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Since you are using implicit flow, azure AD will not return refresh token in implicit flow for you to refresh the token. As per this documentation, here is the recommended approach for this.
A JavaScript application has another mechanism at its disposal for
renewing access tokens without repeatedly prompting the user for
credentials. The application can use a hidden iframe to perform new
token requests against the authorization endpoint of Azure AD: as long
as the browser still has an active session (read: has a session
cookie) against the Azure AD domain, the authentication request can
successfully occur without any need for user interaction.
Source:stackexchange.com