2011年7月18日
每當(dāng)提到Exeption就會(huì)有人跳出來說“Exception not use for flow control”，那到底是什么意思呢？什么情況下Exception就算控制流程了，什么時(shí)候就該拋出Exception了呢？
首先什么是Exception？
Definition: An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions.
再看什么是“流程”？如果流程是指程序的每一步執(zhí)行，那異常就是控制流程的，它就是用來區(qū)分程序的正常流程和非正常流程的，從上面異常的定義就可以看出。因此為了明確我們應(yīng)該說”不要用異常控制程序的正常流程“。如何定義正常流程和非正常流程很難，這是一個(gè)主觀的決定，沒有一個(gè)統(tǒng)一的標(biāo)準(zhǔn)，只能根據(jù)實(shí)際情況。網(wǎng)上找個(gè)例子：
bool isDouble(string someString) {
try {
double d = Convert.ParseInt32(someString);
} catch(FormatException e) {
return false;
}
return true;
}
這個(gè)程序其實(shí)不是想convert數(shù)字，而是想知道一個(gè)字符串是否包含一個(gè)數(shù)字，通過判斷是不是有異常的方式來決定返回true還是false，這是個(gè)Smell，這種應(yīng)該算”異常控制了正常流程“。我們可以通過正則表達(dá)式或其他方式來判斷。
另外Clean Code上一個(gè)例子：
try {
MealExpenses expenses = expenseReportDAO.getMeals(employee.getID());
m_total += expenses.getTotal();
} catch(MealExpensesNotFound e) {
m_total += getMealPerDiem();
}
MealExpensesNotFound異常影響了正常的計(jì)算m_total的業(yè)務(wù)邏輯。對(duì)于這種情況可以通過一下方式改進(jìn)：
public class PerDiemMealExpenses implements MealExpenses {
public int getTotal() {
// return the per diem default
}
}
以上兩個(gè)例子是比較明顯的異常控制正常流程，Smell很明顯，不會(huì)有很大爭(zhēng)議，但是實(shí)際情況中可能有很多例子沒有這么明顯，因?yàn)槎际侵饔^判定的。比如一下代碼，算不算異常控制正常流程？
public int doSomething()
{
doA();
try {
doB();
} catch (MyException e) {
return ERROR;
}
doC();
return SUCCESS;
}
看到這樣一段程序，如果沒有上下文，我們無法判斷。但是如果doSomething是想讓我們回答yes or no，success or error，我們不應(yīng)該通過有無異常來判斷yes or no，success or error，應(yīng)該有個(gè)單獨(dú)的方法來判斷，這個(gè)方法就只做這一件事情。如果doSometing是執(zhí)行一個(gè)操作，那么在這個(gè)過程中我們假定是不會(huì)出現(xiàn)問題的，否則拋出異常是比較合理的。
1. Java Class Loading Mechanism
首先當(dāng)編譯一個(gè)Java文件時(shí)，編譯器就會(huì)在生成的字節(jié)碼中內(nèi)置一個(gè)public，static，final的class字段，該字段屬于java.lang.Class類型，該class字段使用點(diǎn)來訪問，所以可以有：
java.lang.Class clazz = MyClass.class
當(dāng)class被JVM加載，就不再加載相同的class。class在JVM中通過（ClassLoader，Package，ClassName）來唯一決定。ClassLoader指定了一個(gè)class的scope，這意味著如果兩個(gè)相同的包下面的class被不同的ClassLoader加載，它們是不一樣的，并且不是type-compatible的。
JVM中所有的ClassLoader（bootstrap ClassLoader除外）都是直接或間接繼承于java.lang.ClassLoader抽象類，并且人為邏輯上指定了parent-child關(guān)系，實(shí)現(xiàn)上child不一定繼承于parent，我們也可以通過繼承它來實(shí)現(xiàn)自己的ClassLoader。
