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, in which case they don’t need to have a side effect to be useful. For example, the function Math.max takes any amount of number arguments 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 part of a plus expression:
console.log(Math.min(2, 4) + 100);
// → 102
The next chapter explains how to write your own functions.
Source: Haverbeke Marijn (2018), Eloquent JavaScript: A Modern Introduction to Programming,
No Starch Press; 3rd edition.