Percentage chance of saying something?

I do this all the time with my discord bots

const a = Math.floor(Math.random() * 11);
    if (a >= 8) { // 20% chance
        /*
          CODE HERE
        */
    } else { // 80% chance
        /*
          CODE HERE
        */
    }

You can change 11 to 101 if you want.

The reason why it has an extra one is so it does 1 - 10 instead of 1 - 9 (or 1 - 100 instead of 1 - 99)

For cases like this it is usually best to generate one random number and select the case based on that single number, like so:

int foo = Math.random() * 100;
if (foo < 80) // 0-79
    sendMessage("hi");
else if (foo < 85) // 80-84
    sendMessage("bye");
else // 85-99
    sendMessage("test");

I made a percentage chance function by creating a pool and using the fisher yates shuffle algorithm for a completely random chance. The snippet below tests the chance randomness 20 times.

var arrayShuffle = function(array) {
   for ( var i = 0, length = array.length, swap = 0, temp = ''; i < length; i++ ) {
      swap        = Math.floor(Math.random() * (i + 1));
      temp        = array[swap];
      array[swap] = array[i];
      array[i]    = temp;
   }
   return array;
};

var percentageChance = function(values, chances) {
   for ( var i = 0, pool = []; i < chances.length; i++ ) {
      for ( var i2 = 0; i2 < chances[i]; i2++ ) {
         pool.push(i);
      }
   }
   return values[arrayShuffle(pool)['0']];
};

for ( var i = 0; i < 20; i++ ) {
   console.log(percentageChance(['hi', 'test', 'bye'], [80, 15, 5]));
}

Yes, Math.random() is an excellent way to accomplish this. What you want to do is compute a single random number, and then make decisions based on that:

var d = Math.random();
if (d < 0.5)
    // 50% chance of being here
else if (d < 0.7)
    // 20% chance of being here
else
    // 30% chance of being here

That way you don't miss any possibilities.