为什么for-of不能跳过稀疏数组的空插槽？ [JavaScript的]

Question

As the title says, why does for-of run the loop body with the loop variable bound to undefined for indices not in the Array , while other iteration constructs ( forEach() , for-in , etc.) don't?

Clarification : As many misunderstood the question

It is not about:

• iterating over TypedArray s (which cannot be sparse) or any other class
• how to "correctly" iterate over a sparse Array (every other method seems to work in the expected way)
• skipping undefined elements in an Array

---

Is the following informal description found on MDN incorrect?

The for...of statement [...] invokes a custom iteration hook with statements to be executed for the value of each distinct property of the object.

Ie it is also invoked for non-existent properties.

 const sparse = [0, 1, 2] // Was [0, , 2], but some are unfamiliar with this syntax // or think it creates the array [0, undefined, 2] delete sparse[1] for (let e of sparse) console.log('for-of', e) // Contrast with: sparse.forEach(e => console.log('forEach', e)) for (let i in sparse) console.log('for-in', sparse[i]) console.log('map', sparse.map(e => e)) // Note, prints incorrectly in the snippet // console, check browser console // etc. 

Is this the intended behaviour ( Yes ) & why was it designed in this way?

Answer 1

for..of calls the Array iterator method, which is described in the spec .

(2) Let iterator be ObjectCreate(%ArrayIteratorPrototype%, «‍[[IteratedObject]], [[ArrayIteratorNextIndex]], [[ArrayIterationKind]]»).

(4) Set iterator's [[ArrayIteratorNextIndex]] internal slot to 0.

Then, when the iterator is iterated over, in 22.1.5.2.1 %ArrayIteratorPrototype%.next: :

(6) Let index be the value of the [[ArrayIteratorNextIndex]] internal slot of O.

(10) If index ≥ len, then

(10) (a) Set the value of the [[IteratedObject]] internal slot of O to undefined.

(10) (b) Return CreateIterResultObject(undefined, true).

(11) Set the value of the [[ArrayIteratorNextIndex]] internal slot of O to index+1 .

(create iterator result object whose value is array[index] )

In other words - the iterator iterates starting at index 0, and increments the index by 1 every time .next() is called. It does not check to see if the array actually has an item at that index (which a sparse array will not) - it just checks that the index is less than the .length of the array.

With for..in , on the other hand, all enumerable properties are iterated over, and an array's own enumerable properties do not include sparse array indicies.

 const sparse = [0, , 2]; console.log(sparse.hasOwnProperty('0')); console.log(sparse.hasOwnProperty('1')); 

So yes, this is intended behavior.