In a single page app, all of the decisions about what view/subview to render occurs on the client and does not require a trip back to the server.

This means that the client has to be able to authenticate the currently logged in user and access its data without going back to the server which means it has to be stored locally.

JWTs are good because they allow for the client to be responsible for keeping track of the permissions of the currently logged in user.

However JWTs require a secret key to be decrypted which means it can't happen on the frontend with the same key that the server uses, say for its csrf protection

Because of this it's clear that we need to have unencrypted way of viewing the local authentication data for use by the application logic

Special care needs to be made to prevent someone interacting with the developers console to be able to change the local authentication data in order to gain access to restricted parts of the code by elevating their permissions

Since we have to at least talk to the server once in order to login in and authenticate the credentials, we can save a copy of the authentication information in a way that prevents it from being easily changed by the console.

Closures to the rescue!

Create a function that is available globally in the success handler of the xhr request which will scope the value of the data returned by the server in the closure. The global scope of the closure makes the unencrypted authentication data readable outside that request and "safely" in memory in the sense that a malicious user can not modify it. 

If you wanted to add it to some kind of store that was accessible to the developer console (like redux) then we could not prevent them from easily modifying the data. However, hope is not lost. We can let them modify the data so long as we have some way of invalidating it afterwards.

In that case, we need some way of comparing it to a known valid state corresponding to the logged in user which we create when the user logs in and verifying that it is the same

In most cases, to achieve persistence, an additional request is required when the user logs in which decrypts the JWT. The server would respond with the authentication information which is in turn stored locally. This data is protected by the above algorithm since changing the value of the data used to authenticate invalidates it when comparing to the source of truth.

Authenticating the current user for the role admin then looks something like return isEqual(currentAuthInfo, sourceOfTruth) && intersection(auth.roles, [targetRoles])).length targetRoles)).length  > 0