How can I generate a radom number for twice?

So guys I am trying to make a function that returns a random number (which I need that code to be executed excluesively) generate the same number for different two functions. Basically I am going to call a function that returns a random number but when I call that function again, I need it to be the same number as in the previous function (I am not very good at javascript.). I have these codes, but of course it generates some another number in each function:

function gen() {
    return Math.floor(Math.random() * 21) + 40;
}

function chan() {
    var rand = gen();
}

function sell() {
    var rand = gen();
}