Rotate matrix clockwise 90

rotateMatrix.js

function rotateMatrix (matrix = []) {
  return matrix[0].map(
    (_row, i) => getRotatedRow(i, matrix)
  )
}

function getRotatedRow (i, matrix) {
  return matrix.map(
    (_row, y) => matrix[(matrix.length - 1) - y][i]
  )
}

module.exports = rotateMatrix

rotateMatrix.test.js

let rotateMatrix = require('../rotateMatrix')

describe('rotateMatrix', () => {
  it('rotate a 2x matrix', () => {
    let matrix = [
      [1, 2],
      [3, 4]
    ]

    let rotated = rotateMatrix(matrix)

    expect(rotated).toEqual([
      [3, 1],
      [4, 2]
    ])
  })

  it('rotate a 3x matrix', () => {
    let matrix = [
      [1, 2, 3],
      [4, 5, 6],
      [7, 8, 9]
    ]

    let rotated = rotateMatrix(matrix)

    expect(rotated).toEqual([
      [7, 4, 1],
      [8, 5, 2],
      [9, 6, 3]
    ])
  })
  it('rotates a rectangular matrix', () => {
    let matrix = [
      [1, 2, 3, 4, 5],
      [6, 7, 8, 9, 10]
    ]

    let rotated = rotateMatrix(matrix)

    expect(rotated).toEqual([
      [6, 1],
      [7, 2],
      [8, 3],
      [9, 4],
      [10, 5]
    ])
  })
})