JVM ClassLoder架構(gòu)，從上到下依次為parent-child關(guān)系：
- Bootstrap ClassLoader - 啟動(dòng)類加載器，主要負(fù)責(zé)加載核心Java類如java.lang.Object和其他運(yùn)行時(shí)所需class，位于JRE/lib目錄下或-Xbootclasspath指定的目錄。我們不知道過多的關(guān)于Bootstrap ClassLoader的細(xì)節(jié)，因?yàn)樗且粋€(gè)native的實(shí)現(xiàn)，不是Java實(shí)現(xiàn)，所以不同JVMs的Bootstrap ClassLoader的行為也不盡相同。調(diào)用java.lang.String.getClassLoder() 返回null。
- sun.misc.ExtClassLoader - 擴(kuò)展類加載器，負(fù)責(zé)加載JRE/lib/ext目錄及-Djava.ext.dirs指定目錄。
- sun.misc.AppClassLoader - 應(yīng)用類加載器，負(fù)責(zé)加載java.class.path目錄
另外，還有一些其他的ClassLoader如：java.net.URLClassLoader，java.security.SecureClassLoader，java.rmi.server.RMIClassLoader，sun.applet.AppletClassLoader- 用戶還可以自己繼承java.lang.ClassLoader來實(shí)現(xiàn)自己的ClassLoader，用來動(dòng)態(tài)加載class文件。
ClassLoader特性：
- 每個(gè)ClassLoader維護(hù)一份自己的命名空間，同一個(gè)ClassLoader命名空間不能加載兩個(gè)同名的類。
- 為實(shí)現(xiàn)Java安全沙箱模型，默認(rèn)采用parent-child加載鏈結(jié)構(gòu)，除Bootstrap ClassLoader沒有parent外，每個(gè)ClassLoader都有一個(gè)邏輯上的parent，就是加載這個(gè)ClassLoader的ClassLoader，因?yàn)镃lassLoader本身也是一個(gè)類，直接或間接的繼承java.lang.ClassLoader抽象類。
java.lang.Thread中包含一個(gè)public的方法public ClassLoader getContextClassLoader()，它返回某一線程相關(guān)的ClassLoader，該ClassLoader是線程的創(chuàng)建者提供的用來加載線程中運(yùn)行的classes和資源的。如果沒有顯式的設(shè)置其ClassLoader，默認(rèn)是parent線程的Context ClassLoader。Java默認(rèn)的線程上下文加載器是AppClassLoader。
ClassLoader工作原理：
了解ClassLoader工作原理，先來看一個(gè)ClassLoader類簡(jiǎn)化版的loadClass()方法源碼
1 protected Class<?> loadClass(String name, boolean resolve)
2 throws ClassNotFoundException
3 {
4 synchronized (getClassLoadingLock(name)) {
5 // First, check if the class has already been loaded
6 Class c = findLoadedClass(name);
7 if (c == null) {
8 long t0 = System.nanoTime();
9 try {
10 if (parent != null) {
11 c = parent.loadClass(name, false);
12 } else {
13 c = findBootstrapClassOrNull(name);
14 }
15 } catch (ClassNotFoundException e) {
16 // ClassNotFoundException thrown if class not found
17 // from the non-null parent class loader
18 }
19
20 if (c == null) {
21 // If still not found, then invoke findClass in order
22 // to find the class.
