As we’ve seen, Math is a grab bag of number-related utility functions, such as Math.max (maximum), Math.min (minimum), and Math.sqrt (square root).
The Math object is used as a container to group a bunch of related functionality. There is only one Math object, and it is almost never useful as a value. Rather, it provides a namespace so that all these functions and values do not have to be global bindings.
Having too many global bindings “pollutes” the namespace. The more names have been taken, the more likely you are to accidentally overwrite the value of some existing binding. For example, it’s not unlikely to want to name something max in one of your programs. Since JavaScript’s built-in max function is tucked safely inside the Math object, we don’t have to worry about overwriting it.
Many languages will stop you, or at least warn you, when you are defining a binding with a name that is already taken. JavaScript does this for bindings you declared with let or const but—perversely—not for standard bindings nor for bindings declared with var or function.
Back to the Math object. If you need to do trigonometry, Math can help.
It contains cos (cosine), sin (sine), and tan (tangent), as well as their inverse functions, acos, asin, and atan, respectively. The number n (pi)—or at least the closest approximation that fits in a JavaScript number—is available as Math.PI. There is an old programming tradition of writing the names of constant values in all caps.
function randomPointOnCircle(radius) {
let angle = Math.random() * 2 * Math.PI;
return {
x: radius * Math.cos(angle),
y: radius * Math.sin(angle)};
}
console.log(randomPointOnCircle(2));
// → {x: 0.3667, y: 1.966}
If sines and cosines are not something you are familiar with, don’t worry. When they are used in this book, in Chapter 14, I’ll explain them.
The previous example used Math.random. This is a function that returns a new pseudorandom number between zero (inclusive) and one (exclusive) every time you call it.
console.log(Math.randomQ);
// → 0.36993729369714856
console.log(Math.randomQ);
// → 0.727367032552138
console.log(Math.randomQ);
// → 0.40180766698904335
Though computers are deterministic machines—they always react the same way if given the same input—it is possible to have them produce numbers that appear random. To do that, the machine keeps some hidden value, and whenever you ask for a new random number, it performs complicated computations on this hidden value to create a new value. It stores a new value and returns some number derived from it. That way, it can produce ever new, hard-to-predict numbers in a way that seems random.
If we want a whole random number instead of a fractional one, we can use Math.floor (which rounds down to the nearest whole number) on the result of Math.random.
console.log(Math.floor(Math.random() * 10));
// → 2
Multiplying the random number by 10 gives us a number greater than or equal to 0 and below 10. Since Math.floor rounds down, this expression will produce, with equal chance, any number from 0 through 9.
There are also the functions Math.ceil (for “ceiling,” which rounds up to a whole number), Math.round (to the nearest whole number), and Math.abs, which takes the absolute value of a number, meaning it negates negative values but leaves positive ones as they are.
Source: Haverbeke Marijn (2018), Eloquent JavaScript: A Modern Introduction to Programming,
No Starch Press; 3rd edition.