0 in [1,2, ,4,5]
→
2 in [1,2, ,4,5]
→
"length" in [1,2, ,4,5]
→
"a" in {a:1, b:2, d:3}
→
"c" in {a:1, b:2, d:3}
→
d instanceof Date
→
d instanceof Object
→
d instanceof Array
→
[1,2, ,4,5] instanceof Array
→