turing-ish machine in js

turing-ish.js

const X_B = {
  "B,s1": ["X", "R", "s2"],
  "B,s2": ["B", "L", "s3"],
  "X,s3": ["B", "R", "s4"],
  "B,s4": ["B", "L", "s1"],
}

var tape = ["B", "B"],
    head = 0,
    state = "s1";

for(i = 0; i < 8; i++) {
  console.log("  " + state + ": " + tape.join(""));
  console.log("      " + Array(head).fill(" ").join("") + "^");

  var value = tape[head],
      key = [value, state].join(","),
      [next_value, next_action, next_state] = X_B[key];

  tape[head] = next_value;
  head += next_action == "R" ? 1 : -1;
  state = next_state;
}