Suppose we have 3 arrays:
var arr1 = [undefined, undefined, undefined];
var arr2 = [, , ,];
var arr3 = new Array(3);
Are all of them identical in JavaScript?
If we wants to use undefined as default value for fixed size array, can we use them interchangeably?
The second two are the same (though the second one might cause issues on some browsers), but the first is different.
The first one
var arr1 = [undefined, undefined, undefined];
makes explicit assignments to the first three indexes in the array. Even though you're assigning undefined, the fact that indexes 0, 1, and 2 are targets of assignment operations means that those elements of the array will be treated as "real" by methods like .reduce():
[undefined, undefined, undefined].reduce(function(c) { return c + 1; }, 0);
// 3
[,,].reduce(function(c) { return c + 1; }, 0);
// 0
Many of the Array.prototype methods skip uninitialized array elements.
[]for an empty array. Relying on the array length seems like a really fragile design pattern.