How to Generate Embed Token in pure JavaScript

Hi,

I am new to Power Bi and any help would be appreciated. So I want to do the following
I want to show reports in my Angular application that angular application is the multi-tenant application of users that do not belong to my organization. So I have next set up

My Angular app is talking whit my API that API is using the token-based system to create token and manage tenets. I want for my users to see reports that I created so my API will talk with Power BI API to get token and pass it to Angular App and angular application will emend report from power bi, I was wondering what is the best practices to do this. Let's I want to use "Master user" or "Service principal" to authenticate to Power BI, Do each time I generate token with my "Master user" I always get new token and old token is squished or old token is still valid until expiration time?