It is not possible to resolve conflicts of package-lock.json in GitHub's merge tool and you need to do a manual merge.
- Update the masterbranch with the latest changes:git checkout master git pull
- Merge your feature branch into master:
 You will see something like the following message:git merge mybranchAuto-merging package-lock.json CONFLICT (content): Merge conflict in package-lock.json Auto-merging package.json CONFLICT (content): Merge conflict in package.json Automatic merge failed; fix conflicts and then commit the result.
- Open your editor (e.g. VSCode) and:
- Carefully resolve conflicts in package.json(if there is any)
- Ignore the conflicts in package-lock.json
 
- Carefully resolve conflicts in 
- Install packages, which will re-generate package-lock.json:npm install
- "Test drive" your application to make sure the conflicts in package.jsonhave been resolved correctly.
- If the application is able to start up (i.e. there are no missing dependencies), add all changes and finish the merge:
git add --update git commit⚠️ Make sure not to commit the*.origfiles!
- If everything looks fine, push to GitHub:
git push
@DaveVodrazka I've also opened a feedback suggestion with the npm team which you may be interested in: npm/feedback#777.