perimeter.js

function getPerimeter(matrix) {
    let sum = 0;
    for(let i = 0; i < matrix.length; i++) {
        let row = matrix[i]
        for(let j = 0; j < row.length; j++) {
            if (isLand(matrix, i, j)) {
                sum += 4;
                if (isLand(matrix, i - 1, j)) {
                    sum--;
                }
                if (isLand(matrix, i + 1, j)) {
                    sum--;
                }
                if (isLand(matrix, i, j - 1)) {
                    sum--;
                }
                if (isLand(matrix, i, j + 1)) {
                    sum--;
                }
            }
        }
    }
    return sum;
}

function isLand(matrix, i, j) {
    if (i < 0 || i >= matrix.length || j < 0 || j >= matrix[0].length) {
        return false;
    }
    return matrix[i][j] == 1;
}

getPerimeter([
    [0,1,0,0],
    [1,1,1,0],
    [0,1,0,0],
    [1,1,0,0]
])