How to assign a value to new attribute to array json objects in java script?

The array map method returns a new array computed by calling the predicate in your case (order) => { order.orderCustomerCount; return undefined; } (verbose return, functions are equivalent) for each element of the array, and storing the return at the same position in a new array.

[1, 2, 3, 4, 5].map((n) => 2 * n);
// returns [2, 4, 6, 8, 10]

In your case, you return undefined in the predicate, because any arrow function has a default return of undefined.

In your case a more appropriate solution would be the array forEach method.

listOrders.docs.forEach((order)=> {
   order.orderCustomerCount = 1;
});

This method only calls the predicate for each element of the array. Because arrays and objects are reference types, the value of the elements are mutated to add the property.

Additionally, you might want to consider whether or whether not you want this. In my own experience, when using objects and arrays, in order to increase predictability, I rarely use assignments, instead I construct new arrays of new objects to ensure the source is not mutated.

let newOrders = listOrders.docs.map((order) => {
   let newOrder = {
       ...order, 
       // copy all attributes (properties) of order
       orderCustomerCount: 1,
       // add orderCustomerCount property equal to 1
   };

   return newOrder;
})

Note the spread syntax.

By doing this, the original object is kept as it were, and newOrders along with the elements of the array can be modified independently from it.

the .map() function creates & returns a new array. You need to do the following:

listOrders.docs = listOrders.docs.map((order) => {
  order.orderCustomerCount = 1
  /* 
     NOTE: This line is important because if you don't return 
     anything from this function, then your array will have an 
     undefined item in it.
  */
  return order; 
})