Can not seem to figure out why my matrix multiplication function is not working

Question

let multiply = function(a, b) {
  let c = [];
  for(let row = 0; row < 4; row++){
    for(let col = 0; col < 4;col++){
      for(let i = 0; i < 4; i++){
        c[col + (4*row)] += a[i][row] * b[col][i];
      }
    }
  }

The function takes two four by four matrixes and outputs an array of length 16. It does not seem to be giving me the correct array.

Answer 1

This should do the trick

let multiply = function(a, b) {
  let c = [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0];
  for(let row = 0; row < 4; row++){
    for(let col = 0; col < 4; col++){
      for(let i = 0; i < 4; i++){
        c[col + (4*row)] += a[row][i] * b[i][col];
      }
    }
  }
  return c;
}

Notice that you need to initialize the output array and your row x col multiplication was wrong; you need:
a[row][i] * b[i][col]
instead of
a[i][row] * b[col][i]
The function should also return the resulting array.
You can give it a try using JSFiddle: https://jsfiddle.net/1fxLpucg/