UndefinedVsNull.js

//-------------------------------------------------------------------------------------------------------
// Copyright (C) Microsoft. All rights reserved.
// Licensed under the MIT license. See LICENSE.txt file in the project root for full license information.
//-------------------------------------------------------------------------------------------------------

//
// Test Equals operator with abstract equality comparison algorithm (ES3.0: S11.9.1, S11.9.3)
//

if (undefined == null)
    WScript.Echo("Algorithm says equivalent");
else
    WScript.Echo("Objects are not equivalent");

//
// Test Strict Equals operator (ES3.0: S11.9.4)
//

if (undefined === null)
    WScript.Echo("Same instance");
else
    WScript.Echo("Different instances");

if (undefined === undefined)
    WScript.Echo("Same instance");
else
    WScript.Echo("Different instances");

if (null === null)
    WScript.Echo("Same instance");
else
    WScript.Echo("Different instances");

function dump(a, index)
{
    var value = a[index];
    if (value === undefined)
    {
        WScript.Echo("'undefined'");
    }
    else if (value === null)
    {
        WScript.Echo("'null'");
    }
    else
    {
        WScript.Echo(value);
    }
}

//
// Create an array and grow it, ensuring that all empty slots are properly set to 'undefined'
//

var a = new Array(2);

dump(a, 0);
dump(a, 1);

dump(a, 10);
a[10] = 'A';
dump(a, 10);

dump(a, 5);