The .. operator is the descendant operator; you can use it in place of the normal . member-access operator :

E4X is designed so that you can perform most common XML manipulations using language syntax. E4X also defines methods you can invoke on XML objects. Here, for example, is the insertChildBefore() method:

Removing attributes and tags is very easy with the standard delete operator :

We can use the above code to trim whitespaces and filter out comments and blank lines.

If a catch clause has no conditional, it behaves as if it has the conditional if true, and it is always triggered if no clause before it was triggered.

A finally block is called after try-catch execution.

The typeof operator returns “object” for all standard JavaScript objects as functions are, and the typeof operator returns “xml”.

The for/each loop is a new looping statement standardized by E4X. E4X (ECMAScript for XML) is a language extension that allows XML tags to appear literally in JavaScript programs and adds syntax and API for operating on XML data.

Iterators that work on finite collections throw StopIteration from their next() method when there are no more values to iterate. StopIteration is a property of the global object in JavaScript 1.7. Its value is an ordinary object (with no properties of its own) that is reserved for this special purpose of terminating iterations. Note, in particular,that StopIteration is not a constructor function like TypeError() or RangeError().

An iterable object represents a collection of values that can be iterated. An iterable object must define a method named __iterator__() (with two underscores at the start and end of the name) which returns an iterator object for the collection.

The functions defined in this way behave exactly like functions defined with curly braces and the return keyword.

The above code snippet sorts an array in reverse numerical order.

Variables declared with var are defined throughout the enclosing function. Variables declared with let are defined only within the closest enclosing block (and any blocks nested within it, of course).

The above code returns 1,4,9,16,25 which is the square of the first five natural numbers. Notice the usage of let keyword in the above code snippet.

In the above code snippet, p is not defined. Hence, it gives a ReferenceError.

Since x is a let variable and since x is undefined, so x+1 is NaN. Thus, the above code snippet prints NaN.

The above code snippet swaps the value of the two variables.

The client-side document object defines methods that allow complete control over page content.

ADsafe was one of the first security subsets proposed) It was created by Douglas Crockford (who also defined The Good Parts subset).ADsafe relies on static verification only, and it uses JSLint as its verifier. It forbids access to most global variables and defines an ADSAFE variable that provides access to a secure API, including special-purpose DOM methods. ADsafe is not in wide use, but it was an influential proof-of-concept that influenced other secure subsets.