CSE 154

Intro to JavaScript

Preview Slides

Overview

Introduction to JavaScript

• History and motivation
• Getting started with JS (linking to a .js script file and interacting with it in the browser)
• Introduction to data types, conditionals, and loops
• Introduction to JS functions
• Summary: Java vs. JS vs. Python
• Adding JS to an HTML page!

What We've Learned So Far

How to write content for a webpage using HTML

How to add styles to a webpage using CSS and linking a CSS file to an HTML file

How to inspect the HTML and CSS of web pages in the browser

Webpage Behavior with JavaScript

Now that we know how to add content and styles to a web page, lets explore how to add responsive behavior

These slides will introduce the basics of the JavaScript language:

• History/Terminology
• JavaScript vs. Java comparisons
• Variables and Types
• Loop and Conditionals

Soon, we'll use these building blocks to dynamically update what you see on a web page in response to clicks, text input, timers, etc.

Terminology: Client-Side Scripting

Your browser is a "client" program that knows how to makes requests to a server (e.g. the UW web server). The server takes the request from each client and returns a response (e.g. HTML files, CSS files, images, etc.).

Client-side JavaScript is ran in the browser after the page is returned from server.

Often, this code provides instructions for websites to listen to events and manipulate the page (DOM) as a result.

So... What is JavaScript?

JavaScript vs. Java

JavaScript is interpreted, Java is compiled. (huh?)

JavaScript has more "relaxed" syntax and rules

• Fewer and "looser" data types (both integers and doubles are considered Numbers)
• Variables don't need to be declared
• Errors are often silent (a few exceptions)
• JS is similar to Python in these ways

JavaScript's key construct is the function rather than the object/class.

JS is contained within a web page and integrates with its HTML/CSS content

Following Along

As an interpreted programming language, JS is great to interact with a line at a time (similar to Python, but very different than Java). Where do you start?

The easiest way to dive in is with the browser's Console tab in the same inspector tool you've used to inspect your HTML/CSS.

Until we learn functions to interact with the HTML DOM with JS, we recommend experimenting with the following code examples using this console to get comfortable with the fundamental syntax and behavior of the language. Try the recommended practice problems along the way!

Our First JavaScript Statement: alert

alert("message");

JS (template)

            alert("IE6 detected. Suck-mode enabled.");

JS (example)

A JS command that pops up a dialog box with a message - not ideal in practice, but a good debugging tool when first learning JS

Output in JS: console.log

Used to output values to the browser console (can view in your browser's dev tools under "Console"). You can think of this as System.out.println in Java or print in Python.

Often preferred over alert to debug JS programs.

console.log("message");

JS (template)

console.log("The answer is: " + 42);

JS (example)

We'll often use it just to get familiar with the language/test things out in the Chrome Console.

Comments (same as Java)

// single-line comment

/* multi-line comment */

JS

Identical to Java's comment syntax

Recall: 3 comment syntaxes

• HTML: <!-- comment -->
• CSS/Java/JS: /* comment */
• Java/JS: // comment

For functions and program files, we'll use JSDoc commenting, which will be introduced when we learn how to write JS functions (next lecture).

Practice: commentSyntax, commentary

Variables and types

let name = expression;

JS (template)

let level = 23;
let accuracyRate = 0.99;
let name = "Pikachu";

JS (example)

Variables are declared with the let keyword (case-sensitive). You may also see var used instead of let - this is an older convention, and you should use let in this class.

Types are not specified, but JS does have types ("loosely-typed")

• Number, Boolean, String, Array, Object, Function, Null, Undefined
• Can find out a variable's type by calling typeof
• Note: Type conversion isn't always what you expect...

A Note about Declaring Types in JavaScript

If you've programmed in a statically-typed language like Java, you will recall that when declaring variables, you must specify their type which must always stay the same.

boolean isValid = "hello!"; // error

Java

In a dynamically-typed language like JavaScript, you don't need to specify the type (just use let or const) and you may change the type the variable refers to later in execution.

let isValid = true; // no error
isValid = "hello!";
isValid = 1;

JS

This may seem to imply fewer errors in JS, but it's not uncommon to run into subtle (silent) bugs in your JS programs as a result!

"Constants" in JavaScript

Since ECMA6, JavaScript has a special keyword to declare "constant" values. You should use these over the let keyword for variables that are not intended to ever be updated (similar to constants in Java).

Use UPPER_CASING naming conventions to denote const in JS.

let month = 12;
month = 1; // no error

const COOLEST_CLASS = "CSE154";
COOLEST_CLASS = "clazz"; // error

JS

These can help avoid some of the bugs mentioned on the previous slide (an error occurs when trying to reassign a const).

