How to add a new key value pair in existing JSON object using JavaScript?

If you want to add new key and value to each of the key of json object and then you can use the following code else you can use the code of other answers –

Object.keys(json).map(
  function(object){
    json[object]["newKey"]='newValue'
});

 const Districts=[
  {
    "District": "Gorkha",
    "Headquarters": "Gorkha",
    "Area": "3,610",
    "Population": "271,061"
  },
  {
    "District": "Lamjung",
    "Headquarters": "Besisahar",
    "Area": "1,692",
    "Population": "167,724"
  }
]
Districts.map(i=>i.Country="Nepal")
console.log(Districts)

If you have JSON Array Object Instead of a simple JSON.

const Districts= [
  {
    "District": "Gorkha",
    "Headquarters": "Gorkha",
    "Area": "3,610",
    "Population": "271,061"
  },
  {
    "District": "Lamjung",
    "Headquarters": "Besisahar",
    "Area": "1,692",
    "Population": "167,724"
  }
]

Then you can map through it to add new keys.

Districts.map(i=>i.Country="Nepal");

Your object is only a JavaScript object and not JSON. You can either use brackets notation or the dot notation like

json["workbookInformation"]["new_key"] = "new_value";

var json={
    "workbookInformation": {
     "version": "9.1",
    "source-platform": "win"
    }
    
    }

json.workbookInformation.new_key = 'new_value';
console.log(json);

This should work:

json.workbookInformation.new_key = 'new_value';

json.workbookInformation.key = value;