Interesting JavaScript array length problem code description_javascript skills

First point:

Result: lennon.length is 0

Cause:
Increasing array elements in the following way will not change the length of the array
arr["a"]=3;
Second point:


foo2.length is 4 again

All array objects without assignment will be defined as undefined
JavaScript can automatically resize arrays. This automatic resizing mechanism makes out-of-bounds arrays more likely to occur and harder to detect. So pay extra attention to situations like this.