24 c = findClass(name);
25 }
26 }
27 if (resolve) {
28 resolveClass(c);
29 }
30 return c;
31 }
32 }
首先查看該class是否已被加載，如果已被加載則直接返回，否則調(diào)用parent的loadClass來加載，如果parent是null代表是Bootstrap ClassLoader，則有Bootstrap ClassLoader來加載，如果都未加載成功，最后由該ClassLoader自己加載。這種parent-child委派模型，保證了惡意的替換Java核心類不會(huì)發(fā)生，因?yàn)槿绻x了一個(gè)惡意java.lang.String，它首先會(huì)被JVM的Bootstrap ClassLoader加載自己JRE/lib下的，而不會(huì)加載惡意的。另外，Java允許同一package下的類可以訪問受保護(hù)成員的訪問權(quán)限，如定義一個(gè)java.lang.Bad，但是因?yàn)閖ava.lang.String由Bootstrap ClassLoader加載而java.lang.Bad由AppClassLoader加載，不是同一ClassLoader加載，仍不能訪問。
2. Hotswap - 熱部署
即不重啟JVM，直接替換class。因?yàn)镃lassLoader特性，同一個(gè)ClassLoader命名空間不能加載兩個(gè)同名的類，所以在不重啟JVM的情況下，只能通過新的ClassLoader來重新load新的class。
1 public static void main(String[] args) throws InterruptedException, MalformedURLException {
2 IExample oldExample = new Example();
3 oldExample.plus();
4 System.out.println(oldExample.getCount());
5
6 Hotswap hotswap = new Hotswap();
7 while (true) {
8 IExample newExample = hotswap.swap(oldExample);
9 String message = newExample.message();
10 int count = newExample.plus();
11 System.out.println(message.concat(" : " + count));
12 oldExample = newExample;
13 Thread.sleep(5000);
14 }
15 }
16
利用hotswap替換就的Example，每5秒鐘輪詢一次，swap方法實(shí)現(xiàn)如下：
1 private IExample swap(IExample old) {
2 try {
3 String sourceFile = srcPath().concat("Example.java");
4 if (isChanged(sourceFile)) {
5 comiple(sourceFile, classPath());
6 MyClassLoader classLoader = new MyClassLoader(new URL[]{new URL("file:"+classPath())});
7 Class<?> clazz = classLoader.loadClass("Example");
8 System.out.println(IExample.class.getClassLoader());
9 IExample exampleInstance = ((IExample) clazz.newInstance()).copy(old);
10 System.out.println(exampleInstance.getClass().getClassLoader());
11 return exampleInstance;
12 }
13 } catch ...
24 return old;
25 }
這里必須將exampleInstance轉(zhuǎn)型為IExample接口而不是Exmaple，否則會(huì)拋出ClassCastExecption，這是因?yàn)閟wap方法所在類Hotswap是有AppClassLoader加載的，而且加載Hotswap的同時(shí)會(huì)加載該類引用的Exmaple的symbol link，而Example是MyClassLoader加載的，不同的ClassLoader加載的類之間直接用會(huì)拋出ClassCastException, 在本例中ClassLoader實(shí)現(xiàn)如下：
1 public class MyClassLoader extends URLClassLoader {
2
3 public MyClassLoader(URL[] urls) {
4 super(urls);
5 }
6
7 @Override
8 public Class<?> loadClass(String name) throws ClassNotFoundException {
9 if ("Example".equals(name)) {
10 return findClass(name);
11 }
12 return super.loadClass(name);
13 }
14 }
而對(duì)IExample我們還是調(diào)用super的loadClass方法，該方法實(shí)現(xiàn)仍是JVM的parent-child委派方式，因此最終由AppClassLoader加載，加載Hotswap時(shí)加載的symbol link也是由AppClassLoader加載的，因此能夠成功。
此外再熱部署時(shí)，被替換的類的所有引用及狀態(tài)都要遷移到新的類上，本例中只是很簡(jiǎn)單的調(diào)用copy函數(shù)遷移了count的狀態(tài)。
Tomcat的jsp熱部署機(jī)制就是基于ClassLoader實(shí)現(xiàn)的，對(duì)于其類的熱部署機(jī)制是通過修改內(nèi)存中的class字節(jié)碼實(shí)現(xiàn)的。
Resource：
1. java.lang.IllegalThreadStateException: process hasn't exited1 public static void main(String[] args) {
2 try {
3 Process process = Runtime.getRuntime().exec("javac");
4 System.out.println(process.exitValue());
5 } catch (IOException e) {
6 e.printStackTrace();
7 }
8 }
exec方法創(chuàng)建了一個(gè)native的進(jìn)程，并返回該process的對(duì)象，如果進(jìn)程還沒有返回，調(diào)用exitValue方法就會(huì)出現(xiàn)此異常，因?yàn)樵摲椒]有阻塞，其實(shí)現(xiàn)如下：
1 public synchronized int exitValue() {
2 if (!hasExited) {
3 throw new IllegalThreadStateException("process hasn't exited");
4 }
5 return exitcode;
6 }
2. waitFor方法
1 public static void main(String[] args) {
2 try {
3 Process process = Runtime.getRuntime().exec("javac");
4 int result = process.waitFor();
5 System.out.println(result);
6 } catch (IOException e) {
7 e.printStackTrace();
8 } catch (InterruptedException e) {
9 e.printStackTrace();
10 }
11 }
waitFor方法會(huì)一直阻塞直到native進(jìn)程完成，并返回native進(jìn)程的執(zhí)行結(jié)果。如果native進(jìn)程無法執(zhí)行完成，waitFor方法將一直阻塞下去，其實(shí)現(xiàn)如下：
1 public synchronized int waitFor() throws InterruptedException {
2 while (!hasExited) {
3 wait();
4 }
5 return exitcode;
6 }
該程序在jdk1.7 windows下測(cè)試工作正常，返回2; 但是jdk1.4 windows下測(cè)試出現(xiàn)hang。JDK documention的解釋是
The methods that create processes may not work well for special processes on certain native platforms,
such as native windowing processes, daemon processes, Win16/DOS processes on Microsoft Windows,or shell scripts.
