nodejs exported variable is not reusable on second hit

Question

I am exporting a variable which is an array from file1.js:

// file1.js
const myArray = [
    {
        "name": "Judah Hendricks",
        "email": "diam@lobortis.net",
        "hobbies": [
            {
                "sports": "tennis",
                "recent": true
            },
            {
                "sports": "football",
                "recent": false
            },
            {
                "sports": "volleyball",
                "recent": false
            }
        ]
    },
    {
        "name": "Jakeem Olsen",
        "email": "at@tellus.org",
        "hobbies": [
            {
                "sports": "swimming",
                "recent": false
            },
            {
                "sports": "running",
                "recent": true
            }
        ]
    }
];
module.exports = { myArray };

and I am using it inside a route:

//main.js
const { myArray } = require('./file1.js');
router.get('/getData', (req, res) => {
    dataBase.users.findOne({ email: req.user.email }, function(error, data){
        if (data){
            const myDataClone = [...myArray];
            let newData = [];
            newData = myDataClone.map( dt => {
                dt.oldHobbies = []
                for (let i = 0; i < dt.hobbies.length; i++) { // Cannot read property 'length' of undefined
                    if (dt.hobbies[i].recent) {
                        dt.recentHobby = dt.hobbies[i];
                    } else {
                        dt.oldHobbies.push(dt.hobbies[i]);
                    }
                }
                delete dt.hobbies;
                return dt;
            })
            return res.status(200).json({userInfo: newData})
        } else {
            // do another
        }
    });
}

when I hit this route for the first time, it returns the expected data. my problem is when I revisit this route for the second time, I get Cannot read property 'length' of undefined for the dt.hobbies . Could somebody help me understand what is wrong here and how to fix it?

You would have to do a deep copy of the array, you are duplicating the array itself, but inside the copy, there are references to the original objects — Akxe, Jun 09 '18 at 20:35

CertainPerformance · Accepted Answer · 2018-06-09T20:51:22.043

3

The problem is here:

const myDataClone = [...myArray];

You're only shallow cloning the array, not deep cloning it; although the myDataClone array is different from myArray, the objects inside both arrays refer to the same objects in memory. So, when you

delete dt.hobbies;

the original object's hobbies property gets deleted as well, which means when you try to access the hobbies property of the object later, errors are thrown.

Fix it by making a deep clone at the beginning instead:

const myDataClone = JSON.parse(JSON.stringify(myArray));

As mentioned here, JSON.parse(JSON.stringify()) is probably the quickest way to deep clone objects. There are other methods like adding jQuery as a dependancy and using jQuery.extend, or writing your own custom cloning function, but serializing and deserializing works just fine.

edited Jun 09 '18 at 20:51

answered Jun 09 '18 at 20:35

CertainPerformance

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i see, thanks for solution. as @Akxe mentioned, it would be nice if some deep copy algo was suggested – Bonnard Jun 09 '18 at 20:43
1

If they are just plain objects json parse/stringify outperforms any deep clone algorithm – lleon Jun 09 '18 at 20:50
Does it really? @lleon – Akxe Jun 09 '18 at 22:54

nodejs exported variable is not reusable on second hit

1 Answers1