[root]$ cat for_in.js
var str = "hello world";   // [1] 修改为: var str = new String();  [2]处没有
打印任何内容
for (var x in str) {
    console.log(x);   // [2]
}

var base_obj = {
    "foo" : "bar",
    "bar" : "foo"
};

if (typeof Object.create !== 'function') {
    //Object.create = function (o) {
    //    var F = function () {};
    //    F.prototype = o;
    //    return new F();
    //}
}

var derived_obj = Object.create(base_obj);
derived_obj.xyz = function() {
    1 + 1 == 2;
};

for (var x in derived_obj) {
    console.log(x);     //[3]
    console.log(derived_obj.hasOwnProperty(x)  ? "Own property" : "Base
property");
}

-----------------------------------------------
[1]处修改为new String()，输出没有任何显示
[2]输出显示为: "0, 1, 2, 3, ...., 10"
[3]输出显示为:
xyz
Own property
foo
Base property
bar
Base property

请问:
(1) [1]处定义的为啥不是String object?  感觉是char类型数组。
(2) [1]处做修改，为何for..in不能打印出String的所有properties以及methods?
(3) 为何不鼓励用new String(), 但可以是new Constructor()?  w3schools的说法
是:"Avoid String, Number, and Boolean objects. They complicate your code and
slow down execution speed." 会使得写代码变复杂，还有影响执行速度。

不明白之处，还得多多请教。
--
修改:saynothing FROM 125.120.149.*
FROM 125.120.149.*