AJAX - The Complete Reference

PART IV

Appendix A: JavaScript Quick Reference 581

It is also possible to define functions using an object style with new and the Function()

constructor.

var myFun = new Function("x","alert('Hi '+x) ");

myFun("Thomas");

Given they are objects like everything else, there are a variety of useful properties you

might explore. For example, you can check how many arguments a function expects by

accessing its length property:

functionName.length

The argument values, in addition to being placed in the declared parameters upon

invocation, are accessible via the functionName.arguments[] array. This array holds the

actual values passed to the function, so it may hold a different number of arguments than

the function expects. With such a feature, you can define variable argument functions that

can work with arbitrary amounts of passed data.

The with Statement

As a convenience for handling object paths, JavaScript supports the with statement.

with ( objectExpression )

statement(s)

The object that objectExpression evaluates to is placed at the front of the scope chain

while statement executes. Statements in statement can therefore utilize methods and

properties of this object without explicitly using the property-accessing operator. An

example of the with statement is shown here:

with (document)

{

write("hello ");

write("world ");

write("last modified on " + lastModified);

}

Some JavaScript pundits quite dislike the with statement, given its ambiguity. For

example, in the last expression if we had a user-defined function called write, would it

invoke that within the with or the standard document.write() method? While we could

certainly look at how the scope chain is consulted, the immediate readability problems of

with is clear even in the simplest of cases.

Exceptions

You can catch programmer-generated and runtime exceptions as shown in Table A-19, but

you cannot catch JavaScript syntax errors, though you may handle them in some browsers

using window.onerror.