method DOMMatrixReadOnly.multiply

The multiply() method of the DOMMatrixReadOnly interface creates and returns a new matrix which is the dot product of the matrix and the otherMatrix parameter. If otherMatrix is omitted, the matrix is multiplied by a matrix in which every element is 0 except the bottom-right corner and the element immediately above and to its left: m33 and m34. These have the default value of 1. The original matrix is not modified.

MDN Reference