数组中。Length方法不是数组项的计数，而是最高索引。即使该项被设置为undefined

var a = [];
a.length;   // === 0
a[10];      // === undefined
a[10] = undefined;
a.length;   // === 11
a.pop();    // === undefined
a.length;   // === 10

这种行为很难与语言设计错误区分开来。

知道生活中有()和没有()，以及在哪里划定界限。 你可以用throw语句创建自定义错误来故意停止javascript运行时。

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)。

JSLint http://www.JSLint.com/

Javascript对象和函数都是一等公民，回调，别忘了还有事件和JQuery。

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