Ah. Yeah … changing the control value inside of a for() loop is … not recommended. 
Although, you’re right… You can manipulate the control variable in Blocks. Interesting. I wonder if TypeScript likes variables by reference. 
//% hidden prevents the code snippet from appearing in the toolbox when switching to a typing language. TypeScript and Python have their own ways to handle for() loops; no need to clutter the toolbox with redundant snippets.