The created subprocess does not have its own terminal or console. All its standard io (i.e. stdin, stdout, stderr)
operations will be redirected to the parent process through three streams (getOutputStream(), getInputStream(),
getErrorStream()). The parent process uses these streams to feed input to and get output from the subprocess. Because some native platforms only provide limited buffer size for standard input and output streams,
failure to promptly write the input stream or read the output stream of the subprocess may cause
the subprocess to block, and even deadlock.
所以，出現(xiàn)hang時(shí)，及時(shí)的flush標(biāo)準(zhǔn)輸入輸出或者錯(cuò)誤流能夠消除hang，如上面的javac，我們知道redirect到stderr中，所以解決hang后的代碼
1 public static void main(String[] args) {
2 try {
3 Process process = Runtime.getRuntime().exec("echo 'abc'>b.txt");
4 BufferedReader reader = new BufferedReader(new InputStreamReader(process.getErrorStream()));
5 String line;
6 while((line=reader.readLine())!=null){
7 System.out.println(line);
8 }
9 int result = process.waitFor();
10 System.out.println(result);
11 } catch (IOException e) {
12 e.printStackTrace();
13 } catch (InterruptedException e) {
14 e.printStackTrace();
15 }
16 }
3. exec() is not a command line 并不是所有的command line命令都可以用exec 1 public static void main(String[] args) {
2 try {
3 Process process = Runtime.getRuntime().exec("echo 'abc'>a.txt");
4 BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
5 String line;
6 while((line=reader.readLine())!=null){
7 System.out.println(line);
8 }
9 int result = process.waitFor();
10 System.out.println(result);
11 } catch (IOException e) {
12 e.printStackTrace();
13 } catch (InterruptedException e) {
14 e.printStackTrace();
15 }
16 }
結(jié)果為：
1 'abc'>a.txt
2 0
并沒有將創(chuàng)建a.txt，而從命令行執(zhí)行
"echo 'abc'>a.txt"卻正確創(chuàng)建了a.txt
Extreme programming is a set of simple and concrete practices than combine into an agile development process.
- Whole Team - Cutomers, managers and developers work closely with one another. So they are all aware of one another's problem and are collaborating to solve the problem. The customers can be a group of BAs, QAs or marketing persons in the same company as developers. The customers can be the paying customer. But in an XP project, the customer, however defined, is the member of and available to the team.
- User Stories - In order to plan a project, we must know something about the requirement, but we don't need to know very much. We need to know only enough about a requirement to estimate it. You may think that in order to estimate the requirement, you need to know all the details. That's not quite true. You have to know that there are details and you have to know roughly the kinds of details, but you don't have to know the sepecifics. Since the sepecific details of the requirements are likely to change with time, especially once the customers begin to see the system together. So when talking about requirements with the customer, we write some key words on a card to remind us of the conversation. A user story is memonic token of an ongoing conversation about a requirement. Every user story should have an estimation.
- Short Cycles - An XP project delivers working software every iteration - one or two weeks. At the end of each iteration, the sytem is demonstrated to the customer or the stakeholder in order to get their feedbacks. Every release there is a major delivery that can be put into production.
The Planning Game - The customers decide how important a feature is and the developers decide how much effort the feature will cost to implement. So the customers will select a collection of stories based on the priority and the previous iterations' velocity.
