javascript - 如何在 JavaScript ES6 中功能性地遍历矩阵？

Question

这是我为 Hackerrank 上的“Staircase”挑战提交的代码:

function staircase(n) {
    // fill() fills out `undefined` items, which helps to initiate new n*n matrix 
    let matrix = Array(n).fill().map(() => Array(n).fill());

    matrix.forEach((curArr, idY) => {
        curArr.forEach((curVal, idX) => {
            curVal = (idX + idY + 1 >= n) ? '#' : ' ';
            process.stdout.write(curVal);
        }, [])
        console.log();
    }, []);
}

当我使用staircase(6)进行测试时，我确实得到了预期的结果:

     #
    ##
   ###
  ####
 #####
######

但是，当我修改代码以检查 matrix 时，如下所示:

function staircase(n) {
    // fill() fills out `undefined` items, which helps to initiate new n*n matrix 
    let matrix = Array(n).fill().map(() => Array(n).fill());

    matrix.forEach((curArr, idY) => {
        curArr.forEach((curVal, idX) => {
            curVal = (idX + idY + 1 >= n) ? '#' : ' ';
        }, [])
    }, []);

    console.log(matrix);
}

这是我得到的:

[ [ undefined, undefined, undefined, undefined, undefined, undefined ],
  [ undefined, undefined, undefined, undefined, undefined, undefined ],
  [ undefined, undefined, undefined, undefined, undefined, undefined ],
  [ undefined, undefined, undefined, undefined, undefined, undefined ],
  [ undefined, undefined, undefined, undefined, undefined, undefined ],
  [ undefined, undefined, undefined, undefined, undefined, undefined ] ]

所以我的问题是:

1. 为什么matrix没有被修改？
2. 是否有任何纯函数式方法可以解决此问题(即仅使用 map() ？)

Answer 1

这就是我要做的。

const array = (length, lookup) => Array.from({ length }, (_, i) => lookup(i));

const string = (separator, length, lookup) => array(length, lookup).join(separator);

const staircase = n =>
    console.log(
        string("\n", n, i =>
            string("", n, j =>
                i + j < n - 1 ? ' ' : '#')));

staircase(9);

除了 console.log 之外，staircase 函数是纯函数。