Question

Showing a dialog box or writing text to the screen is a side effect. A lot of functions are useful because of the side effects they produce. Functions may also produce values, and in that case, they don’t need to have a side effect to be useful. For example, the function Math.max takes any number of number values and gives back the greatest.

console.log(Math.max(2, 4));
// → 4

When a function produces a value, it is said to return that value. Anything that produces a value is an expression in JavaScript, which means function calls can be used within larger expressions. Here a call to Math.min , which is the opposite of Math.max , is used as an input to the plus operator:

console.log(Math.min(2, 4) + 100);
// → 102

The next chapter explains how to write your own functions.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.