Functions

Objectives

• Describe a situation where you would use:

  • A function with a return value

  • A function without a return value

  • A function with a parameter

• Describe the difference between printing and returning

• Write a function with and without a return value

• Write a function with a parameter

• Write a function that operates on two or more parameters

• Identify the scope of variables inside and outside functions

Defining a function

A function is a module that can store and invoke code. When writing repetitive code, we can isolate code into functions in order to reduce repetition. For example, if we needed to say "Hello World" to the screen multiple times, we can create a function like so.

const greeting = function() {
    console.log("Hello World");
}

greeting();

Note that a function is assigned to a variable, and we can call the function by taking the variable name and appending parentheses to the end of the function variable.

Parts of a function

const functionName = function() {
    //CODE
}

We can also create functions that accept parameters, and use those parameters as variables in the function.

Defining a function with a parameter

const greeting = function(taco) {
    // anything inside of here will execute when called
    console.log("Good morning", taco);
}

let name = "Josh"
let name2 = "Brian"
greeting(name);
greeting(name2);

Defining a function with two parameters

Functions can have multiple parameters, separated by commas.

const greeting = function(taco, stuff) {
    // anything inside of here will execute when called
    console.log("Good morning", stuff, taco);
    console.log("taco:", taco);
    console.log("stuff:", stuff);
}

let name = "Josh"
let name2 = "Brian"
greeting(name, name2);
greeting(name2, name);

Printing and returning are different

Note that functions can have input via parameters. They can also have output as return values. Returning values from a function is denoted by the keyword return. Also, return values are optional.

Note that printing something to the screen using console.log is not the same as returning values.

const multiply = function(num1, num2) {
    console.log("inside the function");
    // return result = num1 * num2;
    return num1 * num2
}

let firstNum = 2;
let secNum = 3;
let taco = multiply(firstNum,secNum);

console.log(firstNum + " multiplied by " + secNum + " is " + taco )

// With a return value
const returnHello = function (name) {
    return("Hello, " + name)
}

console.log("with a return value:", returnHello("jane") );

// Without a return value
const returnHello2 = function(name) {
    console.log("inside returnHello2: Hello, " + name);
}
returnHello2("nachos");
console.log("without a return value:", returnHello2("taco") ); //will show as undefined

Declaring functions

There are two different ways to declare a function

const multiply = function(a, b) {
    return a * b;
}

function multiply(a, b) {
    return a * b;
}

The difference between these two is that the first one is defined at run-time, meaning that if we try to call the function before it's declared, an error will be thrown:

multiply(2, 2); // ERROR

const multiply = function(a, b) {
    return a * b;
}

The second declaration is defined at parse-time, so we can call the function wherever we'd like.

multiply(2, 2); // success

function multiply(a, b) {
    return a * b;
}

Despite being more flexible, the former declaration that assigns the function to a variable is more common when developing Node applications.

Exercises

1. What is the return value of this function when called?

const lightsabers = function(num) {
    console.log('I have ' + num + ' lightsabers.');
}

lightsabers(2);

1. How would the function above be modified if the user wanted to pass in an object of lightsabers, like this one?

const myLightsaberCollection = {
    blue: 1,
    green: 3
}

const lightsabers = function(lightsaberCollection) {
    //code here
}

lightsabers(myLightsaberCollection);

// Output
// I have 1 blue lightsaber
// I have 3 green lightsabers