AO-parameters-arguments.js

/**
 * Abstract representation of ES3
 * or ES5-non-strict sharing of
 * passed formal parameters and
 * corresponding indexes of the
 * arguments object
 *
 * Implemented in ES5, non-strict
 * tested in BESEN rev. 109
 * 
 * @author Dmitry A. Soshnikov (C) 2010
 */

print("==Exact===\n");

function foo(a, b) {
    
    print(arguments.length); // 1
    print(arguments.callee.length); // 2
    
    print(a); // 10

    arguments[0] = 20;
    print(a); // 20

    a = 30;
    print(arguments[0]); // 30

    b = 100;
    print(arguments[1]); // undefined
    
    delete a; // false    

    delete arguments[0]; // true
    print(arguments[0]); // undefined
    print(a); // 30

    arguments[0] = 50;
    print(arguments[0]); // 50
    print(a); // 30

    a = 40;
    print(a); // 40
    print(arguments[0]); // 50

}

// call function, passing
// only first parameter
foo(10);

// Abstract description of handling
// variable/activation object in ES3, for details see:
// http://dmitrysoshnikov.com/ecmascript/chapter-2-variable-object/

print("\n==Abstract===\n");

// 1. Entering the context stage

var AO = Object.defineProperties({}, {
    arguments: {
        value: Object.defineProperties({}, {
            // create accessor only for the
            // passed argument - "a", index 0
            0: {
                get: function getA() {
                    return AO.a;
                },
                set: function setA(value) {
                    AO.a = value;
                },
                configurable: true
            },
            // length is a number of
            // the actual arguments passed
            length: {value: 1, writable: true, configurable: true},
            // current function
            callee: {value: foo, writable: true, configurable: true}
        })
    },
    // create formal parameters
    a: {value: 10, writable: true},
    b: {value: undefined, writable: true},
});

// 2. Code execution stage

// use with to emulate identifier
// resolution in function's AO

with (AO) {
    
    print(arguments.length); // 1
    print(arguments.callee.length); // 2

    print(a); // 10

    arguments[0] = 20;
    print(a); // 20

    a = 30;
    print(arguments[0]); // 30

    // "b" is not passed to
    // function, so it's not shared
    b = 100;
    print(arguments[1]); // undefined, as should be
    
    // can't delete formal parameter,
    // because it's not configurable
    // {DontDelete} == true ES3,
    // [[Configurable]] == false ES5
    delete a; // false

    // but can delete corresponding
    // arguments' property; however,
    // deleting it, we lose accessor
    delete arguments[0]; // true
    print(arguments[0]); // undefined
    print(a); // 30

    // arguments[0] now is a
    // simple data property
    arguments[0] = 50;
    print(arguments[0]); // 50
    print(a); // 30

    a = 40;
    print(a); // 40
    print(arguments[0]); // 50

}

// This correct behavior currently
// is broken in Chrome, for details
// see http://code.google.com/p/chromium/issues/detail?id=52484