javascript - CommonJS 模块模式

Question

这是我从

Flux architecture

var AppDispatcher = require('../dispatcher/AppDispatcher');
var EventEmitter = require('events').EventEmitter;
var TodoConstants = require('../constants/TodoConstants');
var assign = require('object-assign');

var CHANGE_EVENT = 'change';

var _todos = {}; // collection of todo items

/**
 * Create a TODO item.
 * @param {string} text The content of the TODO
 */
function create(text) {
  // Using the current timestamp in place of a real id.
  var id = Date.now();
  _todos[id] = {
    id: id,
    complete: false,
    text: text
  };
}

/**
 * Delete a TODO item.
 * @param {string} id
 */
function destroy(id) {
  delete _todos[id];
}

var TodoStore = assign({}, EventEmitter.prototype, {

  /**
   * Get the entire collection of TODOs.
   * @return {object}
   */
  getAll: function() {
    return _todos;
  },

  emitChange: function() {
    this.emit(CHANGE_EVENT);
  },

  /**
   * @param {function} callback
   */
  addChangeListener: function(callback) {
    this.on(CHANGE_EVENT, callback);
  },

  /**
   * @param {function} callback
   */
  removeChangeListener: function(callback) {
    this.removeListener(CHANGE_EVENT, callback);
  },

  dispatcherIndex: AppDispatcher.register(function(payload) {
    var action = payload.action;
    var text;

    switch(action.actionType) {
      case TodoConstants.TODO_CREATE:
        text = action.text.trim();
        if (text !== '') {
          create(text);
          TodoStore.emitChange();
        }
        break;

      case TodoConstants.TODO_DESTROY:
        destroy(action.id);
        TodoStore.emitChange();
        break;

      // add more cases for other actionTypes, like TODO_UPDATE, etc.
    }

    return true; // No errors. Needed by promise in Dispatcher.
  })

});

它说的地方

There are a few important things to note in the above code. To start, we are maintaining a private data structure called _todos. This object contains all the individual to-do items. Because this variable lives outside the class, but within the closure of the module, it remains private — it cannot be directly changed from outside of the module. This helps us preserve a distinct input/output interface for the flow of data by making it impossible to update the store without using an action.

粗体部分我不清楚。 js 解释器如何知道所有这些代码都在模块闭包内而不是在全局范围内？模块闭包从哪里开始，从哪里结束？

据我所知

The scope of a variable declared with var is its current execution context, which is either the enclosing function or, for variables declared outside any function, global.

有什么解释吗？

Answer 1

你实际上错过了你引用的摘录中的最后一行:

module.exports = TodoStore;

CommonJS 是一个 API，用于定义使用以下约定的模块:

• 每个文件定义一个模块并在隔离环境中执行；也就是说，它定义的变量在模块外部不可用。
• 为了允许导入其他模块，模块可以使用全局变量require，允许它导入其他模块
• 以同样的方式，变量 module 可用于模块，以便它可以设置其 exports 属性来定义模块应导出的内容；您在模块 a.js 中为 module.exports 设置的值正是 require('./a') 将返回的值。

每个实现 CommonJS 的 JS 环境都必须知道这些规则。这当然包括 Node.js，但也包括 Browserify 和 Webpack 等 bundler ，它们将打包您的代码，以便遵守这些约定。

这样，您可以控制要导出模块的哪一部分。

附言:请注意，您还可以使用 exports var 来定义导出，并且它的使用与 module.exports 略有不同。参见 this StackOverflow question and answer详情