SOURCE

const traverseArray = function (arr) {
    let res = [];

    const getOutlineEl = (restArr, res) => {
        res.push(...restArr.shift());
        for (let i = 0; i < restArr.length; i++) {
            res.push(restArr[i].pop())
        }
        res.push(...restArr.pop().reverse())
        for (let i = restArr.length - 1; i >= 0; i--) {
            res.push(restArr[i].shift());
        }
        if (restArr.length) {
            return getOutlineEl(restArr, res)
        } else {
            return res;
        }
    }

    return getOutlineEl(arr, res);
} 

const length = 4;
const testArr = Array.from(new Array(length)).map((_, index) => Array.from(new Array(length)).map((_, insideIndex) => length * index + insideIndex + 1));
console.log(testArr);
console.log(traverseArray(testArr));