Functions

What is a function?

A function is a group of instructions that you can execute at a later time.

function someNameHere() {
	// Do something
	// then do something else
}

// adding the () at the end is what instructs the computer to execute the
// function
someNameHere(); // this will run the function 

You name it with the keyword function.

Arguments

You can pass in arguments which help make the function reusable.

function add(a,b) {
  a+b; // will result in 8 when add(5,3) is called
}

add(5,3); 

`Return` keyword

Functions usually have a return keyword at the end. Without it, the function won’t return any value.

function add(a,b) {
  return a+b; // will return the result of the expression (in this case "5+3")
}

var result = add(5,3); // without the "return" keyword above, the value of
// "result" would be "undefined" 

Passing a function as an argument (listeners)

Ocasionally you’ll need to pass a function to another function. Event listeners are a common example for this.

// el is an element on the page we've defined earlier 
// when I click on the element, the function named "someFunction" will be
// executed
el.addEventListener('click', someFunction);


// DON'T DO THIS. INCORRECT. 
el.addEventListener('click', someFunction());
// this will run with the RESULT of someFunction because of the ()
// -> el.addEventListener('click', 10);

function someFunction() {
  return 5+5;
}
 

Exercises. Now you try it.

1) Create a function named subtract.