How do I implement a cycle-through array with a generator function

You need a loop in your generator code, otherwise there is only one yield happening:

function* stepGen(steps){
  let index = 0;
  while (true) {
    yield steps[index];
    index = (index+1)%steps.length;
  }
}

let gen = stepGen(['one', 'two', 'three']); // pass array to make it more reusable
console.log(gen.next().value); 
console.log(gen.next().value);
console.log(gen.next().value);
console.log(gen.next().value);
console.log(gen.next().value);

Alternatively you can also use yield* which yields values from an iterable, one by one:

function* stepGen(steps){
  while (true) yield* steps;
}

let gen = stepGen(['one', 'two', 'three']);
console.log(gen.next().value); 
console.log(gen.next().value);
console.log(gen.next().value);
console.log(gen.next().value);
console.log(gen.next().value);