underscore源码解析（object）

//用来变对象的。可以没有value直接传list,把list变一个对象
//也可以传长度和list一样的value，来构成数组
_.object = function(list, values) {
    var result = {};
    for (var i = 0, length = list && list.length; i < length; i++) {
        if (values) {
            result[list[i]] = values[i];
        } else {
            result[list[i][0]] = list[i][1];
        }
    }
    return result;
};

//获取对象的所有键集合，有原生的nativeKeys,就用原生。
//貌似ie9以下有个枚举bug，实例的toString会覆盖原型上的toString,toString就变成不可枚举了,都没试出来过。。。。
//!({toString: null}).propertyIsEnumerable('toString');
_.keys = function(obj) {
    if (!_.isObject(obj)) return [];
    if (nativeKeys) return nativeKeys(obj);
    var keys = [];
    for (var key in obj) if (_.has(obj, key)) keys.push(key);
    // Ahem, IE < 9.
    if (hasEnumBug) collectNonEnumProps(obj, keys);
    return keys;
};


// 所有键列出来，实例和原型的
_.allKeys = function(obj) {
    if (!_.isObject(obj)) return [];
    var keys = [];
    for (var key in obj) keys.push(key);
    // Ahem, IE < 9.
    if (hasEnumBug) collectNonEnumProps(obj, keys);
    return keys;
};

// 实例键的值组成一个数组
_.values = function(obj) {
    var keys = _.keys(obj);
    var length = keys.length;
    var values = Array(length);
    for (var i = 0; i < length; i++) {
        values[i] = obj[keys[i]];
    }
    return values;
};

//类似map,只不过obj是对象，用iteratee迭代每个键值对，返回新数组
_.mapObject = function(obj, iteratee, context) {
    iteratee = cb(iteratee, context);
    var keys =  _.keys(obj),
        length = keys.length,
        results = {},
        currentKey;
    for (var index = 0; index < length; index++) {
        currentKey = keys[index];
        results[currentKey] = iteratee(obj[currentKey], currentKey, obj);
    }
    return results;
};

//把一个对象转变为一个[key, value]形式的数组。
_.pairs = function(obj) {
    var keys = _.keys(obj);
    var length = keys.length;
    var pairs = Array(length);
    for (var i = 0; i < length; i++) {
        pairs[i] = [keys[i], obj[keys[i]]];
    }
    return pairs;
};

// 对象键值对互换位置，要保证键值对中值的唯一性
_.invert = function(obj) {
    var result = {};
    var keys = _.keys(obj);
    for (var i = 0, length = keys.length; i < length; i++) {
        result[obj[keys[i]]] = keys[i];
    }
    return result;
};
//把对象中的函数堆到一个数组中，函数名按字母a-z排序
_.functions = _.methods = function(obj) {
    var names = [];
    for (var key in obj) {
        if (_.isFunction(obj[key])) names.push(key);
    }
    return names.sort();
};
//对象的扩展，扩展传入的所有属性，包括原型链
_.extend = createAssigner(_.allKeys);

////对象的扩展，扩展传入的所有属性，不包括原型链
_.extendOwn = _.assign = createAssigner(_.keys);

// Return a copy of the object only containing the whitelisted properties.
//返回obj,包含符合条件的键值对
_.pick = function(object, oiteratee, context) {
    var result = {}, obj = object, iteratee, keys;
    if (obj == null) return result;
    if (_.isFunction(oiteratee)) {
        keys = _.allKeys(obj);
        iteratee = optimizeCb(oiteratee, context);
    } else {
        keys = flatten(arguments, false, false, 1);
        iteratee = function(value, key, obj) { return key in obj; };
        obj = Object(obj);
    }
    for (var i = 0, length = keys.length; i < length; i++) {
        var key = keys[i];
        var value = obj[key];
        if (iteratee(value, key, obj)) result[key] = value;
    }
    return result;
};

// Return a copy of the object without the blacklisted properties.
//返回obj,过滤掉符合条件的键值对
_.omit = function(obj, iteratee, context) {
    if (_.isFunction(iteratee)) {
        iteratee = _.negate(iteratee);
    } else {
        var keys = _.map(flatten(arguments, false, false, 1), String);
        iteratee = function(value, key) {
            return !_.contains(keys, key);
        };
    }
    return _.pick(obj, iteratee, context);
};

