javascript forEach数组回调时间？

Question

So im learning about the different array iterating methods and ran into something odd:

[1,2,3].forEach(function(element,index,arr){
        console.log(element,index);
    console.log(arr);
    arr.shift();
})

So you would think this would produce:

1,0
[1,2,3]
2,1
[2,3]
3,2
[3]

however you get this:

1 0
[1, 2, 3]
3 1
[2, 3]

Im printing off 3 despite have only shifted off at the end? Is this because the arr callback happens at the beginning of the next item or something? Since it should be doing shift() on the array AFTER? I realize this is a "bad case" of using this method, im just curious why this happens.

Answer 1

The .length of the array is changed. The element adjacent to element at index 0 becomes index 0 following .shift() call.

Answer 2

The answer to your question is described in the documentation of shift method.

As per MDN from here

The shift method removes the element at the zeroeth index and shifts the values at consecutive indexes down, then returns the removed value. If the length property is 0, undefined is returned.

So, once the shift operation is done the Original array is changed from [1,2,3] to [2,3] and the value of index increments from 0 to 1 . Next, iteration the third iteration of the loop is Not required as the length of the array (2) has been traversed.