Question

I want to make this into a function:

var meanPoints = d3.mean(data, function(d) {return d['total points'] });
console.log(meanPoints);

This works and gives me the mean of the total point of my json file.
But if i try to make a function like so:

function meanVak(vak) {
d3.mean(data, function(d) {return d [vak] });
}

var meanPoints = meanVak('total points');
console.info(meanPoints);

It comes back as undefined.

Answer 1

Because you don’t have return statement:

function meanVak(vak) {
    return d3.mean(data, function(d) {return d [vak] });
}

Side note: by default functions without return statement return undefined.