@wallinglee we saw a similar question when 6.0.0 of the driver was released (MongoDB NodeJS Driver 6.0.0 Released - #3 by Warren_James) .
The nullish coalescing assignment has been supported since Node.js 15, so it would be worth verifying that you’re indeed running your code in the version of the runtime you’ve indicated.
If you add a console.log(process.version) to the code this should be fairly easy to validate.