This means you are telling to iterate through the outer array right?
Means if the array length is 4 you have to run the for loop 3 times.

Now consider the question filteredArray([[3, 2, 3], [1, 6, 3], [3, 13, 26], [19, 3, 9]], 3)

initially i = 0 and length = 4, 0<4 - true - enter loop
arr[0] contains 3 so you are splicing the array so the length of array is not 4 anymore its 3 now.

i = 1, length = 3, 1<3 - true enter loop
arr[0] contains 3 so you are splicing the array so the length will become 2

i = 2, length = 2, 2<2 false exit loop

as you can see you are stopping at index 1 and not iterating the entire array

also check this condition , you are iterating the inner array and you are splicing the outer array every time you meet the given value in the inner array.

syntax of splice is Array.splice(startIndex,noOfElementsToBeDeleted)

when arr.splice is executed it cuts out a portion of this array thus its length will be no more stable
eg : if you have array = [ 0 , 1 ]
imagine a for loop looping through this array then for i = 0 and array.length will return 2 and a[0] is cut out using array.splice
the new array will be array = [1]
for i = 1 it will stop since array.length will change (it will decrease by 1) and cause this error