Cannot destructure property `variableName` of 'undefined' or 'null'

Question

The original error say's: Cannot destructure property 'firstime' of 'undefined' or 'null'.

I am developing web-base desktop application for Windows pc using node.js and Electron. I am trying to persist some data in user data directory, I found the idea and using the same approach in this link.

Writing and fetching data works fine, however the error occurred at the first time of fetching the data.

here is the code for UserPreferences class

const electron = require('electron');
const path = require('path');
const fs = require('fs');

class UserPreferences {
    constructor(opts) {
            const userDataPath = (electron.app || electron.remote.app).getPath('userData');
            this.path = path.join(userDataPath, opts.configName + '.json');
            this.data = parseDataFile(this.path, opts.defaults);
            console.log(userDataPath); 
        }
    get(key) {
            return this.data[key];
        }
    set(key, val) {
        this.data[key] = val;
        fs.writeFileSync(this.path, JSON.stringify(this.data));
    }
}

function parseDataFile(filePath, defaults) {
    try {
        return JSON.parse(fs.readFileSync(filePath));
    } catch (error) {
        return defaults;
    }
}

module.exports = UserPreferences;

and here's the function for using the UserPreferences class

function isFirstTime() {
            try{
                const userAccount = new UserPreferences({
                configName: 'fipes-user-preferences', // We'll call our data file 'user-preferences'
                defaults: {

                    user: { firstime: true, accountid: 0, profileid: '' }
                }
                });

                var { firstime, accountid, profileid } = userAccount.get('user');

                if (firstime === true) {  //check if firstime of running application
                    //do something
                } else {
                    //do something
                }   
            }catch(err){
                console.log(err.message);
            }
        }

the error occurred on the line where I am checking weather the firstime is true or false.

Answer 1

Here is a version of UserPreferences which will be more natural to use as you write your code. You can create it like you see in isFirstTime.

console.debug(userPreferences[accountId]);
userPreferences[accountId] = 1;

This is preferred because there is no reason for a developer not to treat UserPreferences as an object. Another good idea would be separating the writing to the file into a separate flush method, in case you are updating preferences often.

const electron = require("electron");
const fs = require("fs");
const path = require("path");

class UserPreferences {
    constructor(defaultPrefs, pathToPrefs) {
        const app = electron.app || electron.remote.app;
        this.pathToPrefs = path.join(app.getPath("userData"), pathToPrefs + ".json");

        try {
            this.store = require(this.pathToPrefs);
        }
        catch (error) {
            this.store = defaultPrefs;
        }
        return new Proxy(this, {
            get(target, property) {
                return target.store[property];
            },
            set(target, property, value) {
                target.store[property] = value;
                fs.writeFileSync(target.pathToPrefs, JSON.stringify(target.store));
            }
        });
    }
}

module.exports = UserPreferences;

Here is a pure version of isFirstTime, that should do what you want, while maintaining a more robust method of checking for isFirstTime. The check can also be changed so check whether lastSignIn is equal to createdAt (with appropriate defaults, of course).

function isFirstTime() {
    const account = new UserPreferences({
        user: {
            accountId: 0,
            createdAt: new Date(),
            lastSignIn: null,
            profileId: ""
        }
    }, "fipes-user-preferences");

    const {lastSignIn} = account;

    return lastSignIn === null;
}

Answer 2

First of all do not declare a object like var { firstTime, .. } like this. if you do this firstTime will be a property of an anonymous object. That you can never access elsewhere. Check what is the output of userAccount.get('user') function, output contain some object like { firstime: true, accountid: "test", profileid: "test" } then try this. Hope this helps you.

var result=userAccount.get('user');
if(result.firstTime===true){
  //your code
}