1.10Lesson 1.10: Functions in JavaScript

The idea

function double(number) {
  return number * 2;
}

console.log(double(4));
console.log(double(10));

8
20

Several parameters

Separated by commas, like in pseudocode:

function area(base, height) {
  return base * height;
}

console.log(area(3, 4));

12

Without return

A function can also do something without returning anything:

function greet(name) {
  console.log("Hello, " + name);
}

greet("Ana");

Hello, Ana

If you use its result, you'll see undefined: that's JavaScript's word for "no value".

The short form

In other people's code you'll see functions written with an arrow, =>:

const double = (number) => number * 2;

It does the same as the one above. Here we use function until writing functions feels natural.

Try it

function isEven(n) {
  if (n % 2 === 0) {
    return true;
  } else {
    return false;
  }
}

function isEven(n) {
  return n % 2 === 0;
}

Summary

• function defines a function; return gives back the result and ends it.
• Without return, the function returns undefined.
• The arrow => is another way to write them.