The Iteration Plan - Once an iteration has been started, the business agrees not to change the definition or priority of the stories in that iteration. The order of the stories within the iteration is a technical decision. The developers may work on the stories serially or concurrently. It depends but it's a technical decision.
The Release Plan - A release is usually three months. Similarly, the business or customer selecte collections of user stories and determines the priority based on their buget. But release are not cast in stone. The business can change or reorder at any time. They can write new stories, cancel stroies or change the priority. However, the business should not to change an interation that has been started.
Acceptance Tests - The detail about the user stories are captured in the form of acceptance tests specified by the customer. The acceptance tests are written by BAs and QAs immediately before or concurrently with the implementation of that story. They are written in a scripting form that allow them to be run automatically and repeatlly. These tests should be easy to read and understand for customers and business people. These tests become the true requirement document of the project. Once the acceptance test passes, it will be added the body of passing acceptance tests and is never allowed to fail. So the system is migrated from one working state to another.
Pair Programming - One memeber of each pair drives the keyboard and types the code. The other member of the pair watches the code being typed, finding errors and improvements.
The roles change frequently. If the driver gets tired or stuck, the pair grabs the keyboard and starts to drive. The keybord will move back and forth between them serval times in one hour. The resulting code is designed and authored by both memebers.
Pair memebership changes frequently. A reasonable goal is to change pair partner at least once per day. They should have worked on about everything that was going on in this iteration. Also it is very good for knowledge transfer.
Collective Ownership - A pair has right to check out any module and imporve it. No particular pair or developer are individually responsible for one particular module. Every developer of the project has fully responsible for each line of the code. So don't complain about the code may be written by the other pair. Let's try to improve it. Since it is our code.
Open Workspace - The team works together at one table or in one room. Each pair is within earshot of every other pair. Each has the opportunity to hear when another pair is in trouble. Each konws the state of the other. The sound in this room is a buzz of conversation. One might think that this would be a noise and distracting envrionment. But study suggested, working in a "war room" envrionment may increase productivity.
Simple Design - An XP team makes its designs as simple and expressive as they can be. Furthermore, the team narrows its focus to consider only the stories that are planned for the current iteration, not worrying about stories to come. The team migrates the design of the system from iteration to iteration to be the best design for the stories that the system currently implements.
- consider the simplest thing that could possibly work. - XP teams always try to find the simplest possible design option fot the current batch of stories.
- You aren't going to need it. - That means an XP team introduces one technology or one infrastructure before it is strictly needed. Yeah, but we know we are going to need that database one day. We are going to have to support multiple threads one day. So don't we need to put the hooks in for those things? The team puts the technology or infrastucture in only if it has proof or at least very compelling evidence. that putting it in now will be more cost-effective than waiting.
- Once and only once. XPers don't tolerate duplication of code. Wherever they find it. they remove it. The best way to remove redundancy is to create abstractions. After all, if two things are similar, some abstraction can be from them.
Continuous Integration
Sustainable Pace
Test-Driven Development
Refactoring
Metaphor
- 第一范式 - 列的原子性，數(shù)據(jù)庫表中的每一列都是不可再分的基本數(shù)據(jù)項(xiàng)
- 單一字段有多個(gè)有意義的值：比如 （people，address）其中address包括street，city，country，以逗號(hào)分割。想要查詢住在某一city的people很不容易
