将mocha.js和chai.js基本但正确地使用beforeEach（）或afterEach（）

我想使用mocha / chai测试与二进制搜索树相关的代码。在这里，我正在测试公共insert方法。我想在每个语句之前使用beforeEach()和/或afterEach()挂钩重置测试环境，it()这样我就不必完全重复基础知识。但是，我不断收到各种错误。

规格

describe("BinarySearchTree insert function", function() {

  beforeEach(function() {
    var binarySearchTree = new BinarySearchTree();
    binarySearchTree.insert(5);
  });

  it("creates a root node with value equal to the first inserted value", function () {
    expect(binarySearchTree.root.value).to.equal(5);
  });

  it("has a size equal to the amount of inserted values", function () {
    binarySearchTree.insert(3);
    expect(binarySearchTree.size).to.equal(2);
  });

  it("returns an error for non-unique values", function () {
    binarySearchTree.insert(3);
    expect(binarySearchTree.insert(3)).to.throw(String);
  });

  it("if inserted value is larger than current node, make or descend to rightChild", function () {
    binarySearchTree.insert(3);
    binarySearchTree.insert(10);
    binarySearchTree.insert(7);
    expect(binarySearchTree.root.rightChild.value).to.equal(10);
  });

});

错误： ReferenceError: binarySearchTree is not defined

实际上，在没有afterEach()重置测试环境之前，我曾预料会出现错误，这不是因为binarySearchTree未定义。如果可能的话，我想仅使用Mocha和Chai（而不是Sinon等其他软件包）来完成此任务。

经过测试的代码

exports.Node = Node;

function Node(value) {
  this.value = value;
  this.leftChild = null;
  this.rightChild = null;
}

exports.BinarySearchTree = BinarySearchTree;

function BinarySearchTree() {
  this.root = null;
  this.size = 0;
}

BinarySearchTree.prototype.insert = function(value) {
  // 1) when root node is already instantiated
  if (this.root === null) {
    // tree is empty
    this.root = new Node(value);
    this.size++;
  } else {
  // 2) nodes are already inserted
    var findAndInsert = function (currentNode) {
      if (value === currentNode.value) {
        throw new Error('must be a unique value');
      }
      // base case
      if (value > currentNode.value) {
        // belongs in rightChild
        if (currentNode.rightChild === null) {
          currentNode.rightChild = new Node(value);
        } else {
          findAndInsert(currentNode.rightChild);
        }
      } else if (value < currentNode.value) {
        // belongs in leftChild
        if (currentNode.leftChild === null) {
          currentNode.leftChild = new Node(value);
        } else {
          findAndInsert(currentNode.leftChild);
        }
      }
    };
    findAndInsert(this.root);
    this.size++;
  }
};

奖励问题...我不确定是否正确测试了抛出的错误（当插入非唯一值时）？