what is the best way to expand the javascript language

I want to use the javascript language Syntax Highlighting but i want to use my custom keyword to alternative the javascript language keyword and use my custom lint
I implement this with ‘overlayMode’ api in codemirror 5
what is the best way to implement this in codemirror6?

You could either add custom decorations on top of the play JavaScript parse tree (iterate the part of the document that’s visible and generate additional decorations for identifiers that match your keywords), or fork lang-javascript to make it properly parse your custom syntax.