区别在于：当属性名有同名的时候，extend直接用source中的值覆盖掉destination中的值；而defaults则会依据destination中的属性值是否为undefined区别对待。

var iceCream = {flavor: "chocolate",sprinkles:undefined};
_.defaults(iceCream, {flavor: "vanilla", sprinkles: "lots"});
console.log("iceCream=" + iceCream.flavor);//chocolate
console.log("sprinkles=" + iceCream.sprinkles);//lots

clone_.clone(object) 
Create a shallow-copied(浅拷贝) clone of the object. Any nested objects or arrays will be copied by reference, not duplicated.

_.clone({name: ‘moe‘});
=> {name: ‘moe‘};

tap_.tap(object, interceptor) 
Invokes interceptor with the object, and then returns object. The primary purpose of this method is to "tap into" a method chain, in order to perform operations on intermediate results within the chain.

_.chain([1,2,3,200])
  .filter(function(num) { return num % 2 == 0; })
  .tap(alert)
  .map(function(num) { return num * num })
  .value();
=> // [2, 200] (alerted)
=> [4, 40000]

has_.has(object, key) 
Does the object contain the given key?

Identical to object.hasOwnProperty(key), but uses a safe reference to the hasOwnProperty function, in case it‘s been overridden accidentally.

_.has({a: 1, b: 2, c: 3}, "b");
=> true

property_.property(key) 
Returns a function that will itself return the key property of any passed-in object.

var moe = {name: ‘moe‘};
‘moe‘ === _.property(‘name‘)(moe);
=> true

matches_.matches(attrs) 
Returns a predicate function that will tell you if a passed in object contains all of the key/value properties present in attrs.

var ready = _.matches({selected: true, visible: true});
var readyToGoList = _.filter(list, ready);

isEqual_.isEqual(object, other) 
Performs an optimized deep comparison between the two objects, to determine if they should be considered equal.

var moe   = {name: ‘moe‘, luckyNumbers: [13, 27, 34]};
var clone = {name: ‘moe‘, luckyNumbers: [13, 27, 34]};
moe == clone;
=> false
_.isEqual(moe, clone);
=> true

isEmpty_.isEmpty(object) 
Returns true if an enumerable object contains no values (no enumerable own-properties). For strings and array-like objects _.isEmpty checks if the length property is 0.

_.isEmpty([1, 2, 3]);
=> false
_.isEmpty({});
=> true

isElement_.isElement(object) 
Returns true if object is a DOM element.

_.isElement(jQuery(‘body‘)[0]);
=> true

isArray_.isArray(object) 
Returns true if object is an Array.

(function(){ return _.isArray(arguments); })();
=> false
_.isArray([1,2,3]);
=> true

isObject_.isObject(value) 
Returns true if value is an Object. Note that JavaScript arrays and functions are objects, while (normal) strings and numbers are not.

_.isObject({});
=> true
_.isObject(1);
=> false

isArguments_.isArguments(object) 
Returns true if object is an Arguments object.

(function(){ return _.isArguments(arguments); })(1, 2, 3);
=> true
_.isArguments([1,2,3]);
=> false

isFunction_.isFunction(object) 
Returns true if object is a Function.

_.isFunction(alert);
=> true

isString_.isString(object) 
Returns true if object is a String.

_.isString("moe");
=> true

isNumber_.isNumber(object) 
Returns true if object is a Number (including NaN).

_.isNumber(8.4 * 5);
=> true

isFinite_.isFinite(object) 
Returns true if object is a finite Number.

_.isFinite(-101);
=> true

_.isFinite(-Infinity);
=> false

isBoolean_.isBoolean(object) 
Returns true if object is either true or false.

_.isBoolean(null);
=> false

isDate_.isDate(object) 
Returns true if object is a Date.

_.isDate(new Date());
=> true

isRegExp_.isRegExp(object) 
Returns true if object is a RegExp.

_.isRegExp(/moe/);
=> true

isNaN_.isNaN(object) 
Returns true if object is NaN.
Note: this is not the same as the native isNaN function, which will also return true for many other not-number values, such as undefined.

_.isNaN(NaN);
=> true
isNaN(undefined);
=> true
_.isNaN(undefined);
=> false

isNull_.isNull(object) 
Returns true if the value of object is null.

_.isNull(null);
=> true
_.isNull(undefined);
=> false

isUndefined_.isUndefined(value) 
Returns true if value is undefined.

_.isUndefined(window.missingVariable);
=> true