t.js

var assert = require('assert');
describe('hoge', function() {

  beforeEach(function() {
    this.foo = 1;
    this.bar = 5;
    THIS = this;
  });

  it('first', function() {
    assert.equal(1, this.foo);
  });

  describe('child scope', function() {

    beforeEach(function() {
      this.foo = 5;
    });

    it('first child', function() {
      assert.equal(this.bar, this.foo);
    });

    it('is prototype', function() {
      assert.equal(THIS, this.__proto__);
    });
  });

  describe('child scope 2', function() {

    it('last child', function() {
      assert.equal(1, this.foo);
    });
  });
});