Is it possible to add an entry to a Javascript object residing in an another file?

Depends on what you mean by "add". If you mean at run-time, and the function you are executing loads after the file containing the object you want to modify, then the answer is yes, though I don't think you need an addExpenses function but could rather just do the following:

expenses.push({day: "11/11/2019", expenses: {biscuit: 20});

But if by "add" you mean such that the entry is saved to the original file on-disk, then the answer is typically no, not in a browser context, though possibly in a node.js context

It's possible using imports and exports. With the little information provided, i created a little rough example to show you how exporting and importing works.

    var expenses = 
    [
      {
        day: "9/11/2019", expenses: {
            pen: 15,
            tea: 32,
            auto: 40,
            juice: 30,
        }
      },
      {
        day: "10/11/2019", expenses: {
            bananas: 50,
            auto: 100,
            tea: 30,
        }
      }
    ]

module.exports = expenses;

In your add-expenses file :

var expenses = require('dir/yourfile')

Now you have access to your object in whatever file you wish

I didn't test it on different files but something like this should work

Basically you create a function who accept an array and a value as params. In the main file you specify your array, expenses and the value you want to insert. To be sure that your array is ok and you don't insert wrong data, you can make a little validation on the object that you want to insert

// ------- expenses.js ------------

// import add-expense.js
const add = require('./add-expense'); 

var expenses = 
[
  {
    day: "9/11/2019", expenses: {
        pen: 15,
        tea: 32,
        auto: 40,
        juice: 30,
    }
  },
  {
    day: "10/11/2019", expenses: {
        bananas: 50,
        auto: 100,
        tea: 30,
    }
  }
]

let newExpense = {day: "11/11/2019", expenses: {biscuit: 20}}
add.addExpenses(expenses, newExpense)
console.log(expenses)



// ---------  add-expense.js -------------
module.exports = {

  addExpenses: function(myArr, value){
    if(isObjectCorrect(value)){
      myArr.push(value);
    }
  },

  isObjectCorrect: function(value) {
      try {
          JSON.stringify(value);
      } catch (e) {
          return false;
      }
      return true;
  }

}