That sounds like you will want to start with https://www.microsoft.com/en-us/learning/mcsa-web-applications-certification.aspx then move on to https://www.microsoft.com/en-us/learning/mcsd-app-builder-certification.aspx. Depending on you level of experience you may actually want to start with https://www.microsoft.com/en-us/learning/mta-summary-certification.aspx 

 

Microsoft has retired their SharePoint specific developer exams. 

 

There are many good free courses at https://mva.microsoft.com/  and @Andrew Connell has a great course on SPfX at https://www.voitanos.io/ 

 

You will also want to get familiar with the Patterns and Practice guidance (PnP) at https://dev.office.com/patterns-and-practices

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