Let's play with randomness

A game’s loot box, a paper fortune, dice. Because the result changes every time you press, you want to try again and again—the thrill behind that is random numbers. With JavaScript, you can make one in a single line.

First, let’s see it in its raw form

console.log(Math.random());

A decimal like 0.8264097… shows up in the console. Run it again, and you get a completely different number. No one can tell what will come up.

Transforming it into a die

A decimal as-is is hard to use, so we process it into “a whole number from 1 to 6.” Here’s the standard formula.

const dice = Math.floor(Math.random() * 6) + 1;

Let’s read it from the inside out.

Math.floor is the tool for “rounding down” (floor = the floor of a room. Remember it as rounding toward the floor). Change the 6 part and you can make “1 to 10” or “1 to 100.” There’s a similar tool, Math.round, which rounds to the nearest, but it’s not suited to processing random numbers—with rounding to the nearest, only the numbers at both ends come up less often, so it won’t make a fair die. Remember to always go with rounding down.

A collection of handy recipes

Once you’ve learned the “multiply, round down, add” pattern, you can make all sorts of random numbers. Here are the forms you’ll use often.

Only the last row has a different shape. The chance of Math.random()’s result being less than 0.5 is exactly half, so for a “50% decision” like a coin’s heads or tails, you can just write this expression directly as an if condition.

Splitting the path into three or more: else if

The if statement from last time was a two-way choice of “if / otherwise.” When you want to split into three or more, like a fortune, you slip an else if (“otherwise, if…”) in between.

if (num === 1) {
  console.log("Great luck!");
} else if (num === 2) {
  console.log("Good luck");
} else {
  console.log("So-so");
}

It checks from the top down, and only the first place that holds true runs.

Let’s see it in action

With a combo of random numbers, else if, and events, your fortune is complete.

Summary of this lesson

1. Math.random() makes a random decimal that’s 0 or greater but less than 1
2. “A whole number from 1 to 6” is Math.floor(Math.random() * 6) + 1—multiply, round down, add
3. When splitting into three or more, slip an else if in between