Javascript 使用 let 时出现问题

Javascript issue when using let

我有以下 js 用于单元测试错误处理程序:

import assert from 'assert';
import deepClone from 'lodash.clonedeep';
import deepEqual from 'lodash.isequal';
import { spy } from 'sinon';
import errorHandler from './index';

function getValidError(constructor = SyntaxError) {
  let error = new constructor();
  error.status = 400;
  error.body = {};
  error.type = 'entity.parse.failed';
  return error;
}

describe('errorHandler', function() {
  let err;
  let req;
  let res;
  let next;
  let clonedRes;
  describe('When the error is not an instance of SyntaxError', function() {
    err = getValidError(Error);
    req = {};
    res = {};
    next = spy();
    clonedRes = deepClone(res);
    errorHandler(err, req, res, next);

    it('should not modify res', function() {
      assert(deepEqual(res, clonedRes));
    });

    it('should call next()', function() {
      assert(next.calledOnce);
    });
  });

  ...(#other test cases all similar to the first)

  describe('When the error is a SyntaxError, with a 400 status, has a `body` property set, and has type `entity.parse.failed`', function() {
    err = getValidError();
    req = {};
    let res = {
      status: spy(),
      set: spy(),
      json: spy()
    };
    let next = spy();
    errorHandler(err, req, res, next);

    it('should set res with a 400 status code', function() {
      assert(res.status.calledOnce);
      assert(res.status.calledWithExactly(400));
    });

    it('should set res with an application/json content-type header', function() {
      assert(res.set.calledOnce);
      assert(res.set.calledWithExactly('Content-Type', 'application/json'));
    });

    it('should set res.json with error code', function() {
      assert(res.json.calledOnce);
      assert(res.json.calledWithExactly({ message: 'Payload should be in JSON format' }));
    });
  });
});

请注意,在 'When the error is a SyntaxError...' 的描述块中,resnextclonedRes 前面有 let

如果没有这些前面的 let,我的测试就会失败。我不明白为什么我需要再次为这些添加 let,而不是为同一块中的 errreq 添加。谁能帮我解释一下?

在严格模式下(以及通常在适当的 linted 代码中),变量必须在赋值之前声明。此外,constlet 变量必须在一个块中声明 一次 ,不能再声明了。重新声明已经声明的 err(或任何其他变量)将引发错误,这就是为什么您应该在 describe('errorHandler' 函数中只看到一次 let <varname>

const describe = cb => cb();

let something;
describe(() => {
  something = 'foo';
});
let something;
describe(() => {
  something = 'bar';
});

describedescribe('errorHandler' 内的 describe('errorHandler' 已经具有对 err.

的范围访问权限

根本不先声明一个变量,以草率的方式分配它会导致它被分配给全局对象,这几乎总是不可取的 。例如:

// Accidentally implicitly referencing window.status, which can only be a string:

status = false;
if (status) {
  console.log('status is actually truthy!');
}

也就是说,尽可能缩小变量范围通常是个好主意 - 仅当您需要外部范围内的值时才分配给外部变量。考虑仅在分配给它们的 describe 内部声明变量,这有一个额外的好处,即允许您使用 const 而不是 let:

describe('When the error is not an instance of SyntaxError', function() {
  const err = getValidError(Error);
  const req = {};
  const res = {};
  const next = spy();
  const clonedRes = deepClone(res);
  errorHandler(err, req, res, next);
  // etc
});
// etc
describe('When the error is a SyntaxError, with a 400 status, has a `body` property set, and has type `entity.parse.failed`', function() {
  const err = getValidError();
  const req = {};
  const res = {
    status: spy(),
    set: spy(),
    json: spy()
  };
  const next = spy();
  // etc