changing value of local variable perform change in value of original global variable

Question

In my node js program in routes i have created array named list. I have assigned value to array from function declared in model. Code for route is:

var express = require('express');
var router = express.Router();
var questionModel = require('../model/questionModel');
var userModel=require('../model/userModel');

/* GET home page. */
router.get('/', function(req, res, next) {



//declare an array and store the json data
var list=questionModel.getAllQuestions();

 for(i=0;i<list.length; i++){
  console.log(list[i].submitter);
   console.log(userModel.getUserById(list[i].submitter)[0]);
   list[i].submitter=userModel.getUserById(list[i].submitter)[0].fname;
   }
  console.log(list);
//respond with the array
res.json(list);

//res.redirect("../question/" + this_id);

});

module.exports = router;

Here in module i am using local variable as in this class project i am not using any database. My all Models fetch value from one global variable.

This code is work fine on first request. But on first request changing value of list[i].submitter locally value of global value change.

Change in global value creates problem when i get second request. On second request value return from questionModel.getAllQuestions is unnecessarily updated.

score 2 · Accepted Answer · edited May 23 '17 at 12:06

Array is always pass by reference in javascript. That's the reason why when you edit the array it affects the original array.

In your case, though you're making a copy of questionModel, still what your getting is a list of objects in your array. The objects reference in both the arrays will refer to same object.

So the changes you make to list[i].submitter is affecting the original object as well.

If you don't want the update to happen, then you need to deep copy the objects inside array as well like below:

function deepCopy (arr) {
    var out = [];
    for (var i = 0, len = arr.length; i < len; i++) {
        var item = arr[i];
        var obj = {};
        for (var k in item) {
            obj[k] = item[k];
        }
        out.push(obj);
    }
    return out;
}

// test case

var questionsModelOriginal = [
    {'a' : 1, submitter : "Rias"},
    {'b' : 2, submitter : "SO"}
    ];
var questionsModelCopy = deepCopy(questionsModelOriginal);

Now if you change the property of questions inside questionsModelCopy, it will not modify the global Questions model.

Refer SO

Yes this is the problem. But what is the solution? – vishal patel Apr 04 '15 at 07:14 — vishal patel, Apr 04 '15 at 07:14

changing value of local variable perform change in value of original global variable

1 Answers1