// Fill in a given object with default properties.
//对象扩展，对象的键是undefined的话就填充，否则就保留原来的
_.defaults = createAssigner(_.allKeys, true);

// Creates an object that inherits from the given prototype object.
// If additional properties are provided then they will be added to the
// created object.
_.create = function(prototype, props) {
    var result = baseCreate(prototype);
    if (props) _.extendOwn(result, props);
    return result;
};

// Create a (shallow-cloned) duplicate of an object.
//克隆对象或数组
_.clone = function(obj) {
    if (!_.isObject(obj)) return obj;
    return _.isArray(obj) ? obj.slice() : _.extend({}, obj);
};

// Invokes interceptor with the obj, and then returns obj.
// The primary purpose of this method is to "tap into" a method chain, in
// order to perform operations on intermediate results within the chain.
//调用interceptor，再传惨obj。返回obj
_.tap = function(obj, interceptor) {
    interceptor(obj);
    return obj;
};

// Returns whether an object has a given set of `key:value` pairs.
//传的键值对否在obj当中
_.isMatch = function(object, attrs) {
    var keys = _.keys(attrs), length = keys.length;
    if (object == null) return !length;
    var obj = Object(object);
    for (var i = 0; i < length; i++) {
        var key = keys[i];
        if (attrs[key] !== obj[key] || !(key in obj)) return false;
    }
    return true;
};

// Perform a deep comparison to check if two objects are equal.
//2个对象是否相等
_.isEqual = function(a, b) {
    return eq(a, b);
};

// Is a given array, string, or object empty?
// An "empty" object has no enumerable own-properties.
//传的东西是否为空
_.isEmpty = function(obj) {
    if (obj == null) return true;
    if (isArrayLike(obj) && (_.isArray(obj) || _.isString(obj) || _.isArguments(obj))) return obj.length === 0;
    return _.keys(obj).length === 0;
};

// Is a given value a DOM element?
//是不是DOM元素
_.isElement = function(obj) {
    return !!(obj && obj.nodeType === 1);
};

// Is a given value an array?
// Delegates to ECMA5's native Array.isArray
//是不是数组
_.isArray = nativeIsArray || function(obj) {
        return toString.call(obj) === '[object Array]';
    };

// Is a given variable an object?
//是不是对象
_.isObject = function(obj) {
    var type = typeof obj;
    return type === 'function' || type === 'object' && !!obj;
};

// Add some isType methods: isArguments, isFunction, isString, isNumber, isDate, isRegExp, isError.
//对每个原生类调用tostring方法，判断是否是原生对象
_.each(['Arguments', 'Function', 'String', 'Number', 'Date', 'RegExp', 'Error'], function(name) {
    _['is' + name] = function(obj) {
        return toString.call(obj) === '[object ' + name + ']';
    };
});

// Define a fallback version of the method in browsers (ahem, IE < 9), where
// there isn't any inspectable "Arguments" type.
//参数对象有个callee属性，指向函数本身，用来判断是不是arguments
if (!_.isArguments(arguments)) {
    _.isArguments = function(obj) {
        return _.has(obj, 'callee');
    };
}

// Optimize `isFunction` if appropriate. Work around some typeof bugs in old v8,
// IE 11 (#1621), and in Safari 8 (#1929).
// 另一种方法用Object.prototype.toString.call(obj)==='[object Function]';
if (typeof /./ != 'function' && typeof Int8Array != 'object') {
    _.isFunction = function(obj) {
        return typeof obj == 'function' || false;
    };
}

// Is a given object a finite number?
_.isFinite = function(obj) {
    return isFinite(obj) && !isNaN(parseFloat(obj));
};

// Is the given value `NaN`? (NaN is the only number which does not equal itself).
_.isNaN = function(obj) {
    return _.isNumber(obj) && obj !== +obj;
};

// Is a given value a boolean?
_.isBoolean = function(obj) {
    return obj === true || obj === false || toString.call(obj) === '[object Boolean]';
};

// Is a given value equal to null?
_.isNull = function(obj) {
    return obj === null;
};

// Is a given variable undefined?
_.isUndefined = function(obj) {
    return obj === void 0;
};

// Shortcut function for checking if an object has a given property directly
// on itself (in other words, not on a prototype).

//http://www.devthought.com/2012/01/18/an-object-is-not-a-hash/防止意外覆盖原型上的方法
_.has = function(obj, key) {
    return obj != null && hasOwnProperty.call(obj, key);
};