Why isn't my function updating my variable in JS?

Question

I'm trying to update biggestIslandLength in my function, but it isn't working. I know a solution would be to use a global variable, but I want to deepen my understanding of JS and want to know how I can get it to work this way. I've attached my code.

// Online Javascript Editor for free
// Write, Edit and Run your Javascript code using JS Online Compiler
   
function max_area_islands_DFS(matrix) {
     let biggestIslandLength = 0;
    const MAX_LENGTH = matrix.length;
    const MAX_WIDTH = matrix[0].length;
    for(let x = 0; x < MAX_LENGTH; x++) {
        for(let y = 0; y < MAX_WIDTH; y++) {
            DFSIslandSearch(matrix, x, y, 0, biggestIslandLength)
        } 
    }
    return biggestIslandLength;
    
    
}

function DFSIslandSearch(matrix, x, y, currentCount, biggestIslandLength) {
    if(x >4 || x < 0 || y > 4 || y < 0) {
        return
    }
    if(matrix[x][y] === 0) {
        return
    }
    matrix[x][y] = 0;
    currentCount += 1;
    biggestIslandLength = Math.max(currentCount, biggestIslandLength);
    DFSIslandSearch(matrix, x + 1, y, currentCount, biggestIslandLength);
    DFSIslandSearch(matrix, x - 1, y, currentCount, biggestIslandLength);
    DFSIslandSearch(matrix, x, y + 1, currentCount, biggestIslandLength);
    DFSIslandSearch(matrix, x, y - 1, currentCount, biggestIslandLength);
}


console.log(max_area_islands_DFS([
  [1, 1, 1, 0, 0],
  [0, 1, 0, 0, 1],
  [0, 0, 1, 1, 0],
  [0, 1, 1, 0, 0],
  [0, 0, 1, 0, 0]
]));

This code returns 0 every time, but it's supposed to return 3. Thanks!