matrix.js

const transpose = (m) => {
  
  return m[0].reduce((acc, _, colIndex) => 
    [...acc, m.map(row => row[colIndex])],
  []);
  
}

const multiplyOneRow = (row1, m2T) => {
  if(row1.length === 3){
    return [
      row1[0]*m2T[0][0] + row1[1]*m2T[0][1] + row1[2]*m2T[0][2], 
      row1[0]*m2T[1][0] + row1[1]*m2T[1][1] + row1[2]*m2T[1][2]
    ];
  }
  
  return [
    row1[0]*m2T[0][0] + row1[1]*m2T[0][1], 
    row1[0]*m2T[1][0] + row1[1]*m2T[1][1]
  ];
}

const multiply = (m1, m2) => {
  const m2T = transpose(m2);
  return m1.map(row=>multiplyOneRow(row, m2T));
}

it('transpose nxn matrix', function() {
   
    const m1 = [
      [1,2,3],
      [4,5,6]
    ];
    
    const transposedM1 = [
      [1,4],
      [2,5],
      [3,6]
   ];
  
  assert.deepEqual(transpose(m1), transposedM1);

});

it('multiply 1x2 matrix with 2x2', ()=> {
  const m1 = [
    [1, 2]
  ];
  
  const m2 = [
    [3, 4],
    [5, 6]
  ];
  
  const expected = [
    [13, 16]
  ]
  
  assert.deepEqual(multiply(m1, m2), expected);
});

it('multiply 3x2 matrix with 2x2', ()=> {
  const m1 = [
    [1, 2],
    [3, 4],
    [5, 6]
  ];
  
  const m2 = [
    [7, 8],
    [9, 10]
  ];
  
  const expected = [
    [25, 28],
    [57, 64],
    [89, 100]
  ]
  
  assert.deepEqual(multiply(m1, m2), expected);
});

it('multiply 2x3 matrix with 3x2', ()=> {
  const m1 = [
    [1, 2, 3],
    [4, 5, 6]
  ];
  
  const m2 = [
    [7, 8],
    [9, 10],
    [11, 12]
  ];
  
  const expected = [
    [58, 64],
    [139, 154]
  ];
  
  assert.deepEqual(multiply(m1, m2), expected);
});