- 用很多字段來表示同一事實(shí)：比如（people，address1，address2，address3），就算我們假設(shè)每個(gè)people最多有三個(gè)地址，當(dāng)想要查詢住在同一地址的people時(shí)也很不容易，因?yàn)橛锌赡躳eople1的address1與people2的address2相同，每一次都要比較3*3次組合
- 第二范式 - 符合第一范式；且表中的屬性必須完全依賴于全部主鍵，消除非主屬性對(duì)主鍵的部分依賴
- 比如（組件ID，供應(yīng)商ID，供應(yīng)商姓名，價(jià)格），組件ID+供應(yīng)商ID為主鍵，價(jià)格完全依賴于全部主鍵，因?yàn)椴煌M件不同供應(yīng)商具有不同價(jià)格，但是對(duì)于供應(yīng)商姓名，則只依賴于供應(yīng)商ID，會(huì)造成對(duì)同一供應(yīng)商ID，名字?jǐn)?shù)據(jù)重復(fù)，而且如果供應(yīng)商改名，需要修改全部數(shù)據(jù)。因此需要單獨(dú)一個(gè)表（供應(yīng)商ID，供應(yīng)商姓名），（組件ID，供應(yīng)商ID，價(jià)格）
- 第三范式 - 非主屬性之間不能有依賴關(guān)系，必須都直接依賴于主屬性，消除傳遞依賴
- 比如（組件ID，制造商姓名，制造商地址），其中組件ID為主鍵，而制造商地址依賴于制造商姓名，需要（組件ID，制造商姓名）和新表（制造商姓名，制造商地址）其中姓名是主鍵
- 比如（訂單ID，組件ID，單價(jià)，數(shù)量，總價(jià)），其中總價(jià)=單價(jià)*數(shù)量，總價(jià)依賴于單價(jià)和數(shù)量，需要去掉總價(jià)欄
- BC范式 - 任何屬性（包括非主屬性和主屬性）不能被非主屬性所決定。第三范式強(qiáng)調(diào)非主屬性不能依賴于其他非主屬性，BC范式是第三范式的加強(qiáng)，強(qiáng)調(diào)“任何屬性”。因此如果滿足第三范式，并且只有一個(gè)主鍵，則一定滿足BC范式
一般，范式越高，表越多，數(shù)據(jù)庫操作時(shí)需要表關(guān)聯(lián)，增加了查詢的復(fù)雜性，降低了查詢性能。因此并不是范式越高越好，要根據(jù)需要進(jìn)行權(quán)衡，第三范式已經(jīng)消除了大部分的數(shù)據(jù)冗余，插入異常，更新異常和刪除異常。
Why?Functional test characteristic
- The number of tests is mostly large.
- Page structure and elements may be changed frequently.
If our tests interact directly with the test driver (selenium,etc).
- UI element locators(Xpath, CSS, element Ids, names, etc) are copuled and repeat throughout the tests logic. It is hard to maintain, refactor and change especially when we change the page structure.
- The fine-grained UI operations hide our actual test intention. After some time we can not easily to identify what we want to do before.
What?A Page Object models the UI elements that your tests interact with as objects within the test code. It decouples the test logic from the fine-grained details of the UI page.
Test -> Page Object -> Test Driver
The driver is the actual executor of browser action, like click, select, type etc. The page object has knowledage of the HTML structure.
Advantages- When UI changes, only responding page object need to be changed. The tests logic will not be affected.
- It makes our tests logic simpler and more readable. Since the tests logic can only focus on its test secinaro rather than the structure of HTML page. As an example, think of login function, it only needs username and password, then do login action. How these are implemented shouldn't matter to the test. We don't care about that it uses a button or link to login.
- Complex interactions can be modeled as methods on the page object, which can be used in multiple times.
Best Practices- Public methods represent the services that the page offers
- Try not to expose the internals of the page. Like OO programming, object just expose the behaviour and hide its internal details.
- Generally don't make assertions, but it is better to do some kind of check to ensure the browser is actually on the page it should be. Since our following tests are all base on the assumption. It can be done by checking something simple like title.
- Methods return other page object. This means that we can effectively model the user's journey through our application.
- Don't need to represent the entire page. It can be just a tab or a navigation bar that your test interacts with.
- Only one place knows the HTML structure of a particular page. UI changes, the fix need only be applied in one place.
- Different results for the same action are modeled as different methods. Since the test knows the expected state.
1.
Small and the indent level should not be greater than one or two.
2.
Do One Thing - Functions should do one thing. They should do it well. They should do it only.
Steps of function are one level of abstraction below the name of the function. Then the function is doing one thing.
Another way to know function is doing more than one thing is if you can extract another function from it.
3.
One level of abstraction per function The stepdown rule - reading code from top to bottom.
switch
4.
Use Descriptive Names Don't be afraid to make a name long.
A long descriptive name is better than a short magic name.
A long descriptive name is better than a long descriptive comment.
Dont't be afraid to spend time choosing a name.
Be consistent in your names.
5.
Function Arguments The ideal number of arguments for a function is zero.
Three arguments should be avoided.
