Tips for golfing javascript code, especially converting while loop to expression
I think the best way to get rid of the curly brackets is to go recursive.



Original function (46 bytes)

t.s=_=&gt;{o="";while(i=t.o()){o+=i+","}return o}


Recursive version (29 bytes)

t.s=_=&gt;(i=t.o())?i+[,t.s()]:i


The i+[,t.s()] trick saves a byte over i+','+t.s().

Because you stop when t.o() returns null, the result of your original version always ends with a comma. In the new version, we actually append null at the end (that's the :i) but it's coerced to an empty string because of the above trick, leading to the same behavior.