UNIQUE Permutations ES6 Generators

permutations.js

const permutations = function*(elements, map = new Map()) {
    if (elements.length === 1) {
      yield elements;
    } else {
      const [first, ...rest] = elements;
      for (const perm of permutations(rest, map)) {
        for (let i = 0; i < elements.length; i++) {
          const start = perm.slice(0, i);
          const rest = perm.slice(i);
          const val = [...start, first, ...rest];
          const key = JSON.stringify(val);
          if (!map.has(key)) {
              map.set(key, val);
              yield val;
          } 
        }
      }
    }
  }

permutations.ts

export function* permutations<T>(
  elements: T[],
  map: Map<string, T> = new Map(),
  serializer = (val: T[]) => JSON.stringify(val)
) {
  if (elements.length === 1) {
    yield elements;
  } else {
    const [first, ...rest] = elements;
    for (const perm of permutations(rest, map, serializer)) {
      for (let i = 0; i < elements.length; i++) {
        const start = perm.slice(0, i);
        const rest = perm.slice(i);
        const val = [...start, first, ...rest];
        const key = serializer(val);
        if (!map.has(key)) {
          map.set(key, val);
          yield val;
        }
      }
    }
  }
}