Appending to an object

How about storing the alerts as records in an array instead of properties of a single object ?

var alerts = [ 
    {num : 1, app:'helloworld',message:'message'},
    {num : 2, app:'helloagain',message:'another message'} 
]

And then to add one, just use push:

alerts.push({num : 3, app:'helloagain_again',message:'yet another message'});

You can do this with Object.assign(). Sometimes you need an array, but when working with functions that expect a single JSON object -- such as an OData call -- I've found this method simpler than creating an array only to unpack it.

var alerts = { 
    1: {app:'helloworld',message:'message'},
    2: {app:'helloagain',message:'another message'}
}

alerts = Object.assign({3: {app:'helloagain_again',message:'yet another message'}}, alerts)

//Result:
console.log(alerts)
{ 
    1: {app:'helloworld',message:'message'},
    2: {app:'helloagain',message:'another message'}
    3: {app: "helloagain_again",message: "yet another message"}
} 

EDIT: To address the comment regarding getting the next key, you can get an array of the keys with the Object.keys() function -- see Vadi's answer for an example of incrementing the key. Similarly, you can get all the values with Object.values() and key-values pairs with Object.entries().

var alerts = { 
    1: {app:'helloworld',message:'message'},
    2: {app:'helloagain',message:'another message'}
}
console.log(Object.keys(alerts))
// Output
Array [ "1", "2" ]

You can use spread syntax as follows..

var alerts = { 
1: { app: 'helloworld', message: 'message' },
2: { app: 'helloagain', message: 'another message' }
 }

alerts = {...alerts, 3: {app: 'hey there', message: 'another message'} }

Now with ES6 we have a very powerful spread operator (...Object) which can make this job very easy. It can be done as follows:

let alerts = { 
   1: { app: 'helloworld', message: 'message' },
   2: { app: 'helloagain', message: 'another message' }
} 

//now suppose you want to add another key called alertNo. with value 2 in the alerts object. 

alerts = {
   ...alerts,
   alertNo: 2
 }

Thats it. It will add the key you want. Hope this helps!!

You should really go with the array of alerts suggestions, but otherwise adding to the object you mentioned would look like this:

alerts[3]={"app":"goodbyeworld","message":"cya"};

But since you shouldn't use literal numbers as names quote everything and go with

alerts['3']={"app":"goodbyeworld","message":"cya"};

or you can make it an array of objects.

Accessing it looks like

alerts['1'].app
=> "helloworld"

Do you have the ability to change the outer-most structure to an array? So it would look like this

var alerts = [{"app":"helloworld","message":null},{"app":"helloagain","message":"another message"}];

So when you needed to add one, you can just push it onto the array

alerts.push( {"app":"goodbyeworld","message":"cya"} );

Then you have a built-in zero-based index for how the errors are enumerated.

Way easier with ES6:

let exampleObj = {
  arg1: {
    subArg1: 1,
    subArg2: 2,
  },
  arg2: {
    subArg1: 1,
    subArg2: 2,
  }
};

exampleObj.arg3 = {
  subArg1: 1,
  subArg2: 2,
};

console.log(exampleObj);

{
arg1: {subArg1: 1, subArg2: 2}
arg2: {subArg1: 1, subArg2: 2}
arg3: {subArg1: 1, subArg2: 2}
}

As an alternative, in ES6, spread syntax might be used. ${Object.keys(alerts).length + 1} returns next id for alert.

let alerts = { 
    1: {app:'helloworld',message:'message'},
    2: {app:'helloagain',message:'another message'}
};

alerts = {
  ...alerts, 
  [`${Object.keys(alerts).length + 1}`]: 
  { 
    app: `helloagain${Object.keys(alerts).length + 1}`,message: 'next message' 
  } 
};

console.log(alerts);

I'm sorry but i can't comment your answers already due my reputation!...so, if you wanna modify the structure of your object, you must do like Thane Plummer says, but a little trick if you do not care where to put the item: it will be inserted on first position if you don't specify the number for the insertion.

This is wonderful if you want to pass a Json object for instance to a mongoDB function call and insert a new key inside the conditions you receive. In this case I gonna insert a item myUid with some info from a variable inside my code:

// From backend or anywhere
let myUid = { _id: 'userid128344'};
// ..
// ..

  let myrequest = { _id: '5d8c94a9f629620ea54ccaea'};
  const answer = findWithUid( myrequest).exec();

// ..
// ..

function findWithUid( conditions) {
  const cond_uid = Object.assign({uid: myUid}, conditions);
  // the object cond_uid now is:
  // {uid: 'userid128344', _id: '5d8c94a9f629620ea54ccaea'}
  // so you can pass the new object Json completly with the new key
  return myModel.find(cond_uid).exec();
}

[Javascript] After a bit of jiggery pokery, this worked for me:

 let dateEvents = (
            {
                'Count': 2,
                'Items': [
                    {
                        'LastPostedDateTime': {
                            "S": "10/16/2019 11:04:59"
                        }
                    },
                    {
                        'LastPostedDateTime': {
                            "S": "10/30/2019 21:41:39"
                        }
                    }
                ],
            }
        );
        console.log('dateEvents', dateEvents);

The problem I needed to solve was that I might have any number of events and they would all have the same name: LastPostedDateTime all that is different is the date and time.