Random number generator with non-equal distribution of numbers

I know that I can use

var rolls = [];
for (var i=0; i<100; i++) {
  rolls.push(Math.floor(6 * Math.random()) + 1);
}

to get 100 rolls with a single die.

But what if it is a magic die, where each number doesn't show up equally?

So instead of each number showing up 1/6th of the time, the numbers 1-4 each shows up 10% of the time whereas 5 shows up 20% of the time and the remaining 6 shows up 100%-20%-4*10% = 40% of the time.

How do you make such a random number generator where the distribution can easily be adjusted?