Program Flow: Functions

Now that you have an understanding of the basic data types and operations, we should try to make our programs a little more interesting. We will do this by manipulating the way in which the JavaScript program executes. One way we can accomplish this is through the use of functions.

Functions

Functions allow us to repeat tasks that involve a similar sequence of steps. You’ve already seen the console.log() function, which allows us to predictably log some output to the console.

Let’s look at an example of a function definition that finds the sum of two numbers:

We start by creating a variable, which we then define as a function by using the function(){} syntax.

Within the parentheses are a set of arguments, which are values that are used or manipulated by the function in some manner. If you have multiple arguments, they are each separated by a comma. A function can also have no arguments.

The code to be executed by the function lies within the curly braces ({}). If you expect the function to give back some value, it should include a return statement (which is done by using the keyword return), followed by the value you want to be returned. If you don’t expect your JavaScript function to return a value, you do not have to include a return statement.

You may notice if you run the code above, nothing happens. In order to make use of our function, we must invoke or call the function. A function is called like this:

functionName();

The parentheses together, (), are an operator that initiates a function call. If you have arguments for the function to make use of, you must include those within the parenthesis.

Let’s make use of the sum function we just created:

In this example, we make use of two different functions:

• The sum() function, where we pass two numbers as arguments.
• The console.log() function, where we supply the newly created variables as arguments.

In both cases where we use sum(), numbers are passed, so the value returned will also be a number. This number is then placed into the console using console.log().

Why are functions important?

The most compelling reason to use functions is that they allow us to reuse code and create modules to perform procedures we plan on using repeatedly. Though the example above isn’t much more useful than just using the + operator, you will see that functions are very useful as your code starts to get more complex.

Exercise

Write a function named subtract, that takes two values as arguments and returns the result of subtracting one value from the other.