Flag Arguments - can be split into two function.
Aruments Objects - group the variables and abstract concept if possible.
Verbs and keywords - function and argument should form a very nice verb/noun pair.
6.
Have no side effects7.
Command Query Separation - Functions should either do something or answer something, but not both.
8.
Prefer Exceptions to returning error codes. Extract try/catch blocks, error handling is one thing.
9.
DRY - don't repeat yourself - especially the duplicated logic.
一，全局變量
1.全局變量可以被任何部分在任意時(shí)間改變，復(fù)雜化，降低可靠性
2.可能與子程序變量名相同，沖突可能導(dǎo)致程序無法運(yùn)行，難以調(diào)試
三種聲明全局變量模式
1.脫離任何函數(shù)的var foo ＝ value
2.直接添加屬性至全局對(duì)象，全局對(duì)象是所有全局變量的容器，在web瀏覽器中全局對(duì)象名為window，window.foo = value
3.直接未經(jīng)聲明的變量 － 隱式的全局變量 foo ＝ value
二，沒有塊作用域，有函數(shù)作用域
函數(shù)中定義的參數(shù)和變量在函數(shù)外部不可見，而在一個(gè)函數(shù)中任何位置的定義的變量在該函數(shù)的任何地方可見。
function f1(){
var a=1;
function f2(){
if(false){
var a=2; //變量基于函數(shù)，而非基于語句塊，沒有塊作用域
}
console.log(a); //undefined 未定義
}
f2();
console.log(a);//1
}
f1();
大多數(shù)語言中，一般聲明變量都是在第一次用到它的地方，在javascript中是一個(gè)壞習(xí)慣，因?yàn)闆]有塊作用域，更好的是在每個(gè)函數(shù)開頭聲明所有變量。
三，return語句返回一個(gè)值，值表達(dá)式必須和return在同一行上
return {
status:true
};
返回包含status成員元素的對(duì)象。
如果用
return
{
status:true
};
會(huì)返回undefined。
四，保留字不能用來命名變量或函數(shù)，當(dāng)保留字被用作對(duì)象字面量的鍵值時(shí)，必須用引號(hào)，而且不能用點(diǎn)表示法，必須使用括號(hào)表示法。
var object = {case:value}; //非法
var object = {'case':value}; //ok
object.case = value; //非法
object['case'] = value; //ok
各瀏覽器對(duì)保留字使用限制上實(shí)現(xiàn)不同，以上語法在FF中合法，但其他瀏覽器不合法；再有不同的保留字行為也不相同。類似int/long/float等保留字在各瀏覽器中都可以做變量名及對(duì)象字面量的鍵值。但不建議使用任何保留字。
五，typeof － 識(shí)別運(yùn)算數(shù)類型的字符串
但typeof null返回'object'，更好的檢測(cè)null的方法： my_value === null
對(duì)于正則表達(dá)式 typeof /a/，一些瀏覽器返回'object'，另一些返回'function'
六，parseInt － 將字符串轉(zhuǎn)換為整數(shù)
1.此函數(shù)遇到非數(shù)字時(shí)就停止解析，即parseInt('16')和parseInt('16 abc')產(chǎn)生結(jié)果相同，都是16.
2.如果字符串第一個(gè)字符是0，則基于八進(jìn)制解析，而不是十進(jìn)制。八進(jìn)制中沒有8，9數(shù)字，所以parseInt('08')和parseInt('09')結(jié)果為0。此函數(shù)可以接收一個(gè)基數(shù)作為參數(shù)，parseInt('08',10)結(jié)果為8。
七，浮點(diǎn)數(shù)
0.1+0.2不等于0.3
八，NaN
function isNumber(value){
return typeof value === 'number' && isFinite(value);
}
isFinite篩調(diào)NaN和Infinity，但是isFinite會(huì)試圖把它的運(yùn)算數(shù)轉(zhuǎn)換為一個(gè)數(shù)字，如果運(yùn)算數(shù)事實(shí)上不是一個(gè)數(shù)字，就會(huì)報(bào)錯(cuò)，所以加上typeof value === 'number'。
九，假值
0，NaN，''，false，null，undefined全部等于假，但它們是不可互換的。