## Topics

Functions Arguments Parameters Return-Values

## Functions

• a variable is a name for a chunk of data
• a function is a name for a chunk of code

## Why would you want to name a chunk of code?

Perhaps...

• you have some code you want to run again and again
• you want to do the same operation on different values
• you want to keep your code organized

## Function example

Here's a not very useful function:

``````function add(x, y) {
let sum = x + y;
return sum;
}
``````
• `function` means "define a function"
• `add` is the name of the function
• `x, y` are the parameters of the function (also called arguments)
• `sum` is a local variable of the function
• `sum` is also the function's return value because of the magic word return

## Call Me, Maybe

You call a function by its name, plus parentheses:

``````function add(x, y) {
let sum = x + y;
return sum;
}

``````

## Lab: One Nation, Divisible

One number is divisible by another if you divide them and the remainder is 0.

Write a function called `divisible` that takes two numbers, and returns `true` if the first number is divisible by the second number, and `false` otherwise.

``````divisible(100, 10)    //=> true
divisible(100, 7)     //=> false
divisible(3333, 11)   //=> true
divisible(99, 12)     //=> false
``````

## Rant!!!

Here is a function that takes an "opinion" as input, and as output returns a VERY FORCEFUL statement of that opinion.

``````function rant(opinion) {
let strongOpinion = opinion.toUpperCase();
return strongOpinion + '!!!';
}

rant('i like pizza');  #=> 'I LIKE PIZZA!!!'
``````

The variable `strongOpinion` is called a local variable and can only be used inside the function.

## Lab: Capitalize

Please write a function that capitalizes a word.

For instance,

`capitalize('tomato')` returns `'Tomato'`

A solution is on the next slide, but try to solve it on your own. Remember that there are many string operations available to you...

## Capitalize

``````function capitalize(word) {
let firstLetter = word;
let restOfWord = word.slice(1);
return firstLetter.toUpperCase() + restOfWord.toLowerCase();
}

console.log(capitalize('smith'));
console.log(capitalize('MACGUYVER'));
``````

The variables `firstLetter` and `restOfWord` are called local variables and can only be used inside the `capitalize` function.

## Passing Variables to Functions

When you pass a variable to a function, that variable's value is assigned to a parameter.

The variable and parameter names do not need to match!

``````function rant(opinion) {
let strongOpinion = opinion.toUpperCase();
return strongOpinion + '!!!';
}

let feeling = "I feel great";
let strongFeeling = rant(feeling);
``````
Outside the function Inside the function Value
`feeling` `opinion` `"I feel great"`
`strongOpinion` `"I FEEL GREAT"`
`strongFeeling` `"I FEEL GREAT!!!"`

## Four Function Syntaxes

WARNING: JavaScript has many ways to define a function.

This is the standard, original, retro function syntax:

``````function add(x,y) { return x + y; }
``````

The following are all roughly equivalent to the above:

``````let add = function(x,y) { return x + y; };
``````
``````let add = (x,y) => { return x + y; };
``````
``````let add = (x,y) => x + y;
``````
• Note that these new forms are anonymous:
• there is no name between `function` and `(x,y)`
• the name of the function is the name of the variable that points to it