CODEWITHSUNDEEP
javascriptnode.js
Sujeet
Sujeet

Reputation: 3810

Using this.property returns undefined inside a method on prototype

In the following snippet I made an event emitter which executes all the event when called. I used a function constructor and added on and emit methods on it.
The even transfer updates the objects property amount.
But when I access this.amount inside on it returns undefined.
emitter.js

function Account(n) {
    this.balance=n;
    this.events = {}
}
Account.prototype.on =  function(type, listener){
    this.events[type] = this.events[type] || [];
    this.events[type].push(listener);
}

Account.prototype.emmit =  function(type){
    if(this.events[type]) {
        this.events[type].forEach(listener => {

            listener();
        });
    }
    console.log("Amount transfered. Total Balance is " + this.balance);
}
module.exports = Account;

app.js

const Account = require('./emitter');

const myAccount = new Account(5);
myAccount.on('transfer', function(amount = 10){
    this.balance += amount;
    console.log(amount + ' Rs. has been transferrered! Balance is ' + this.balance);
});

myAccount.on('transfer', function(amount = 30){
    this.balance += amount;
    console.log(amount + 'Rs. has been transferrered! Balance is ' + this.balance);
});

myAccount.on('transfer', function(amount = 40){
    this.balance += amount;
    console.log(amount + 'Rs. has been transferrered! Balance is ' + this.balance);
});
myAccount.emmit('transfer');

This is what I have already tried.
I think that this.events is already an object so this inside it will refer to the current object not the parent object.
So tried accessing the property myAccount.amount. Still it returns undefined
I also tried storing reference of this in a variable that and instead of accessing property in this.amount, i tried that.amount still undefined.

Upvotes: 0

Views: 114

Answers (1)

c-smile
c-smile

Reputation: 27460

Your are not passing 'this' to your listener callback function.

Change listener() invocation to this:

listener.call(this);

Upvotes: 2

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