Sometimes you need a random number from javascript. In this case I needed to get a random integer to select a CSS class referencing a header image from an array to randomly show a header graphic.

Here’s the snippet:

Math.random() generates a “floating-point, pseudo-random number” in the range of 0 and 1. For example:

>>> Math.random()

0.5031280189773967

>>> Math.random()

0.5024755050937924

To get an integer for use as an array index another javascript method is required: Math.floor()

For example – to generate a random integer between 0 and 6:

>>> Math.floor(Math.random() * (6 - 0 + 1));

4

>>> Math.floor(Math.random() * (6 - 0 + 1));

6

>>> Math.floor(Math.random() * (6 - 0 + 1));

2

Or put it all in a function for easy re-use and to get a random number of any size:

// Returns a random integer between min and max
// Using Math.round() will give you a non-uniform distribution!
function getRandomInt(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

Reference:
Math.random – JavaScript | MDN.