The important difference to note between these two syntaxes is that in the first, the property name is an identifier, and in the second, the property name is a string. You'll see why this is so important shortly.

In C, C++, Java, and similar strongly typed languages, an object can have only a fixed number of properties, and the names of these properties must be defined in advance. Since JavaScript is a loosely typed language, this rule does not apply: a program can create any number of properties in any object. When you use the . operator to access a property of an object, however, the name of the property is expressed as an identifier. Identifiers must be typed literally into your JavaScript program; they are not a datatype, so they cannot be manipulated by the program.

On the other hand, when you access a property of an object with the [] array notation, the name of the property is expressed as a string. Strings are JavaScript datatypes, so they can be manipulated and created while a program is running.
还没写完，待我醒来再细说！

var r = new Rectangle(2, 3);
r.hasOwnProperty("width");   // true: width is a direct property of r
r.hasOwnProperty("area");    // false: area is an inherited property of r
"area" in r;                 // true: "area" is a property of r

delve a little into the history of how it came to be like that

delve [delv]

v. 探究, 钻研; 挖, 掘; 搜索; 挖, 掘; 刨

2: sooner or later

3: generated

we first need to remind ourselves why XML has proved such a success and generated so much excitement.

generate [gen·er·ate || 'dʒenəreɪt]

v. 产生, 导致, 发生

4:

bespoke [be·spoke || bɪ'spəʊk]

adj. 预定的; 定制的

bespeak [be·speak || bɪ'spiːk]

v.                  预定; 证明; 预约; 表示

5: ado

ado [a·do || ə'duː]

n. 纷扰, 骚扰; 费力, 麻烦; 忙乱; 无谓的纷扰

6：effect

effect [ef·fect || ɪ'fekt]

n. 结果, 效果, 影响

v. 造成; 招致; 产生; 实现, 达到

7: bewildering

Don't worry if this example seemed a bit bewildering

bewilder [be·wil·der || bɪ'wɪldə]

v. 使迷惑; 使昏乱; 使不知所措

8: minimizes

This minimizes the work that needs to be done at display time and is ideal when the same displayed page is presented to very many users.

minimize (Amer.) ['min·i·mize || 'mɪnɪmaɪz]

v.                  将...减到最少

9:occasionally

It's occasionally useful to have a stylesheet as the input or output of a transformation.

occasionally [oc'ca·sion·al·ly || ə'keɪʒnəlɪ]

adv. 偶尔, 间或

10: compromise

The xhtml output method, as one might expect, is a compromise between the xml and html output methods

compromise [com·pro·mise || 'kɒmprəmaɪz]

n. 妥协, 折衷案, 折衷

v. 互让解决; 放弃; 连累, 危及; 泄露; 妥协, 让步

11: inefficient

inefficient [,inef'fi·cient || ‚ɪnɪ'fɪʃnt]

adj. 无效率的, 无能的

12: efficient

efficient [ef'fi·cient || -nt]

adj. 生效的; 能干的; 有效率的