What is a function?

Functions allow you to group together some code, give this code a name, and reuse it later, addressing it by the name you gave it. Let's see an example:

function sum(a, b) {
  var c = a + b;
  return c;
}

The parts that make up a function are shown as follows:

• The function statement.
• The name of the function, in this case sum.
• The function parameters, in this case a and b. A function can take any number of parameters, separated by commas.
• A code block, also called the body of the function.
• The return statement. A function always returns a value. If it doesn't return a value explicitly, it implicitly returns the value undefined.

Note that a function can only return a single value. If you need to return more values, you can simply return an array that contains all of the values you need as elements of this array.

The preceding syntax is called a function declaration. It's just one of the ways to create a function in JavaScript, and more ways are coming up.

Calling...