1351. Count Negative Numbers in a Sorted Matrix

Given a m * n matrix grid which is sorted in non-increasing order both row-wise and column-wise.
Return the number of negative numbers in grid.

• Example 1:
  Input: grid = [[4,3,2,-1],[3,2,1,-1],[1,1,-1,-2],[-1,-1,-2,-3]]
  Output: 8
  Explanation: There are 8 negatives number in the matrix.

• Example 2:
  Input: grid = [[3,2],[1,0]]
  Output: 0

• Example 3:
  Input: grid = [[1,-1],[-1,-1]]
  Output: 3

• Example 4:
  Input: grid = [[-1]]
  Output: 1

var countNegatives = function(grid) {
    let num = 0;
    grid.flat().forEach(i => { 
      if(i<0) 
        num++; 
    })
    return num
};