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.

const 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
const objectOrder = {
  'HeartRate': null,
  'HeartRateRegularity': null,
}
  
const addObjectResource = Object.assign(objectOrder, addObjectResource);

The two items you wanted to be 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).

I like the approved answer by Chamika Sandamal. Here’s a simple function that uses their same logic with a little be of freedom to change the order as you need it.

function preferredOrder(obj, order) {
    var newObject = {};
    for(var i = 0; i < order.length; i++) {
        if(obj.hasOwnProperty(order[i])) {
            newObject[order[i]] = obj[order[i]];
        }
    }
    return newObject;
}

You give it an object, and an array of the key names you want, and returns a new object of those properties arranged in that order.

var data = {
    c: 50,
    a: 25,
    d: 10,
    b: 30
};

data = preferredOrder(data, [
    "a",
    "b",
    "c",
    "d"
]);

console.log(data);

/*
    data = {
        a: 25,
        b: 30,
        c: 50,
        d: 10
    }
*/

I’m copying and pasting from a big JSON object into a CMS and a little bit of re-organizing of the source JSON into the same order as the fields in the CMS has saved my sanity.

You can use Object.keys():

var obj = {
    a: 1,
    b: 2,
    c: 4
};

var new_obj = {};

Object.keys(obj)
.sort(function(a, b) {
    /** Insert your custom sorting function here */
    return a - b;
})
.forEach(function(key) {
    new_obj[key] = obj[key];
});

obj = new_obj;

If you do not want to create a new object, you can use the following code snippet.

function orderKey(obj, keyOrder) {
    keyOrder.forEach((k) => {
        const v = obj[k]
        delete obj[k]
        obj[k] = v
    })
}

here is a codepen: https://codepen.io/zhangyanwei/pen/QJeRxB

I think that’s not possible in JavaScript.

You can create an array which will contain the field names in your order and you can iterate through this array and fetch the fields from the actual object.

Just refer to the object keys in the order that you like:

aKeys = [
  addObjectResponse[0].DateTimeTaken,
  addObjectResponse[0].Weight,
  addObjectResponse[0].Height,
  ...etc...
]

function orderKeys(obj, keys){
    const newObj = {};
    for(let key of keys){
        newObj[key] = obj[key];
    }
    return newObj;
}