But it is certainly something which is nice to know since it can be the cause of some pretty nasty bugs.

/Karl

E.g.

function foo(a, b)
{
bar(arguments);
alert(a + b);
}

function bar(args)
{
args[0] = args[0] + 1;
}

foo(1, 2); // alerts 4 in IE

Not sure what use this might be in practice!

For the formal reference:

In ECMA-263, 10.1.8 Arguments Object it says: … an arguments object is created and initialised as follows:

…
* For each non-negative integer, arg, less than the value of the length property, a property is created with name ToString(arg) and property attributes {DontEnum}. The initial value of this property is
the value of the corresponding actual parameter supplied by the caller. The first actual parameter
value corresponds to arg=0, the second to arg=1, and so on. **In the case when arg is less than the
number of formal parameters for the Function object, this property shares its value with the
corresponding property of the activation object. This means that changing this property changes the corresponding property of the activation object and vice versa.**

My guess is:-

1. We know that if you change a value in the arguments array, e.g. arguments[1] = “New Value”, then the local variable for position 1 will be updated. I.e. in your case “proto would now contain the string “New Value”.

2. Your “slice(arguments, 0, 0)” inserts a new value in position arguments[0], movement the original “conf” (index 0) and “proto” (index 1) values up one index position in the arguments array.

3. This movement is equivalent to assigning a new value, and so the “proto” variable will now contain the “conf” contents.

4. This movement up occurs before the “function” object you insert at the start is called. So by the time the function is called, the specifc “proto” variable (on the activation/variable object captured by the closure of the inner function) would have changed. So “proto” would point to the wrong value when it came to be used in the inner function.

I believe this is due to how Javascript’s closures work.