Changing the order of the Object keys....

var addObjectResponse = [{
    'DateTimeTaken': '/Date(1301494335000-0400)/',
    'Weight': 100909.090909091,
    'Height': 182.88,
    'SPO2': '222.00000',
    'BloodPressureSystolic': 120,
    'BloodPressureDiastolic': 80,
    'BloodPressurePosition': 'Standing',
    'VitalSite': 'Popliteal',
    'Laterality': 'Right',
    'CuffSize': 'XL',
    'HeartRate': 111,
    'HeartRateRegularity': 'Regular',
    'Resprate': 111,    
    'Temperature': 36.6666666666667,
    'TemperatureMethod': 'Oral',    
    'HeadCircumference': '',    
}];

This is a sample object which i am getting from back end, now i want to change the order of the object. I don't want to sort by name or size... i just want to manually change the order...

If you create a new object from the first object (as the current accepted answer suggests) you will always need to know all of the properties in your object (a maintenance nightmare).

Use Object.assign() instead.

*This works in modern browsers -- not in IE or Edge <12.

 let addObjectResponse = {
        'DateTimeTaken': '/Date(1301494335000-0400)/',
        'Weight': 100909.090909091,
        'Height': 182.88,
        'SPO2': '222.00000',
        'BloodPressureSystolic': 120,
        'BloodPressureDiastolic': 80,
        'BloodPressurePosition': 'Standing',
        'VitalSite': 'Popliteal',
        'Laterality': 'Right',
        'CuffSize': 'XL',
        'HeartRate': 111,
        'HeartRateRegularity': 'Regular',
        'Resprate': 111,    
        'Temperature': 36.6666666666667,
        'TemperatureMethod': 'Oral',    
        'HeadCircumference': '',    
    };

    // Create an object which will serve as the order template
    let objectOrder = {
        'HeartRate': null,
        'HeartRateRegularity': null,
    }

    addObjectResource = Object.assign(objectOrder, addObjectResource);

Now the two items you wanted ordered are in order, and the remaining properties are below them.

Now your object will look like this:

{           
            'HeartRate': 111,
            'HeartRateRegularity': 'Regular',
            'DateTimeTaken': '/Date(1301494335000-0400)/',
            'Weight': 100909.090909091,
            'Height': 182.88,
            'SPO2': '222.00000',
            'BloodPressureSystolic': 120,
            'BloodPressureDiastolic': 80,
            'BloodPressurePosition': 'Standing',
            'VitalSite': 'Popliteal',
            'Laterality': 'Right',
            'CuffSize': 'XL',
            'Resprate': 111,    
            'Temperature': 36.6666666666667,
            'TemperatureMethod': 'Oral',    
            'HeadCircumference': '',    
}

I wrote this small algorithm which allows to move keys, it's like jQuery .insertAfter() method. You have to provide:

//currentKey: the key you want to move
//afterKey: position to move-after the currentKey, null or '' if it must be in position [0]
//obj: object


function moveObjectElement(currentKey, afterKey, obj) {
    var result = {};
    var val = obj[currentKey];
    delete obj[currentKey];
    var next = -1;
    var i = 0;
    if(typeof afterKey == 'undefined' || afterKey == null) afterKey = '';
    $.each(obj, function(k, v) {
        if((afterKey == '' && i == 0) || next == 1) {
            result[currentKey] = val; 
            next = 0;
        }
        if(k == afterKey) { next = 1; }
        result[k] = v;
        ++i;
    });
    if(next == 1) {
        result[currentKey] = val; 
    }
    if(next !== -1) return result; else return obj;
}

Example:

var el = {a: 1, b: 3, c:8, d:2 }
el = moveObjectElement('d', '', el); // {d,a,b,c}
el = moveObjectElement('b', 'd', el); // {d,b,a,c}

You can't order JavaScript object key/value pairs. It's stored in its own internal format, so you should never rely on the order of that. In JS, everything is an Object, even an Array. So sometimes you can introduce bugs when using array notation and object notation together (for x in var)

2020 Update

The answer below was correct at time of writing in 2011. However, since ES6, enumeration order has been specified as part of the language. Here's a nice article summarising this: https://2ality.com/2015/10/property-traversal-order-es6.html

Original answer

Properties of an object in JavaScript do not have an order. There may appear to be an order in some browsers but the ECMAScript specification defines object property enumeration order as being implementation-specific so you should not assume one browser's behaviour will be the same as another's. Chrome, for example, does not use the same ordering as some other browsers: see this lengthy bug report for at least as much discussion of this issue as you could possibly want.

If you need a specific order, use an array, or two arrays (one for keys and one for values).