SPFX- Using Custom JS in SPFX

Hi,

 

I have created a footer using custom js which fetches all the terms inside the termstore and displays it in the footer for the page and i have referenced the custom js and css files in the custom masterpage for Sharepoint online. This works well for classic pages. How can i use the js for creating the footer for modern pages using SPFX. I have created a basic footer using the link:

 

https://docs.microsoft.com/en-us/sharepoint/dev/spfx/extensions/get-started/using-page-placeholder-with-extensions

 

How can i enhance it to use the custom js to get terms from the termset and display it in the footer.

 

Please help...