javascript - js array traversal issues

Question

Does anyone know how to get the variable names in the array? For example, [a, b, c] wants to get the values ​​​​of a, b, c and a, b, c respectively by traversing.

Answer 1

> arr=[]

> arr['a']=1

> arr['b']=2

> arr['c']=3

> for(k in arr)console.log(k,arr[k])
a 1
b 2
c 3

> arr
[ a: 1, b: 2, c: 3 ]

Answer 2

No.

Answer 3

for in ?