这是一个只有三行的js库

/**
 * Require the given path.
 *
 * @param {String} path
 * @return {Object} exports
 * @api public
 */
function require(path, parent, orig) { //全局require方法支持
  var resolved = require.resolve(path); //是否已注册过?
  // lookup failed
  if (null == resolved) { //没有注册直接抛出异常
    orig = orig || path;
    parent = parent || 'root';
    var err = new Error('Failed to require "' + orig + '" from "' + parent + '"');
    err.path = orig;
    err.parent = parent;
    err.require = true;
    throw err;
  }
  var module = require.modules[resolved]; //module指向注册过的模块
  // perform real require()
  // by invoking the module's
  // registered function
  if (!module.exports) { //如果模块没有暴露出接口
    module.exports = {};
    module.client = module.component = true;
    module.call(this, module.exports, require.relative(resolved), module); //执行模块, 暴露接口
  }
  return module.exports; //require最后返回该模块的全部接口
}

/**
 * Registered modules.
 */
require.modules = {}; //用于存储注册的模块

/**
 * Registered aliases.
 */
require.aliases = {}; //用户存储模块依赖

/**
 * Resolve `path`.
 *
 * Lookup:
 *
 *   - PATH/index.js
 *   - PATH.js
 *   - PATH
 *
 * @param {String} path
 * @return {String} path or null
 * @api private
 */
require.resolve = function(path) { //用来查找该名字的模块是否已注册
  if (path.charAt(0) === '/') path = path.slice(1);
  var index = path + '/index.js';
  var paths = [ //这个路径有如下五种查找方式, 都会指向同一个模块
    path,
    path + '.js',
    path + '.json',
    path + '/index.js',
    path + '/index.json'
  ];
  for (var i = 0; i < paths.length; i++) {
    var path = paths[i];
    if (require.modules.hasOwnProperty(path)) return path; //存在即返回路径
  }
  if (require.aliases.hasOwnProperty(index)) {
    return require.aliases[index]; //或返回模块
  }
};

/**
 * Normalize `path` relative to the current path.
 *
 * @param {String} curr
 * @param {String} path
 * @return {String}
 * @api private
 */
require.normalize = function(curr, path) { //
  var segs = [];
  if ('.' != path.charAt(0)) return path;
  curr = curr.split('/');
  path = path.split('/');
  for (var i = 0; i < path.length; ++i) {
    if ('..' == path[i]) {
      curr.pop();
    } else if ('.' != path[i] && '' != path[i]) {
      segs.push(path[i]);
    }
  }
  return curr.concat(segs).join('/');
};

/**
 * Register module at `path` with callback `definition`.
 *
 * @param {String} path
 * @param {Function} definition
 * @api private
 */
require.register = function(path, definition) { //用于注册path
  require.modules[path] = definition;
};

/**
 * Alias a module definition.
 *
 * @param {String} from
 * @param {String} to
 * @api private
 */
require.alias = function(from, to) { //用于注册依赖
  if (!require.modules.hasOwnProperty(from)) {
    throw new Error('Failed to alias "' + from + '", it does not exist');
  }
  require.aliases[to] = from;
};

/**
 * Return a require function relative to the `parent` path.
 *
 * @param {String} parent
 * @return {Function}
 * @api private
 */
require.relative = function(parent) {
  var p = require.normalize(parent, '..');
  /**
   * lastIndexOf helper.
   */
  function lastIndexOf(arr, obj) {
    var i = arr.length;
    while (i--) {
      if (arr[i] === obj) return i;
    }
    return -1;
  }

  /**
   * The relative require() itself.
   */
  function localRequire(path) {
    var resolved = localRequire.resolve(path);
    return require(resolved, parent, path);
  }

  /**
   * Resolve relative to the parent.
   */
  localRequire.resolve = function(path) {
    var c = path.charAt(0);
    if ('/' == c) return path.slice(1);
    if ('.' == c) return require.normalize(p, path);
    // resolve deps by returning
    // the dep in the nearest "deps"
    // directory
    var segs = parent.split('/');
    var i = lastIndexOf(segs, 'deps') + 1;
    if (!i) i = 0;
    path = segs.slice(0, i + 1).join('/') + '/deps/' + path;
    return path;
  };

  /**
   * Check if module is defined at `path`.
   */
  localRequire.exists = function(path) {
    return require.modules.hasOwnProperty(localRequire.resolve(path));
  };
  return localRequire;
};
//将该模块引入浏览器的基础, 通过这个名字注册模块进require.module里面
require.register("isarray/index.js", function(exports, require, module){
module.exports = Array.isArray || function (arr) {
  return Object.prototype.toString.call(arr) == '[object Array]';
};
//描述依赖
require.alias("isarray/index.js", "isarray/index.js");