JavaScript in 60 Seconds


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.


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.

// 1. create a function named subtract // 2. call the function // 3. Ensure it returns 5 function someName() { }