Dmitry, I was referring to these tests:
http://webreflection.blogspot.com/2010/02/arguments-callee-call-and-apply.html
Looks like all JS engines avoid creating arguments, unless necessary. V8 is the only engine where performance doesn’t suffer but only if there’s no arguments passed.

The arguments object is not automatically created every time the function is called, the JavaScript engine will only create it on-demand, if it’s used

Which exactly “JavaScript engine” is meant? Where did you get this exact info? Although, it can be true in some implementations (yep, it’s the good optimization as all needed info about the context is available on parsing the code, so there’s no need to create arguments object if it was not found on parsing), but as you know ECMA-262-3 statements, that arguments object is created each time on entering the execution context.

Consider this example that illustrates the second case (B changing values behind A’s unsuspecting back)

Nothing is wrong, nothing is odd. It’s the same if you simply pass an object (or an array to A and then will pass this argument to B). Moreover, arguments name can be used (although, it will be an error in “strict mode” of ES5):

function foo(arguments) {
console.dir(arguments); // {a: 10, b: 20}
bar(arguments);
// oops? why “oops”? that’s the common
// ECMAScript behavior
console.dir(arguments); // {a: 100, b: 20}
}

function bar(args) {
args.a = 100;
}

foo({a: 10, b: 20});

Dmitry.

B(arguments);
to
B([obj, ar]);

it has the same effect, so I guess this isn’t specifically a problem with the ‘arguments’ object… thoughts?