实现原理

浅拷贝 jQuery.extend(clone, copy)

实现原理：效果查看

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： 效果查看

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;
};