I recently ran into a problem where Emmet autocomplete would work for files ending with the .jsx extension but break break for files with the .js extension.

To solve it, do the following

1. Click on the settings icon on the bottom left corner.
   

2. Click on the document icon to access the settings json file
   

3. Add the following lines to the end of the settings.json file
   

"emmet.includeLanguages": {
    "javascript": "javascriptreact"
  },
  "emmet.triggerExpansionOnTab": true

and voila, Your autocomplete should work as expected.