Nested contexts for tests

contexts.js

describe('RedBlackTree', () => {
  beforeEach(() => {
    // Set-up code
  });
  
  it('does this', () => {
    // Asserts
  });
  
  it('does that', () => {
    // Asserts
  });
  
  describe('when empty', () => {
    beforeEach(() => {
      // Set-up code for the "empty" case
    });
    
    // it-s with asserts
  });
  
  describe('insertion', () => {
    // Some set-up with beforeEach
    
    describe('happy case', () => {
      // Additional set-up with beforeEach
      
      // Asserts
    });
    
    describe('when heavily unbalanced', () => {
      // Additional set-up with beforeEach
      
      // Asserts
    });
  });
  
  // ... and so on
});