refactor: replace deprecated String.prototype.substr()

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What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update [x] Bug fix [ ] New feature [ ] Other, please explain:

What changes did you make? (Give an overview)

String.prototype.substr() is deprecated so we replace it with String.prototype.slice() which works similarily but isn't deprecated. .substr() probably isn't going away anytime soon but the change is trivial so it doesn't hurt to do it.

Which issue (if any) does this pull request address?

Is there anything you'd like reviewers to focus on?

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