chore: replace deprecated String.prototype.substr()

[CommanderRoot]

What:

Replacing usage of .substr() with .slice()

Why:

String.prototype.substr() is deprecated.

Checklist:

• [ ] Documentation N/A
• [ ] Added/updated unit tests N/A
• [x] Code complete

[iChenLei]

Thanks for your contribution, @matthew-dean sir, what'r you opinion ? Do we need replace substr with slice now? (To be honest, I never use substr on my own code.

[matthew-dean]

Oh interesting, I had no idea it was deprecated. However, I'm wary of this drop-in replace with .slice. The MDN document doesn't say that it's a drop-in replacement, and gives a different polyfill.