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Generate a random number with JavaScript

michelc profile image Michel Updated on ・3 min read

The "Math.random()" method

To create a random number, we have to use the Math.random() function. This method returns a "pseudo" random number which is:

  • greater than or equal to 0.0
  • strictly less than 1.0

So, if you want to get a card number to play (an integer number between 1 and 52), it is necessary:

  • Multiply the result of Math.random() by 52, which gives us a number between 0.0 and 51.999...
  • Round this number to the lower integer value with the function Math.floor() to have an integer between 0 and 51 (included)
  • Add 1 to this value to have a number between 1 and 52
// Get an integer between 1 and 52
const cardNumber = 1 + Math.floor(52 * Math.random());

Note: We talk about "pseudo" random number, because for a computer, it's not easy to reproduce chance, so it's easier to simulate.

The "Math.seedrandom()" module

Aside from my real work (C# + Oracle), I have a side project and I run a solitaire games website: Solitaire-Play, with solitaires like Klondike, Canfield, Freecell, Golf...

Each game provides a hint system. In most cases, I just present the cards that are playable according to the rules of the game. But with Klondike Solitaire, I tried to do a little better and have an algorithm that "thinks" to optimize the chances of winning.

During the development and to check that what I write has a positive impact, I have a program that tests the success rate of the games played by following only the advice of my program.

To have enough significant values, I test the results for 1000 games. The problem is that I never get exactly the same percentage of success each time I run a test set. This is "normal", since each time it's 1000 completely random games.

So I searched whether it was possible to "force" random and that my tests always run the same 1000 games. This would allow me to better evaluate the improvements I add to my hint system.

Luckily, I found David Bau's "seedrandom.js" package. This code makes the result of the Math.random() method "predictable".

As in my case I only use it for testing, I can use directly the function Math.seedrandom("seed") before my 1000 tests. In the end, my test program now looks like the following code:

function Test_Run (seed) {
  if (seed) Math.seedrandom(seed);
  var play_count = 0;
  var win_count = 0;
  for (var i = 0; i < 1000; i++) {
    if (game.state() > 0) win_count++;
  console.log(play_count + " games => "
          + "{ win: " + win_count
          + ", lost: " + (play_count - win_count)
          + ", percent: " + (Math.round(win_count * 1000 / play_count) / 10)
          + " }");

I tryed with several values for "seed" and got the following results:

  • Test_Run("michel") => 41% of games won
  • Test_Run("chance") => 41.2% of games won
  • Test_Run("randomize") => 39.3 of games won

By playing my system as best I can, I can therefore hope to win 4 out of 10 games. Which is already not bad for "if ... else" code. And so, by using the "randomize" string as initialization value, I will have a better view of the progress made by my hint system as I develop it. The goal being of course to achieve at least a 50% success rate...

This post was originally published on my blog.
Cover image : XKCD #221 - Random Number.

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michelc profile



French developper, from QB Compiler 2, Visual Basic and C# for work. Also like testing Ruby/Sinatra and JavaScript/Node.


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