Interesting JavaScript array length problem code description_javascript skills

First point:

Copy code The code is as follows:

var lennon =Array();
lennon["name"]="windy";
lennon["year"]="1989";
lennon["living"]=true;
alert( lennon.length);

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:

Copy code Code As follows:

var foo2 = [];
foo2['3'] = 3;
foo2['age'] = 20;
debugger;
alert (foo2.length);

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.