I woke up a few weekends ago, after a night of working homework problems, and realized that I could probably fit a Turing machine into a tweet.... Sure enough, it works!

Here's a gist with an example program and many improvements:
https://gist.github.com/mrrrgn/3200044be3fd31f4c3b5

t=(c,k)=>{i=0;s='q0';while(true){p=c[s+k[i]];if(p[0]=='t'||p[0]=='f'){return p[0]};;k[i]=p[0];if(p[1]=='R'){i++}else{i--};s=p[2]+p[3];}};

— Morgan Phillips (@mrrrgn) August 9, 2015