Web API Authentication from JavaScript

I've implemented web API authentication using this link "https://msdn.microsoft.com/en-us/library/mt595797.aspx" and it's working fine.

But my question is, what I have to do if I want to skip the (“authContext.login();”  or redirection process )part which will take me to the dynamics 365 login page?

To rephrase the question, I want to pass the credential using  javascript web API service so that I can skip the manual login part and continue doing CRUD operations with CRM. How can I achieve this?