Mastering JavaScript Functional Programming - Second Edition

Troublesome functions

Finally, some functions also cause problems. For instance, Math.random() is impure: it doesn't always return the same value, and it would certainly defeat its purpose if it did! Furthermore, each call to the function modifies a global seed value, from which the next random value will be calculated. 

The fact that random numbers are actually calculated by an internal function, and are therefore not random at all (if you know the formula that's used and the initial value of the seed), implies that pseudorandom would be a better name for them. 

For instance, consider the following function that generates random letters ("A" to "Z"):

const getRandomLetter = () => {  const min = "A".charCodeAt(); const max = "Z".charCodeAt(); ...