islandPerimeter.js

/**
 * @param {number[][]} grid
 * @return {number}
 */
var islandPerimeter = function(grid) {
    // Initialize perimeter to 0
    var perimeter = 0;

    // Loop through each cell in the grid
    for (var i = 0; i < grid.length; i++) {
        for (var j = 0; j < grid[i].length; j++) {

            // If the cell is land (1)
            if (grid[i][j] === 1) {

                // Add 4 to the perimeter (assuming it's a standalone cell)
                perimeter += 4;

                // If there's a cell above this one and it's also land, subtract 2 from the perimeter
                // (because this cell shares an edge with the one above it)
                if (i > 0 && grid[i - 1][j] === 1) {
                    perimeter -= 2;
                }

                // If there's a cell to the left of this one and it's also land, subtract 2 from the perimeter
                // (because this cell shares an edge with the one to its left)
                if (j > 0 && grid[i][j - 1] === 1) {
                    perimeter -= 2;
                }
            }
        }
    }

    // Return the calculated perimeter
    return perimeter;
};