Question

non sparse array are contiguous in nature and from 0 to length-1, all index should return true for ‘in’ operator. It should be true also for empty element.

obj1 = {x:1,y:2};
obj2 = Object.create(obj1);
obj2.z = 5;
var arr = [obj1,obj2,1,,2];

console.log(arr.length); //5
console.log(3 in arr);//false

index 3 is valid.
Why is it returning false?

Note: I am using latest firefox.

It should return false only for sparse array which doesn’t have a specified index number 3.

Update: index 3 is valid so it should return true. in operator doesn’t validate value of at the indexed position, it just validate the validity of index not the value

Answer 1

3 in arr is false because arr[3] is never set (and thus the array is indeed sparse).

Note the double comma in

var arr = [obj1,obj2,1,,2];

– if you make that

var arr = [obj1,obj2,1,2];

then 3 in arr becomes true.

See page 63 in the specification:

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the
element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another
comma), the missing array element contributes to the length of the Array and increases the index of
subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array,
that element does not contribute to the length of the Array.

– in other words, the behavior of your code is identical to

var arr = [obj1, obj2, 1];
arr[4] = 2;