实现原理
浅拷贝 jQuery.extend(clone, copy)
实现原理：
key值不同: 以clone、copy合集为准
key值相同: value值为基本类型直接覆盖; 为对象或数组若 则以copy为准；
#### 深拷贝 jQuery.extend(true, clone, copy)
实现原理：key值不同: 以clone、copy合集为准
key值相同: value值为基本类型直接覆盖; 为对象或数组 嵌套形式则取两者合集, 直接对象或数组则直接覆盖;
### jQuery 源码片段
/* 继承实现 依赖 */// Object对象var class2type = {};var toString = class2type.toString;// 检测是否为对象本身属性而非继承var hasOwn = class2type.hasOwnProperty; // 获取对象原型函数var getProto = Object.getPrototypeOf;// 对象的实现（函数体）var fnToString = hasOwn.toString;// Object实现 "function Object() { [native code] }"var ObjectFunctionString = fnToString.call( Object );// 是否为纯对象或数组, 目的：是否需要深度遍历对象var isPlainObject = function( obj ) { var proto, Ctor; // obj 是否为对象 if ( !obj || toString.call( obj ) !== "[object Object]" ) { return false; } // obj 对象原型 proto = getProto( obj ); // 是否有原型对象 Object.create( null )无[prototype] if ( !proto ) { return true; } //获取对象构造函数，若为Object直接创建则为 function Object() Ctor = hasOwn.call( proto, "constructor" ) && proto.constructor; //是否为Object直接对象 return typeof Ctor === "function" && fnToString.call( Ctor ) === ObjectFunctionString;}; /* 继承实现 */jQuery.extend = jQuery.fn.extend = function() { var options, name, src, copy, copyIsArray, clone, target = arguments[ 0 ] || {}, i = 1, length = arguments.length, deep = false; // Handle a deep copy situation if ( typeof target === "boolean" ) { deep = target; // Skip the boolean and the target target = arguments[ i ] || {}; i++; } // Handle case when target is a string or something (possible in deep copy) if ( typeof target !== "object" && !jQuery.isFunction( target ) ) { target = {}; } // Extend jQuery itself if only one argument is passed if ( i === length ) { target = this; i--; } for ( ; i < length; i++ ) { // Only deal with non-null/undefined values if ( ( options = arguments[ i ] ) != null ) { // Extend the base object for ( name in options ) { src = target[ name ]; copy = options[ name ]; // Prevent never-ending loop if ( target === copy ) { continue; } // Recurse if we're merging plain objects or arrays if ( deep && copy && ( jQuery.isPlainObject( copy ) || ( copyIsArray = Array.isArray( copy ) ) ) ) { if ( copyIsArray ) { copyIsArray = false; clone = src && Array.isArray( src ) ? src : []; } else { clone = src && jQuery.isPlainObject( src ) ? src : {}; } // Never move original objects, clone them target[ name ] = jQuery.extend( deep, clone, copy ); // Don't bring in undefined values } else if ( copy !== undefined ) { target[ name ] = copy; } } } } // Return the modified object return target;};