Our task is to perform flood fill by updating the values of the four directionally connected cells, which have the same value as the source cell with the target value.

flood-fill.js

function floodFill(grid, sr, sc, target) {
    // last argument is the original value that was in the source cell
    // (which is the value that an adjacent cell must have for the fill to continue)
    // since its our first call we pass null
    // so this value can be initialized
    floodFillRec(grid, sr, sc, target, null) 
    return grid
}

function floodFillRec(grid, sr, sc, target, prevV) {
    if (grid[sr] == null || grid[sr][sc] == null) {
        // check if we went out of bounds
        return;
    }

    // keep the original source cell value to check all connected cells
    const prev = prevV ?? grid[sr][sc];

    if (grid[sr][sc] !== prev || grid[sr][sc] === target) {
        // either value in the cell is not equal to the initial source cell value,
        // or value is the target already
        // in any case, return
        return
    }

    // replace value in the grid
    grid[sr][sc] = target;

    // branch out to the 4 connected cells, left / right and top / bottom
    floodFillRec(grid, sr + 1, sc, target, prev)
    floodFillRec(grid, sr - 1, sc, target, prev)
    floodFillRec(grid, sr, sc + 1, target, prev)
    floodFillRec(grid, sr, sc - 1, target, prev)
}

export { floodFill };

// depth of the recursion tree is n / 4