create new key in object javascript code example

Example 1: javascript add to object

var element = {}, cart = [];
element.id = id;
element.quantity = quantity;
cart.push(element);

// Array of Objects in form {element: {id: 10, quantity: 10} }
var element = {}, cart = [];
element.id = id;
element.quantity = quantity;
cart.push({element: element});

Example 2: js create object with keys

var users = [{userId: "1", name: 'harald'}, {userId: "2", name: 'jamie'}];    
var obj = {};
users.forEach(user => {
  obj = {
    ...obj,
    [user.userId]: user,
  }
})
console.log(obj)
// {
//   1: {userId: "1", name: "harald"}
//   2: {userId: "2", name: "jamie"}
// }