Slice and Splice questions

JavaScript
1

Why cant you do this code:

function frankenSplice(arr1, arr2, n) {
  let localArr = arr2.slice();
  localArr.splice(n, 0, ...arr1);
  return localArr;
}

Like this

function frankenSplice(arr1, arr2, n) {
  let localArr = arr2.slice();
  return localArr.splice(n, 0, ...arr1);
}

Thanks,
Andrej

2

because splice returns an array with the removed elements (in this case returns an empty array), not the array on which it is used

3

This explanation doesn’t make sense to me.
Why does it

4

it returns an empty array because you have removed 0 elements (second argument of splice is 0)

the returned value of splice is the array with the removed elements, you can find it in the documentation about the splice method

6

This explanation doesn’t make sense to me.
Why does it:

7

because splice returns the array with removed elements, as this time the elements removed are 0, it returns an array with 0 elements, so it is empty

8

.slice() makes a copy of an array.
.splice() mutates the array.

Ex:

let evens = [2, 4, 6, 8];

// beginning with 0th index remove 1 element.
evens.splice(0, 1); 
console.log(evens); // [4, 6, 8]

Had you set it to a variable…

let odds = [1, 3, 5, 7, 9];
const removed_elements = odds.splice(0, 1);

console.log(odds); // [3, 5, 7, 9]
console.log(removed_elements); // [1]

In my experience,. most senior developers stay away from mutating. It’s generally better to use a copy and modify the copy instead. It’s far easier to debug.

1 Like