New behavior of arguments in ECMAScript 5

gistfile1.js

/* Section 10.6 Note 1 in ECMA-262, 5th Edition says:
 *
 *   For non-strict mode functions the array index (defined in 15.4) named data 
 *   properties of an arguments object whose numeric name values are less than 
 *   the number of formal parameters of the corresponding function object initially 
 *   share their values with the corresponding argument bindings in the function's 
 *   execution context. This means that changing the property changes the 
 *   corresponding value of the argument binding and vice-versa. 
 *
 * Thus, the following function should work.
 */

function doAdd(num1, num2) {
    if(arguments.length == 1) {
        arguments[1] = 10;
    }
    alert(arguments[0] + num2);
}

/* 
 * I believe this line should work in non-strict ECMAScript 5.
 * In Minefield latest nightly, though, this returns NaN
 * instead of 20. In strict mode, this behavior would be correct.
 */
doAdd(10);        //Should return 20, but returns NaN in Minefield 2010-Oct-30.


doAdd(30, 20);    //50