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The problem is as shown in the picture,I don’t understand why undefined is displayed in the program? The console reports an error?
Your array only has two items, so if you get a[3], the fourth item will of course be undefined...
The same is true for the ob object. It does not have a key of 3, but only a key of 1.
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
Your array only has two items, so if you get a[3], the fourth item will of course be undefined...
The same is true for the ob object. It does not have a key of 3, but only a key of 1.