Number Type

let enrollment = 99;
let medianGrade = 2.8;
let credits = 5 + 4 + (2 * 3);

JS

Integers and real numbers are the same type (no int vs. double). All numbers in JS are floating point numbers.

Same operators: + - * / % ++ -- = += -= *= /= %=

Similar precedence to Java

Many operators auto-convert types: "2" * 3 is 6

Practice!

String type

let nickName = "Sparky O'Sparkz";                  // "Sparky O'Sparks"
let fName = nickName.substring(0, s.indexOf(" ")); // "Sparky"
let len = nickName.length;                         // 15
let name = 'Pikachu';                              // can use "" or ''

JS

Methods: charAt, charCodeAt, fromCharCode, indexOf, lastIndexOf, replace, split, substring, toLowerCase, toUpperCase

More about Strings

Escape sequences behave as in Java: \' \" \& \n \t \\

To convert between Numbers and Strings:

let count = 10;                              // 10
let stringedCount = "" + count;              // "10"
let puppyCount = count + " puppies, yay!";   // "10 puppies, yay!"
let magicNum = parseInt("42 is the answer"); // 42
let mystery = parseFloat("Am I a number?");  // NaN          

JS

To access characters of a String s, use s[index] or s.charAt(index):

let firstLetter  = puppyCount[0];                            // "1"
let fourthLetter = puppyCount.charAt(3);                     // "p"
let lastLetter   = puppyCount.charAt(puppyCount.length - 1); // "!"

JS

Common Bugs when Using Strings

While Strings in JS are fairly similar to those you'd use in Java, there are a few special cases that you should be aware of.

• Remember that length is a property (not a method, as it is in Java)
• Concatenation with +: 1 + 1 is 2, but "1" + 1 and 1 + "1" are both "11"!

Practice: repeat, containsTwice

Math object

            let rand1to10 = Math.floor(Math.random() * 10 + 1);
let three = Math.floor(Math.PI);

JS

Methods: abs, ceil, cos, floor, log, max, min, pow, random, round, sin, sqrt, tan

Properties: E, PI

for loop (same as Java)

for (initialization; condition; update) {
  statements;
}

JS (template)

            let sum = 0;
for (let i = 0; i < 100; i++) {
  sum = sum + i; // same as sum += i;
}

JS (example)

let s1 = "It's a-me, Mario!";
let s2 = "";
for (let i = 0; i < s.length; i++) {
  s2 += s1[i] + s1[i];
}
// s2 stores "IItt''ss  aa--mmee,,  MMaarriioo!!"

JS (example)

if/else Statements (same as Java)

if (condition) {
  statements;
} else if (condition) {
  statements;
} else {
  statements;
}

JS

Identical structure to Java's if/else statements

JavaScript allows almost anything as a condition

Practice: rockPaperScissors

while loops (same as Java)

while (condition) {
  statements;
}

JS

do {
  statements;
} while (condition);

JS

break and continue keywords also behave as in Java but do not use them in this class!

Practice: loopMystery6

Logical Operators

Relational: > < >= <=

Logical: && || !

Equality: == != === !==

• Most logical operators automatically convert types. These are all true:
  • 5 < "7"
  • 42 == 42.0
  • "5.0" == 5
• The === and !== are strict equality tests; checks both type and value: "5.0" === 5 is false. It's usually a good idea to use === instead of ==.
• What does 154 === 154.0 evaluate to?

Helpful JavaScript equality table!

Boolean Type

            let iLikeJS = true;
let ieIsGood = "IE6" > 0; // false
if ("web dev is great") { /* true */ }
if (0) { /* false */ }
if (1) { /* true */ }

JS

Any value can be used as a Boolean

• "falsey" values: false, 0, NaN, "", null, and undefined
• "truthy" values: anything else

Understanding what is "falsey" vs. "truthy" takes patience and practice.

When in doubt, check in the browser console!

Special Values: null and undefined

            let foo = null;
let bar = 9;
let baz;

/* At this point in the code,
 * foo is null
 * bar is 9
 * baz is undefined
 */

JS

undefined: declared but has not yet been assigned a value

null: exists, but was specifically assigned an empty value or null. Expresses intentional a lack of identification.

