In an OutSystems Reactive Application, we can use Client variables.
Client variables are used to store basic data types in reactive applications (they are also used in mobile applications). You can find more information in the Outsystems documentation.
The values of these variables are stored client-side, and thus they are stored in the browser. You can see their values by opening inspect in the browser. Then, click on the tab Application and expand Local Storage. Click on the name of your server, and you will be able to see your Client Variables:
Two other notes: