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function clone( obj ) {

    var copy;

    switch( typeof obj ) {

        case "undefined":

            break;

        case "number":

            copy = obj - 0;

            break;

        case "string":

            copy = obj + "";

            break;

        case "boolean":

            copy = obj;

            break;

        case "object":  //object分为两种情况 对象（Object）和数组（Array）

            if(obj === null) {

                copy = null;

            } else {

                if( Object.prototype.toString.call(obj).slice(8, -1) === "Array") {

                    copy = [];

                    for( var i = 0 ; i < obj.length ; i++ ) {

                        copy.push(clone(obj[i]));

                    }

                } else {

                    copy = {};

                    for( var j in obj) {

                        copy[j] = clone(obj[j]);

                    }

                }

            }

            break;

        default:

            copy = obj;

            break;

    }

    return copy;

}