How to correctly check variables for undefined?

Question

JavaScript

I don’t understand why this happens, but because of this, the server often crashes .......
how can I correctly check for undefined so that such an error does not occur?

Answer 1

You have an error that you are trying to read from res.name, but you have res itself undefined.
To check for undefined, just do this
if (res && (res.name && res.id))