any error messages for easier debugging
console.log(ex.message;);
}
}
</script>
OneSignal has a nice guide with more details on how to send those push notifications using the OneSignal REST API, you can find it here: https://documentation.onesignal.com/docs/transactional-messages
