FreeCodeCamp：使用For循环遍历数组的所有项目

Question

I've got this working block of code from the freecodecamp hint

  function filteredArray(arr, elem) {
  let newArr = [...arr];

  for(let i = 0; i < newArr.length; i++) {
    for(let j = 0; j < newArr[i].length; j++) {
      if(newArr[i][j] == elem) {
        newArr.splice(i,1);
        i--;
        break;
      }
    }
  }

  return newArr;
}

console.log(filteredArray([[3, 2, 3], [1, 6, 3], [3, 13, 26], [19, 3, 9]], 3));

I understand most part of it but not i--; break; i--; break; I really need an explanation as to why i--; break; i--; break; was used.

https://learn.freecodecamp.org/javascript-algorithms-and-data-structures/basic-data-structures/iterate-through-all-an-arrays-items-using-for-loops/

Answer 1

This is a common question when a developer says, this code is skipping indexes.

You are altering the original array and removing the index. So think about it like a stack of blocks. You remove one, all of the ones above it drop down one.

So you are removing an item with splice, so you need to reduce the index by one so you do not skip over the item that fills in the gap.

A [B] C D E <-- before 
   ^
A [C] D E   <-- after

This is why people normally loop from the end to the start to avoid this.