solution.js

/**
 * @param {number} n
 * @return {number[][]}
 */
var generateMatrix = function (n) {
    const arr = Array.apply(null, {
        length: n,
    }).map(() => new Array(n));

    let top = 0;
    let right = n;
    let bottom = n;
    let left = 0;
    let direction = 0;

    let count = 0;

    while (left < right && top < bottom) {
        if (direction === 0) {
            for (let i = left; i < right; i++) {
                arr[top][i] = ++count;
            }
            top++;
        } else if (direction === 1) {
            right--;
            for (let i = top; i < bottom; i++) {
                arr[i][right] = ++count;
            }
        } else if (direction === 2) {
            bottom--;
            for (let i = right - 1; i >= left; i--) {
                arr[bottom][i] = ++count;
            }
        } else {
            for (let i = bottom - 1; i >= top; i--) {
                arr[i][left] = ++count;
            }
            left++;
        }
        direction = (direction + 1) % 4;
    }
    return arr;
};