How to Add a New Object (Key-Value Pair) to an Array in JavaScript

How to add key values pair in object in JAVASCRIPT?

You cannot push on to an object. You have to specify the key and value like so:

Literally

divByThree.key = value

Note, in the above example, key is the literal key name, and so you would access it like:

divByThree.key
divByThree['key']  // note the quotes around key

Dynamically

If you want to create a dynamic key based on a js variable for example, you can do:

const keyName = 'myKey';
divByThree[keyName] = value;

To access the value, you can do:

divByThree[keyName];  // no quotes. Value of keyName variable
divByThree['myKey'];
divByThree.myKey;

How to add a key value pair to an array in Javascript / Jquery

You can use Array.map() to iterate the dataSource array. Use an internal Array.reduce() to iterate the Months, and add missing months to the current object:

const dataSource = [{"location":"France","January":79,"February":81,"March":23},{"location":"Germany","January":98,"February":83},{"location":"Japan","January":96,"March":11}];const Months = ["January","February","March"];
const result = dataSource.map(o =>  Months.reduce((obj, m) => m in obj ? obj : { ...obj, [m]: 100 }, o));
console.log(result);

Map a new key-value pair into an array of objects

map returns new data, so your function should look something like this

loggedInUser: { self: boolean }[];

addUserStatus() {
  const temp = this.userData.map((data, i) => ({ ...data,
    "logged-in": this.loggedInUser[i].self
  }))
  this.userData = temp
}

If you want to modify this.userData directly, you can use other methods like forEach

addUserStatus() {
  this.userData.forEach((data, i) => {
    data["logged-in"] = this.loggedInUser[i].self;
  };
}

Add new key with Value to an array of nested Objects

You can create a small helper function to add ids to the data once you've received it (from whatever source - here I'm just passing it in as a prop to the component), and before you update the state.

Note: in this minimal working example I've initialised state with an empty array rather than null.

const { Component } = React;

// `map` over the data and for each
// object return an object with an id property
function addId(data) {
  return data.map((obj, i) => {
    return { id: i + 1, ...obj };
  });
}

class Example extends Component {

  constructor(props) {
    super();
    this.state = { data: [] };
  }

  // Use the helper function to add ids
  // to the data objects, then set the new state
  componentDidMount() {
    const data = addId(this.props.data);
    console.log(JSON.stringify(data));
    this.setState({ data });
  }

  render() {
    const { data } = this.state;
    if (!data.length) return <div>No data</div>; 
    return (
      <div>
        {data.map(obj => {
          return <div>{obj.id} {obj.name}</div>
        })}
      </div>
    );
  }

};

const data = [
  { name: 'Rita' },
  { name: 'Sue' },
  { name: 'Bob' }
];

ReactDOM.render(
  <Example data={data} />,
  document.getElementById('react')
);

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.2/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.2/umd/react-dom.production.min.js"></script>
<div id="react"></div>

how to take a key value pair from array of objects and create a new object from that key value pair

Try this:

let arr = [
  {name: "abcd", value: "xyz"},
  {name: "pqr", value: "uvw"}
]
const result = {}
arr.forEach((element) => {
  result[element.name] = element.value
})
console.log(result)

Add a Key Value Pair to an array of objects in javascript?

You can simply add properties ("fields") on the fly.

Try

myarray[0].Address = "123 Some St.";

myarray[0]["Address"] = "123 Some St.";

var myarray = [];
myarray.push({    "Name": 'Adam',    "Age": 33});
myarray.push({    "Name": 'Emily',    "Age": 32});
myarray[0]["Address"] = "123 Some St.";
console.log( JSON.stringify( myarray, null, 2 ) );