深入理解jquery中extend的实现

function getOpt(option){
 var _default = {
 name : 'wenzi',
 age : '25',
 sex : 'male'
 }
 $.extend(_default, option);
 return _default;
}
getOpt(); // {name: "wenzi", age: "25", sex: "male"}
getOpt({name:'bing'}); // {name: "bing", age: "25", sex: "male"}
getOpt({name:'bing', age:36, sex:'female'}); // {name: "bing", age: 36, sex: "female"}

function getOpt(option){
 var _default = {
 name : 'wenzi',
 age : '25',
 sex : 'male'
 }
 `$.extend(_default, option);`
 return _default;
}

function getOpt(target, obj1, obj2, obj3){
 $.extend(target, obj1, obj2, obj3);
 return target;
}

var _default = {
 name : 'wenzi',
 age : '25',
 sex : 'male'
}
var obj1 = {
 name : 'obj1'
}
var obj2 = {
 name : 'obj2',
 age : '36'
}
var obj3 = {
 age : '67',
 sex : {'error':'sorry, I dont\'t kown'}
}
getOpt(_default, obj1, obj2, obj3); // {name: "obj2", age: "67", sex: {error: "sorry, I dont't kown"}}

function getOpt(target, obj1, obj2, obj3){
 var result = $.extend(target, obj1, obj2, obj3); 
 return result; // // result即修改后的target值
}

function getOpt(target, obj1, obj2, obj3){
 var result = $.extend({}, target, obj1, obj2, obj3); 
 return result; // // result即为{}修改后的值
}

$.extend({
 _name : 'wenzi',
 _getName : function(){
 return this._name;
 }
})

$._name; // wenzi
$._getName(); // wenzi

var obj = {name:'wenzi', sex:'male'};
var obj1 = obj; // 赋值
obj1.name = 'bing';
console.log(obj.name); // bing

var s = 'hello';
var t = s;
t = 'world';
console.log(s); // hello

_default.sex.error = 'hello world';

var obj = {name:'wenzi', score:80};
var obj1 = {score:{english:80, math:90}}
$.extend(true, obj, obj1);
obj.score.english = 10;
console.log(obj.score.english); // 10
console.log(obj1.score.english); // 80

// 为与源码的下标对应上，我们把第一个参数称为`第0个参数`，依次类推
jQuery.extend = jQuery.fn.extend = function() {
 var options, name, src, copy, copyIsArray, clone,
 target = arguments[0] || {}, // 默认第0个参数为目标参数
 i = 1, // i表示从第几个参数凯斯想目标参数进行合并，默认从第1个参数开始向第0个参数进行合并
 length = arguments.length,
 deep = false; // 默认为浅度拷贝

 // 判断第0个参数的类型，若第0个参数是boolean类型，则获取其为true还是false
 // 同时将第1个参数作为目标参数，i从当前目标参数的下一个
 // Handle a deep copy situation
 if ( typeof target === "boolean" ) {
 deep = target;

 // Skip the boolean and the target
 target = arguments[ i ] || {};
 i++;
 }

 // 判断目标参数的类型，若目标参数既不是object类型，也不是function类型，则为目标参数重新赋值 
 // Handle case when target is a string or something (possible in deep copy)
 if ( typeof target !== "object" && !jQuery.isFunction(target) ) {
 target = {};
 }

 // 若目标参数后面没有参数了，如$.extend({_name:'wenzi'}), $.extend(true, {_name:'wenzi'})
 // 则目标参数即为jQuery本身，而target表示的参数不再为目标参数
 // Extend jQuery itself if only one argument is passed
 if ( i === length ) {
 target = this;
 i--;
 }

 // 从第i个参数开始
 for ( ; i 

// Prevent never-ending loop
if ( target === copy ) {
 continue;
}

var _default = {name : 'wenzi'};
var obj = {name : _default}
$.extend(_default, obj);
console.log(_default);

_default = {name : _default}; 

jQuery.extend = jQuery.fn.extend = function(){}