如何防止更改模块函数返回的对象而更改Javascript中的模块数据本身？

Question

I am working with Node for the first time and I have two modules. One module defines an array of objects, and also has functions that are exported for use in the other module - including lookupbyID, lookupbyLastName, and addEmployee.

My issue is that when I call the functions from module 1 in module 2, which return objects from the original array and assign those objects to a variable, and then I modify that variable, it modifies the original data. Please see the following code:

Module 1:

const us = require('underscore')
var data = [
  {id:1, firstName:'John', lastName:'Smith'},
  {id:2, firstName:'Jane', lastName:'Smith'},
  {id:3, firstName:'John', lastName:'Doe'},
]

exports.lookupByID = function (given_id) {
  var found_id = us.findWhere(data, {id:given_id});
  return found_id;
}

Module 2:

const employeeFunctions = require("./employeeModule");
var id_2_answer = employeeFunctions.lookupByID(2);
id_2_answer.firstName = 'Mary'
console.log(employeeFunctions.lookupByID(2))

As you can see, I changed the name of Jane to Mary. Even though I assigned the object to a variable, changing the variable changed the original object data, which I verified by printing the lookupbyID function a second time.

Can you help me understand why this happens? Can you help me understand possible ways to prevent this from happening? I would like to be able to assign the object to a variable, and be able to change the values within the variable without affecting the original data.

Thank you!

Answer 1

In js objects are assigned by reference so:

var foo = {bar:1};
var baz = foo;
baz.bar; // 1
baz.bar = 2;
foo.bar; // 2

Use Object.assign to clone objects instead:

var foo = {bar:1};
var baz = Object.assign({}, foo);
baz.bar; // 1
baz.bar = 2;
foo.bar; // 1

Answer 2

The short answer is that you can't - you've hit on a key aspect of programming: value vs reference. You can read more about it here but the long and short of it is that both of these variables point to the same object in memory, so changing it for one changes it for the other.

The solution will necessarily involve creating a new object - the neatest way is probably to use Object.assign to copy over values, but bear in mind that in your example, you'll probably need to delete your old entry too.