l-system.ts

/**
 * LSystem implementation.
 * @see https://jsantell.com/l-systems.
 */
class LSystem<T> {
  constructor(
    private readonly productions: Map<T, T>,
    private readonly splitter: (serialized: T) => T[],
    private readonly serializer: (collection: T[]) => T
  ) { }
  
  *iterate(axiom: T, iterations: number) {
    while (iterations--) {
      axiom = this.serializer(this.splitter(axiom).map((item) => {
        return this.productions.get(item) || item;
      }));
      yield axiom;
    }
  }
}

const system = new LSystem(
  new Map([
    ['a', 'ab'],
    ['b', 'c'],
    ['c', 'ba']
  ]),
  (serialized) => serialized.split(''),
  (collection) => collection.join('')
);

for (const iteration of system.iterate('a', 10)) {
  console.log(iteration);
}

Nice!
Micro amendment: yield axiom at the very beginning if the *iterate() in order to see the seed as well.