The plugin API enables you to extract some data from Front, always remember that these data are relative to the teammate using the plugin, if teammate A does not have access to inbox B, then they will not see inbox B when calling fetchInboxes().
Fetching data through the plugin API is asynchronous, this means that you will need to provide the Javascript library with a callback every time you want to retrieve data.
Please also note that the data you extract from these calls is just a snapshot of Front's state when you asked for it, any updates happening afterward will not update the data you received nor call your callback again.