underscore.js 0 －－ 101行

先上代码：

(function() {

  // Baseline setup
  // --------------

  // Establish the root object, `window` (`self`) in the browser, `global`
  // on the server, or `this` in some virtual machines. We use `self`
  // instead of `window` for `WebWorker` support.
  var root = typeof self == 'object' && self.self === self && self ||
            typeof global == 'object' && global.global === global && global ||
            this ||
            {};

  // Save the previous value of the `_` variable.
  var previousUnderscore = root._;

  // Save bytes in the minified (but not gzipped) version:
  var ArrayProto = Array.prototype, ObjProto = Object.prototype;
  var SymbolProto = typeof Symbol !== 'undefined' ? Symbol.prototype : null;

  // Create quick reference variables for speed access to core prototypes.
  var push = ArrayProto.push,
      slice = ArrayProto.slice,
      toString = ObjProto.toString,
      hasOwnProperty = ObjProto.hasOwnProperty;

  // All **ECMAScript 5** native function implementations that we hope to use
  // are declared here.
  var nativeIsArray = Array.isArray,
      nativeKeys = Object.keys,
      nativeCreate = Object.create;

  // Naked function reference for surrogate-prototype-swapping.
  var Ctor = function(){};

  // Create a safe reference to the Underscore object for use below.
  var _ = function(obj) {
    if (obj instanceof _) return obj;
    if (!(this instanceof _)) return new _(obj);
    this._wrapped = obj;
  };

  // Export the Underscore object for **Node.js**, with
  // backwards-compatibility for their old module API. If we're in
  // the browser, add `_` as a global object.
  // (`nodeType` is checked to ensure that `module`
  // and `exports` are not HTML elements.)
  if (typeof exports != 'undefined' && !exports.nodeType) {
    if (typeof module != 'undefined' && !module.nodeType && module.exports) {
      exports = module.exports = _;
    }
    exports._ = _;
  } else {
    root._ = _;
  }

  // Current version.
  _.VERSION = '1.8.3';

  // Internal function that returns an efficient (for current engines) version
  // of the passed-in callback, to be repeatedly applied in other Underscore
  // functions.
  var optimizeCb = function(func, context, argCount) {
    if (context === void 0) return func;
    switch (argCount) {
      case 1: return function(value) {
        return func.call(context, value);
      };
      // The 2-parameter case has been omitted only because no current consumers
      // made use of it.
      case null:
      case 3: return function(value, index, collection) {
        return func.call(context, value, index, collection);
      };
      case 4: return function(accumulator, value, index, collection) {
        return func.call(context, accumulator, value, index, collection);
      };
    }
    return function() {
      return func.apply(context, arguments);
    };
  };

  var builtinIteratee;

  // An internal function to generate callbacks that can be applied to each
  // element in a collection, returning the desired result — either `identity`,
  // an arbitrary callback, a property matcher, or a property accessor.
  var cb = function(value, context, argCount) {
    if (_.iteratee !== builtinIteratee) return _.iteratee(value, context);
    if (value == null) return _.identity;
    if (_.isFunction(value)) return optimizeCb(value, context, argCount);
    if (_.isObject(value) && !_.isArray(value)) return _.matcher(value);
    return _.property(value);
  };

读起underscore.js这一百行, 第一反应：

这什么？

于是还得一行行地看，underscore本身结构并不复杂，大致应该是

var _ = function (obj) {
}
_.attr1 = ...
_.attr2 = ...

所以先一行行看，反正源码上也有官方注释。

var root = typeof self == 'object' && self.self === self && self || typeof global == 'object' && global.global === global && global ||
            this ||
            {};

对应self ｜｜ window
对应node
对应this
都不对应则返回一个空对象

var previousUnderscore = root._;

  // Save bytes in the minified (but not gzipped) version:
  var ArrayProto = Array.prototype, ObjProto = Object.prototype;
  var SymbolProto = typeof Symbol !== 'undefined' ? Symbol.prototype : null;

  // Create quick reference variables for speed access to core prototypes.
  var push = ArrayProto.push,
      slice = ArrayProto.slice,
      toString = ObjProto.toString,
      hasOwnProperty = ObjProto.hasOwnProperty;

  // All **ECMAScript 5** native function implementations that we hope to use
  // are declared here.
  var nativeIsArray = Array.isArray,
      nativeKeys = Object.keys,
      nativeCreate = Object.create;

  // Naked function reference for surrogate-prototype-swapping.
  var Ctor = function(){};

注释确实强大

这样将所有原生方法定义可以免去日后繁杂的书写以及程序访问对象中属性的麻烦。

var _ = function(obj) {
    if (obj instanceof _) return obj;
    if (!(this instanceof _)) return new _(obj);
    this._wrapped = obj;
  };

开始建立underscore主体，第二个if有点类似于直接使用_()对其进行new操作,第一个则看其是否属于underscore对象。

// Export the Underscore object for **Node.js**, with
  // backwards-compatibility for their old module API. If we're in
  // the browser, add `_` as a global object.
  // (`nodeType` is checked to ensure that `module`
  // and `exports` are not HTML elements.)
  if (typeof exports != 'undefined' && !exports.nodeType) {
    if (typeof module != 'undefined' && !module.nodeType && module.exports) {
      exports = module.exports = _;
    }
    exports._ = _;
  } else {
    root._ = _;
  }

  // Current version.
  _.VERSION = '1.8.3';

  // Internal function that returns an efficient (for current engines) version
  // of the passed-in callback, to be repeatedly applied in other Underscore
  // functions.

注释非常清晰

 var optimizeCb = function(func, context, argCount) {
    if (context === void 0) return func;
    switch (argCount) {
      case 1: return function(value) {
        return func.call(context, value);
      };
      // The 2-parameter case has been omitted only because no current consumers
      // made use of it.
      case null:
      case 3: return function(value, index, collection) {
        return func.call(context, value, index, collection);
      };
      case 4: return function(accumulator, value, index, collection) {
        return func.call(context, accumulator, value, index, collection);
      };
    }
    return function() {
      return func.apply(context, arguments);
    };
  };

这个是用来优化函数做的，其中如果没有规定函数内部的context指向，则默认返回func

若指定contxt后，argCount则是用来限定该函数输入的参数有多少个，输入参数数量的不同对应之后不同的函数应用，如果超过，则超过的部分不会当作函数的传入参数。

如果懒，不输入argcount， 则会返回一个可输入任意数量大小的function然而
由于arguments对象与参数存在引用关系，因此没有argCount时得小心。

var builtinIteratee;

  // An internal function to generate callbacks that can be applied to each
  // element in a collection, returning the desired result — either `identity`,
  // an arbitrary callback, a property matcher, or a property accessor.
  var cb = function(value, context, argCount) {
    if (_.iteratee !== builtinIteratee) return _.iteratee(value, context);
    if (value == null) return _.identity;
    if (_.isFunction(value)) return optimizeCb(value, context, argCount);
    if (_.isObject(value) && !_.isArray(value)) return _.matcher(value);
    return _.property(value);
  };

此函数就是optmize的应用了，其中builtinIteratee作用在注释中

// External wrapper for our callback generator. Users may customize
  // `_.iteratee` if they want additional predicate/iteratee shorthand styles.
  // This abstraction hides the internal-only argCount argument.
  _.iteratee = builtinIteratee = function(value, context) {
    return cb(value, context, Infinity);
  };