变量是全局的，除非声明为局部的!!

Bad (DoSomething()只被调用10次):

function CountToTen()
{
  for(i=0; i< 10; i++)
  {
    DoSomething(i);
  }
}

function countToFive()
{
  for(i=0; i<5; i++)
  {
    CountToTen();
  }
}

CountToFive();

好(DoSomething()按预期被调用50次):

function CountToTen()
{
  var i;
  for(i=0; i< 10; i++)
  {
    DoSomething(i);
  }
}

function countToFive()
{
  var i;
  for(i=0; i<5; i++)
  {
    CountToTen();
  }
}

CountToFive();

在Javascript中，性能很重要。

没有智能编译器来优化你的代码，所以你应该更小心，当你写javascript代码比语言像c#， Java…

jQuery。YUI。不是(等等等等)

框架可能很有用，但它们经常隐藏JavaScript和DOM实际工作的细节，这些细节有时很难看。如果你的目标是能够说“我懂JavaScript”，那么在一个框架上投入大量的时间是与此相反的。

这里有一些JavaScript语言的特性，你应该知道它在做什么，不会被发现，但对很多人来说不是很明显:

That object.prop and object['prop'] are the same thing (so can you please stop using eval, thanks); that object properties are always strings (even for arrays); what for...in is for (and what it isn't). Property-sniffing; what undefined is (and why it smells); why the seemingly-little-known in operator is beneficial and different from typeof/undefined checks; hasOwnProperty; the purpose of delete. That the Number datatype is really a float; the language-independent difficulties of using floats; avoiding the parseInt octal trap. Nested function scoping; the necessity of using var in the scope you want to avoid accidental globals; how scopes can be used for closures; the closure loop problem. How global variables and window properties collide; how global variables and document elements shouldn't collide but do in IE; the necessity of using var in global scope too to avoid this. How the function statement acts to ‘hoist’ a definition before code preceding it; the difference between function statements and function expressions; why named function expressions should not be used. How constructor functions, the prototype property and the new operator really work; methods of exploiting this to create the normal class/subclass/instance system you actually wanted; when you might want to use closure-based objects instead of prototyping. (Most JS tutorial material is absolutely terrible on this; it took me years to get it straight in my head.) How this is determined at call-time, not bound; how consequently method-passing doesn't work like you expect from other languages; how closures or Function#bind may be used to get around that. Other ECMAScript Fifth Edition features like indexOf, forEach and the functional-programming methods on Array; how to fix up older browsers to ensure you can use them; using them with inline anonymous function expressions to get compact, readable code. The flow of control between the browser and user code; synchronous and asynchronous execution; events that fire inside the flow of control (eg. focus) vs. events and timeouts that occur when control returns; how calling a supposedly-synchronous builtin like alert can end up causing potentially-disastrous re-entrancy. How cross-window scripting affects instanceof; how cross-window scripting affects the control flow across different documents; how postMessage will hopefully fix this.

请看关于最后两项的答案。

最重要的是，你应该批判性地看待JavaScript，承认由于历史原因，它是一种不完美的语言(甚至比大多数语言都不完美)，并避免它最糟糕的故障点。Crockford在这方面的工作绝对值得一读(尽管我并不100%同意他关于“好的部分”是什么)。

熟悉至少一个Javascript库(Jquery, Prototype等)。 学习如何使用主要浏览器的调试工具(MSIE 7-8, Firefox, Chrome, Safari) 深入了解这个行业:Douglas Crockford的网站是一个宝库，ajajana.com是一个很好的博客，可以让你了解关于Javascript的新的、有趣的或奇怪的想法。还有很多其他的资源，但这些是对我帮助最大的。

如果你不知道以下内容，你就不懂JavaScript:

闭包 基于原型的继承 模块模式 w3c dom的 事件是如何工作的

知道Javascript最初被称为LiveScript，加上“Java”前缀是出于营销目的，而不是因为Java和Javascript有关系(它们并没有关系)。

哦，对于拥有任何版本的David Flanagan的“Javascript:权威指南”(这一信息在第2页)。

．.． 感谢那些之前试图混淆ie4文档的人。all[]和Netscape Navigator 4的文档。在Jquery之前使用layers[]消除了这种痛苦。

编辑:

正如@Kinopiko指出的，JavaScript最初被称为项目Mocha(一些消息来源也认为它被称为项目LiveWire)，但人们普遍认为，该语言(由Brendan Eich编写)在1996年初发布时采用Java前缀之前，计划以LiveScript的形式发布。