matrix_math.js

let mat1 = [
   [ 1, 2 ],
   [ 3, 4 ],
];

let mat2 = [
   [ 5, 6 ],
   [ 8, 9 ]
];

let matN2 = [
  [1],
  [2],
  [3],
  [4]
];

let identityMat = [
  [1, 0, 0, 0],
  [0, 1, 0, 0],
  [0, 0, 1, 0],
  [0, 0, 0, 1]
];

function getMatSize(mat) {
   
  let rows = mat.length;
  
  let cols = mat[0].length;
  
  return { rows, cols };
}

function addMat(m1, m2) {
  
  let m1Rows = m1.length;
  
  let m1Cols = m1[0].length;
  
  let m2Rows = m2.length;
  
  let m2Cols = m2[0].length;
 
  if(m1Cols != m2Cols && m1Rows != m2Rows) {
    
    return null;
  }
  
  var result = [];
  
  
  for(var i = 0; i < m1Rows; i++) {
    
     result[i] = [];
    
    for(var j = 0; j < m2Cols; j++) {
      
       let sum = m1[i][j] + m2[i][j];
      
      result[i][j] = sum;
      
    }
  }
  
  console.log(result);

  return result;
}

function scalarMat(mat, scalar) {
  
   let mRows = mat.length;
  
  let mCols = mat[0].length;
  
   var result = [];
  
   for(var i = 0; i < mRows; i++) {
    
     result[i] = [];
    
    for(var j = 0; j < mCols; j++) {
      
       let sum = mat[i][j] + scalar;
      
        result[i][j] = sum;
    }
     
   }
     console.log(result);
  
     return result;
}

function mulMat(m1, m2) {
  

  let m1Size = getMatSize(m1);
  
  let m2Size = getMatSize(m2);
  
  if(m1Size.cols != m2Size.rows) {
    
    return null;
  }
  
   console.log(m1Size, m2Size);
  
   var result = [];
  
  for(var i = 0; i < m1Size.rows; i++) {
    
     result[i] = [];
    
      for(var j = 0; j < m2Size.cols; j++) {
        

        var sum = 0;
        
        for(var k = 0; k < m1Size.cols; k++) {
          
          let m2RowVal = m2[i][j];
          
          let m1ColVal = m1[i][k];
          
          sum += m1ColVal * m2RowVal;
        }
       
        result[i][j] = sum;
      }
    
  }
  
  console.log(result);
  
  return result;
  
  
}

// addMat(mat1, mat2);
  
// scalarMat(mat2, 2);

 mulMat(identityMat, matN2);