Random index does not return random index in Confirm

[webmaster128]

Expected behaviour

From naming and code comments, this function should return a random index:

const indexByRand = num => Math.floor(num * (words.length - 1));
indexByRand(Math.random());

Actual behaviour

It does not include the last index. This is because Math.random() is strictly < 1.0 and thus num * (words.length - 1) < (words.length - 1) and which makes words.length - 1 impossible.

Even if Math.random() would include 1.0, the last index would be chosen significantly less often then the other indices because only the value 1.0 leads to the last index where the other indices have a whole interval.

Steps to reproduce

Simulate using this node rnd_test.js:

const words_length = 12;
const indexByRand = num => Math.floor(num * (words_length - 1));
const randomIndex = () => indexByRand(Math.random());

let statistics = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]; // number of occurrence for index 0 to 11.

for (let i = 0; i < 10000000; i++) {
    statistics[randomIndex()] += 1;
}

console.log(statistics);

prints

[ 907928,
  910979,
  908350,
  910128,
  909866,
  909620,
  907623,
  908070,
  908512,
  908911,
  910013,
  0 ]

[webmaster128]

When changing the function randomIndex like this, we get proper results. rnd_test.js:

const words_length = 12;
const randomIndex = () => Math.floor(Math.random() * words_length);

let statistics = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];

for (let i = 0; i < 10000000; i++) {
    statistics[randomIndex()] += 1;
}

console.log(statistics);

prints

[ 834966,
  833113,
  832922,
  832819,
  832974,
  833467,
  833171,
  833465,
  833986,
  833847,
  833177,
  832093 ]

This is what StackOverflow also agrees on.