A mental exercise in testing all the permutations of a single line which I didn't complete

jest-test-permutations.ts

export interface ITestCaseSubject {
  subject: string;
}
export interface IEachTestCase extends ITestCaseSubject {
  each: Array<() => any>;
}

export type ArrayMoreThanOne<T> = [T, T, ...T[]];
export function testPermutations(subjectCases: ArrayMoreThanOne<IEachTestCase>) {
  function traverseAll(
    fn: (subject: string, permutation: () => string, subjectIndex: number, permutationIndex: number) => void,
  ): void {
    for (let subjectIndex = 0; subjectIndex < subjectCases.length; subjectIndex++) {
      const subjectCase = subjectCases[subjectIndex];
      const permutations = subjectCase.each;
      for (let permutationIndex = 0; permutationIndex < permutations.length; permutationIndex++) {
        fn(subjectCase.subject, permutations[permutationIndex], subjectIndex, permutationIndex);
      }
    }
  }
  function traverseAllBut(
    skipSubjectIndex: number,
    skipPermutationIndex: number,
    fn: (subject: string, permutation: () => string, subjectIndex: number, permutationIndex: number) => void,
  ): void {
    for (let subjectIndex = 0; subjectIndex < subjectCases.length; subjectIndex++) {
      if (subjectIndex === skipSubjectIndex) continue;
      const subjectCase = subjectCases[subjectIndex];
      const permutations = subjectCase.each;
      for (let permutationIndex = 0; permutationIndex < permutations.length; permutationIndex++) {
        if (permutationIndex === skipPermutationIndex) continue;
        fn(subjectCase.subject, permutations[permutationIndex], subjectIndex, permutationIndex);
      }
    }
  }
  return (runTest: () => void | Promise<void>) => {
    let whenDescription: string[];
    beforeEach(() => {
      whenDescription = [];
      traverseAll((subject, permutation, subjectIndex, permutationIndex) => {
        traverseAllBut(subjectIndex, permutationIndex, (relatedSubject, relatedPermutation) => {
          whenDescription.push(`${relatedSubject} = ${JSON.stringify(relatedPermutation())}`);
        });
      });
    });
    // describe(`when ${whenDescription.join(", ")} and ${subject} = ${JSON.stringify(permutation())}`, () => {
    //   beforeEach(permutation);
    //   runTest();
    //   // it("returns true", async () => {
    //   //   expect(await runTest()).toBe(true);
    //   // });
    // });
  };
}