This is the best, I was struggling with replacing the newly replaced letters, but this case trick does the job!

how did you come up with this?
love it

Didn't expect to see something like this, love it!

Love this solution⭐️⭐️⭐️⭐️⭐️

great readability!

wow genial!

Ok, this was smart!

respect

Really like this trick with lowerCase

Cool, thank you very much for explaining, I get it now.

Because you will overwrite prior replaces. Anything that is lowercase is the new (already replaced/processed) value, and the caps values still need replacing/processing.

example:

function DNAStrand(dna){
  return dna.replace(/A/g, 'T').replace(/T/g, 'A').replace(/C/g, 'G').replace(/G/g, 'C');
}
DNAStrand("AAAA")

The first replace turns "AAAA" into "TTTT" then the second replace turns it right back into "AAAA"

the same thought as you