Slice and Splice-Why the result is reversed?

JavaScript
1

Tell us what’s happening:

Your code so far


function frankenSplice(arr1, arr2, n) {
  // It's alive. It's alive!
  let copyArr1 = arr1.slice(0);
  let copyArr2 = arr2.slice(0);
  
  copyArr1.forEach((x)=>copyArr2.splice(n,0,x));
  return copyArr2;
}

frankenSplice([1, 2, 3], [4, 5], 1);

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.169 Safari/537.36.

Link to the challenge:
https://learn.freecodecamp.org/javascript-algorithms-and-data-structures/basic-algorithm-scripting/slice-and-splice

Can someone tell me why the result of arr1 is reversed???
I got the result as [ 4, 3, 2, 1, 5 ]
Many thanks.

2

Let’s think about what we are trying to achieve with this function:

copyArr1.forEach((x)=>copyArr2.splice(n,0,x));

We’re taking copyArr1 which contains arr1 and iterating or running through each element it contains. Each time you iterate an element you’re splicing that element at nth index of copyArr2 containing arr2

Loop
[1,2,3]
 ^
Add element to index 1 with copyArr[0] at second index without deleting any elements after

This will put the element at the second index of copyArr2: [ 4, 1, 5 ]

The second iteration will do the same thing by adding copyArr1[i] to the second index again:
[ 4, 2, 1, 5 ]

Are you starting to see the pattern? The index isn’t changing to accomdate the order of copyArr1 hence the reverse effect you’re encountering.

So the trick will be how would you go about determining the next index is to properly insert arr1 elements in the correct order? I’ll give you a hint and point you to the forEach() docs on mdn:

The truth lies in what parameter passing with forEach() and how you use it.

3

OH!!! I understand it now!!! Thank you so much!.
Before reading your answer, I just simply used copyArr1.reverse() before running copyArr1.forEach().
I think I know how to modify it in another way. Thank you!!