This blog assumes a global install of mocha, although that can also be installed locally. chai is also required as the assertion library - install as a dev dependency.

Create sm.mocha.js.

1const lib = require("./index");
2const chai = require("chai");
3const { expect } = chai;
4
5describe("spiral matrix", function () {
6  it("should return correct for 2", function () {
7    let target = [
8      [1, 2],
9      [4, 3],
10    ];
11    const res = lib.matrix(2);
12
13    expect(res).to.deep.equal(target);
14  });
15
16  it("should return correct for 3", function () {
17    let target = [
18      [1, 2, 3],
19      [8, 9, 4],
20      [7, 6, 5],
21    ];
22    const res = lib.matrix(3);
23
24    expect(res).to.deep.equal(target);
25  });
26
27  it("should return correct for 0", function () {
28    let target = [];
29    const res = lib.matrix(0);
30
31    expect(res).to.deep.equal(target);
32  });
33});
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Create index.js:

/**
 * Build a 2D matrix based on size n and return
 * integers spiralling down from n*n
 *
 * @param {*} n Matrix size
 */
let matrix = (n) => {
  const mat = [];
  for (let i = 0; i < n; i++) {
    mat.push([]);
  }

  let count = 1;
  let startCol = 0;
  let endCol = n - 1;
  let startRow = 0;
  let endRow = n - 1;

  while (startRow <= endRow && startCol <= endCol) {
    // top row
    for (let i = startCol; i <= endCol; i++) {
      mat[startRow][i] = count;
      count++;
    }
    startRow++;

    // right col down
    for (let i = startRow; i <= endRow; i++) {
      mat[i][endCol] = count;
      count++;
    }
    endCol--;

    // bottow row rtl
    for (let i = endCol; i >= startCol; i--) {
      mat[endRow][i] = count;
      count++;
    }
    endRow--;

    // start col btt
    for (let i = endRow; i >= startRow; i--) {
      mat[i][startCol] = count;
      count++;
    }
    startCol++;
  }
  return mat;
};

module.exports = {
  matrix,
};
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Change into directory and run mocha sm.mocha.js.