Can someone please explain this piece of code to me and why b [2] and b [3] are undefined?

Question

var c = [1,2 3,4];
var b = c.filter (function (a) {
return (a%2);
});
alert (b [0] + b [1]); // outputs 4

Answer 1

It works fine here. Could you kindly try running it? It's one of the quizzes and I don't understand it Cbr✔[ Not Active ]