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Introduction to JS


What is JS

JavaScript is a scripting or programming language that allows you to implement complex things on web pages — every time a web page does more than just sit there and display static information for you to look at — displaying timely content updates, interactive maps, animated 2D/3D graphics, scrolling video jukeboxes, etc. — you can bet that JavaScript is probably involved. It is the third layer of the layer cake of standard web technologies, two of which (HTML and CSS) we have covered (MDN)

How does this compare to what we know

  • There are 3 ways to run JS
    • Browser based JS: Loaded into a browser (Primarily what we are referring to)
    • Backend JS: JS that runs on Backend such as Node
    • JS that runs on the computer such as code that is powering app-app

How to run JS in Browser

  • HTML script tag
// JavaScript code would be here
  • Link to separate JavaScript file
<script src="main.js">

Basic JS topics

  • Google Chrome Tool -- inspect
    • console tab: You can type live JS code into a browser and the browser executes the code
    • source tab: Shows you the JavaScript files loaded into the current page


The JavaScript console is where we can output messages to ourselves (the developer) that are helpful in many ways:

  • Outputs values (the result of some computation)

  • Outputs messages that let us know when some action has happened in our application

  • Outputs errors that are important to us as a developer but not useful to the user

  • console.log

    console.log("Hello World");


Variables in programming languages allow us to store information and give that information a name so we can recall it later.

  • let & const

  • array (approx 00:35:20)

    • ordered list of collection of data
    • index starts at 0
    • values are separated by commas
    • different types of values can be mixed in an array
    • use push to add a value to an array
    • use pop to remove the last index of an array
    • use shift to remove the first index of an array
    • use unshift to insert a value at the beginning of an array
    let instructors = ["Jason", "Toni", "Gavin", "Mark"];
    instructors[("Jason", "Toni", "Gavin", "Mark", "Joebob")];
    instructors[("Jason", "Toni", "Gavin", "Mark")];
    instructors[("Toni", "Gavin", "Mark")];


  • numbers

    • basic math
  • strings

    • use either single or double quote: 'string of characters' or "string of characters"

    • concat: strings and numbers can be concatenated together in JS

      let name = "Gavin"
      let favoriteNumber = 42
      let sentence = name + ' favorite number is ' + favoriteNumber
      -> "Gavin favorite number is 42"
    • use escape character - backslash \\ to use a single quote inside of singe quotes '' or use double quotes

      let sentence = name + '\'s favorite number is ' + favoriteNumber
      -> "Gavin's favoriteNumber is 42"


      let sentence = name + "'s favorite number is " + favoriteNumber
      -> "Gavin's favoriteNumber is 42"
  • interpolation

    • use of backquote ``: What is inside of ${} within backquote gets evaluated

      let newSentence = `${name} favoriteNumber is ${favoriteNumber}`
      -> "Gavin favoriteNumber is 42"
      let newSentence = `${name} favoriteNumber is ${favoriteNumber * 2} or ${favoriteNumber * 3}`
      -> "Gavin favoriteNumber is 84 or 14"

What is the DOM (Document Object Model)

(approx 00:45:00)

  • DOM: Browser's representation of your webpage in memory

  • document - variable that is given by web page browser's JavaScript environment

  • selectors

    • We can use the selectors we learned about with CSS to help us find/target elements on the page

    • querying selectors

    • querySelector > querySelector selects the very first h1 element from document and returns it

      -> <h1>Hello World</h1>
      let header = document.querySelector('h1')
      -> <h1>Hello World</h1>
    • querySelectorAll > given that there are 3 h1 elements in the example document, to select all the h1 elements from document

      let allOurFirstDOM = document.querySelectorAll('h1')
      -> NodeList(3) [h1,h1,h1]
    • NodeList is an array like list that browser gives us to represent collection of nodes.

  • Why do we normally put our <script> tag at the end of our HTML?

    • scripts run right away so only the DOM that has been evaluated so far is available to us
    • So how do we ensure that the DOM is loaded when our JavaScript runs?
    • Use DOMContentLoaded event (see below in Events)
  • Updating (approx 00:53:00)


(approx 1:00:00)

  • data + behavior(actions taken on data) = programming

  • functions are just variables whose value is the code to execute when you call that function

  • named

    function greet(nameOfPerson) {
    console.log('Hello ${nameOfPerson}')
    -> Hello undefined
    -> Hello Gavin
    name = 'Gavin'
    -> Hello Gavin
    name = 'Gavin'
    greet(name + !!!!)
    -> Hello Gavin!!!!
    function greet(nameOfPerson, greeting) {
    console.log(`${greeting}, ${nameOfPerson}`)
    greetFunction('Gavin', 'Howdy')
    -> Howdy, Gavin

    Another way to write function using arrow function (newer way in ES6)

    let greetFunction = (nameOfPerson, greeting) => {
    console.log(`${greeting}, ${nameOfPerson}`)
    greetFunction('Gavin', 'Howdy')
    -> Howdy, Gavin
  • anonymous

    • functions that are dynamically declared at runtime. They're called anonymous functions because they aren't given a name in the same way as normal functions


(approx 1:10:40)

  • JS and DOM are event based world where we are waiting for events to happen and we react to them
  • what is an event
  • In JS we can pass around functions to other functions
  • listening for events
  • attaching code to events
const main = () => {
document.querySelector("h1").textContent += "?";
// when DOMContentLoaded happens, call main
document.addEventListener("DOMContentLoaded", main);

DEMO: a simple "how many times have your clicked this button"

(approx 1:35:00)