TypeError: Cannot read property 'variable' of undefined

Question

I am getting this undefined error in my node js written program. This was the line that was causing the issue

sockets[index].notification = 1;

So in order to resolve this, I did this

if ( typeof sockets[index].notification !== 'undefined' ) {
                              
   sockets[index].notification = 1;
 }

But it still gave the same error

if ( typeof sockets[index].notification_acc_on !== 'undefined' ) {
                                                   ^

TypeError: Cannot read property 'notification' of undefined
    at Socket.<anonymous> (/var/www/html/server.js:506:56)
    at emitOne (events.js:116:13)
    at Socket.emit (events.js:211:7)
    at addChunk (_stream_readable.js:263:12)
    at readableAddChunk (_stream_readable.js:250:11)
    at Socket.Readable.push (_stream_readable.js:208:10)
    at TCP.onread (net.js:597:20)

How can I check and handle this so if the variable isn't set, my program doesn't crash at least? In short I want to prevent the crash

The error states "Cannot read property 'notification' ***of*** undefined". You are reading `notification` of `sockets[index]`. `sockets[index]` is `undefined`. Not `sockets[index].notification`. That being said it sounds like an [off-by-one error](https://stackoverflow.com/questions/2939869/what-is-an-off-by-one-error-and-how-do-i-fix-it). — Ivar, Oct 14 '21 at 19:03

sfuqua · Accepted Answer · 2021-10-14T19:05:34.990

1

It appears that the problem is with sockets[index] being undefined.

if ( typeof sockets[index] !== 'undefined' ) {
                              
   sockets[index].notification = 1;
}

There are other checks you might consider exploring, for example checking for null value, confirming that sockets exists, etc. My response only deals with the immediate problem from the original post.

edited Oct 14 '21 at 19:05

answered Oct 14 '21 at 18:59

sfuqua

5,797
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The variable `sockets` can itself be `undefined`, in which case your code will also fail. – Maksym Shcherban Oct 14 '21 at 19:02
@sfuqua I am sorry so what is the solution here? – mynameisbutt Oct 14 '21 at 19:02
@MaksymShcherban sockets variable has been defined globally. – mynameisbutt Oct 14 '21 at 19:02
In this revision, we check that the object returned by `[index]` is not undefined, whereas in your original you are checking a property _on_ that object. – sfuqua Oct 14 '21 at 19:03
@sfuqua Oh I am sorry got it thank you – mynameisbutt Oct 14 '21 at 19:06

score 0 · Answer 2 · answered Oct 14 '21 at 19:01

0

You reveive this error because either sockets or sockets[index] is undefined. To avoid this error, you can perform the following check:

if (typeof sockets?.[index] !== 'undefined') {
}

answered Oct 14 '21 at 19:01

Maksym Shcherban

742
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okay, Let me implement this. thank you for the answer – mynameisbutt Oct 14 '21 at 19:03

TypeError: Cannot read property 'variable' of undefined

2 Answers2