It's a simple push object to array operation. Please try below

var data=[];

var i;
for (i = 0; i < clients.length; i++) { 
  data.push({
    date:clients.date,
    contact:clients.contact
  });
}

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" ]

JSON stands for Javascript object notation so this could simply be a javascript object

var obj = {employees:[
    {
      firstname:"jerry"
      ... and so on ...
     }
]};

When you want to add an object you can simply do:

object.employees.push({
   firstname: "Mike",
   lastName: "rut"
    ... and so on ....
});

If you are using jQuery, you can use the .extend() jQuery API like:

$.extend(ListData, {"hello": { "label":"Hello", "url":"#hello" }});

If your code is running in the browser and users.json is an output file, I guess you already have access to its content.

Use the push() method.

Also, note the missing commas in your objects.

var obj = {
        username: "James",
        surname: "Brandon",
        id: "[2]"
};

var users = [
  {    
    "username": "Andy",
    "surname": "Thompson",
    "id": [0]
  },
  {    
    "username": "Moe",
    "surname": "Brown",
    "id": [1]
  }
];

users.push(obj);
console.log( JSON.stringify(users) );

Now that you have the updated array of objects you can upload it to the server (check this question) or offer a download to the user (check this other question).

As you have been already told, there is no way to directly update client-side a file in the server. It is also not possible to save it directly into the client filesystem.

In general, If you want to append to file you should use:

fs.appendFile("results.json", json , function (err) {
   if (err) throw err;
   console.log('The "data to append" was appended to file!');
});

Append file creates file if does not exist.

But ,if you want to append JSON data first you read the data and after that you could overwrite that data.

fs.readFile('results.json', function (err, data) {
    var json = JSON.parse(data);
    json.push('search result: ' + currentSearchResult);    
    fs.writeFile("results.json", JSON.stringify(json), function(err){
      if (err) throw err;
      console.log('The "data to append" was appended to file!');
    });
})