Last Class Review

What are Functions?

  • Allows you to group a series of statements together to perform a specific task

  • Functions are used to promote “code reuse”

  • You can control when functions are executed, for example - you can write functions that only get executed (or called) when a user clicks a specific button


// a simple function that greets you with a 'Good Morning' alert

// 1) Declare a function named greeting

function greeting(){
  alert('Good Morning');
};


// 2) Call (or run) the function

greeting();

Declaring a Function

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  • Use the function keyword to declare a function

  • Functions can be given a name

  • The name must be followed by parentheses

  • The opening and closing curly braces indicate a “code block”

  • The statements for your function goes within the code block

  • Simply declaring the function will not run this code, this function must be “called” in order for the code inside the function to be run

Calling a Function

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  • To run the code inside of a function you use the function name followed by parentheses (don’t forget the parentheses!)

  • Now you can call this function as many times as you want

Declaring Functions that need information

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  • Some functions need additional information in order to perform a specific task

  • This additional information is referred to as “parameters”

  • To provide parameters to a function, you specify them inside the parentheses after the parameter name

  • The parameters are used like variables within the function body

  • We use the “return” keyword when we want to retrieve a value from our function, in the case of the example we want to retrieve the result of the multiplying the width times the height

Calling Functions that need information


// Calling the getArea() function with values

getArea(7, 5); // returns 35


// Calling the getArea() function with variables

var doorWidth = 2;
var doorHeight = 8;

getArea(doorWidth, doorHeight); // returns 16

Functions can call other functions

  • Function reusability is key results in cleaner code

  • Reusing functions leverages key programming principle - Don’t Repeat Yourself

  • In the example of below, the surfaceAreaOfCube function calls another function (areaOfSquare) instead of duplicating work that was already done


// Function that calculates area of a square

function areaOfSquare(side){
  return side * side;
};

areaOfSquare(3); // returns 9


// This is a function that calculates the 
// surface area of a cube that *reuses* the areaOfSquare function

function surfaceAreaOfCube(side){
  return 6 * areaOfSquare(side);
};

surfaceAreaOfCube(7); // returns 42