js: matrix multiplication using dot product and transposition

mm.js

mmultiply = function(a,b) {
	return a.map(function(x,i) {
		return transpose(b).map(function(y,k) {
			return dotproduct(x, y)
		});
	});
}

dotproduct = function(a,b) {
	return a.map(function(x,i) {
		return a[i] * b[i];
	}).reduce(function(m,n) { return m + n; });
}

transpose = function(a) {
	return a[0].map(function(x,i) {
		return a.map(function(y,k) {
			return y[i];
		})
	});
}

a = [[1,2,3],[4,5,6]]
b = [[7,8],[9,10],[11,12]]

c = [[1,2,3],[4,5,6],[7,8,9]]
d = [[10,11,12],[13,14,15],[16,17,18]]

JSON.stringify(mmultiply(a,b)) // "[[58,64],[139,154]]"
JSON.stringify(mmultiply(c,d)) // "[[84,90,96],[201,216,231],[318,342,366]]"

Thanks for sharing .. I used them slightly minimized as arrow functions

mmultiply = (a, b) => a.map(x => transpose(b).map(y => dotproduct(x, y)));
dotproduct = (a, b) => a.map((x, i) => a[i] * b[i]).reduce((m, n) => m + n);
transpose = a => a[0].map((x, i) => a.map(y => y[i]));

Great minification!

This is brilliant stuff. I'd been trying to come up with such an expression for matrix multiplication, but my "blocker" was not appreciating the fact that I first needed the "transpose" function, for which your solution is genius. I have a slightly smaller version of dotproduct:
dotproduct = (a, b) => a.reduce(sum, ai, i) => sum + ai * b[i], 0);

guy. i've to thanks you, like a lot! I've past at least 3/4hours to create this function to get the dot product of two array. the problem is that it don't worked with all array. so, your function save me from going crazy and nearly give up. thankkks