A good motivating overview of null vs. undefined

Arrays

let name = [];                          // empty array
let names = [value, value, ..., value]; // pre-filled
names[index] = value;                   // store element

JS (template)

            let types = ["Electric", "Water", "Fire"];
let pokemon = [];        // []
pokemon[0] = "Pikachu";  // ["Pikachu"]
pokemon[1] = "Squirtle"; // ["Pikachu", "Sqiurtle"]
pokemon[3] = "Magikarp"; // ["Pikachu", "Sqiurtle", undefined, "Magikarp"]
pokemon[3] = "Gyarados"; // ["Pikachu", "Sqiurtle", undefined, "Gyarados"]

JS (example)

Two ways to initialize an array

length property (grows as needed when elements are added)

Array methods

let a = ["Mario", "Luigi"]; // [Mario, Luigi]
a.push("Koopatroopa");      // [Mario, Luigi, Koopatroopa]
a.unshift("Bowser");        // [Bowser, Mario, Luigi, Koopatroopa]
a.pop();                    // [Bowser, Mario, Luigi]
a.shift();                  // [Mario, Luigi]
a.sort();                   // [Luigi, Mario]

JS

Array serves as many data structures: list, queue, stack, ...

Methods: concat, join, pop, push, reverse, shift, slice, sort, splice, toString, unshift

• push and pop add/remove from back
• shift and unshift add/remove from front
• shift and pop return the element that is removed

Practice: findMin , switchPairs

Splitting strings: split and join

            let s = "the quick brown fox";
let a = s.split(" "); // ["the", "quick", "brown", "fox"]
a.reverse();          // ["fox", "brown", "quick", "the"]
s = a.join("!");      // "fox!brown!quick!the"

JS

split breaks apart a String into an array using a delimiter

• Can also be used with regular expressions surrounded by /:
  let a = s.split(/[ \t]+/);

join merges an array into a single String, placing a delimiter between them

A Few Last Notes on Typing

As you write JS programs, you may will run into some silent bugs resulting from odd typing behavior in JS. Automatic type conversion, or coersion, is a common, often perplexing, source of JS bugs (even for experienced JS programmers).

Why does it happen? JS was designed to "work" as intuitively as possible without requiring the strict types.

Why is this important to be aware of? You'll be writing programs which use variables and conditional logic. Knowing what is considered truthy/false and how types are evaluated (at a high level) can make you a much happier JS developer (some practice)

Examples of some "less-intuitive" evaluations:

This is worth 3 minutes of your viewing pleasure.

Summary of Java vs. JS vs. Python

From the Console to a .js File

Now, we'll use these building blocks of a new programming language to control the behavior of our pages (which we don't get with HTML/CSS!).

Our Toolbox So Far: HTML/CSS + JS

Image source (MDN) and recommended reading

So far, we have used Atom (or a similar editor) to write HTML/CSS for the structure/styles for our page. We will continue to use Atom to write JavaScript and add behavior.

As always, the webpage is rendered by the browser. We will add a link to our JS from HTML (similar to the idea of linking a CSS file) and the browser will execute the JS after the HTML/CSS has been loaded.

JS/HTML Connection

In the Pre-Check, you were asked to think about some examples of interactive UI elements on webpages.

There are many HTML5 tags to create a UI! We'll see a few as we learn about adding behavior to our webpages.

But HTML tags aren't too helpful if we can't do anything with them.

In general, to add interactivity to our HTML/CSS websites we need to:

1. Link a JS program to our HTML (in the <head>)
2. Identify the elements we want to "listen" to user/page events
3. Identify the events we want to respond to
4. Identify what each response function is
5. Assign the listener elements the functions when the event(s) occurs! own event(s) to listen to

Event-Driven Programming

Unlike Java programs, JS programs have no main; they respond to user actions called events

Event-Driven Programming: writing programs driven by user events

Common Types of JavaScript Events

We'll cover more later (you can find a full list here), but today we'll demo click!

Accessing Elements from the Document in JavaScript

Finally... the fun starts.

Accessing an Element by ID

let name = document.getElementById("id");

JS

• document.getElementById returns the DOM object for an element with a given id (note that you omit the # when giving an id)
• We'll learn about other ways to get DOM elements (e.g. by class) tomorrow and Wednesday!

Back to HTML with Buttons: <button>

            <button id="my-btn">Click me!</button>

HTML

Button's text appears inside tag; can also contain images

To make a responsive button or other UI control:

1. Choose the control (e.g., button) and event (e.g., mouse click) of interest
2. Write a JavaScript function to run when the event occurs
3. Attach the function to the event on the control

Right now this button doesn't do anything when we click it. That's because it doesn't have event handler code attached :( Let's see an example where we do have an event listener attached to a button on our page!

click Event Handler: an Example

<img id="pokeball" src="images/pokeball.jpg" alt="a pokeball" />
<button id="demo-btn">Click me!</button>

HTML

let demoButton = document.getElementById("demo-btn");
demoButton.addEventListener("click", changeImage);

function changeImage() {
  let pokeballImg = document.getElementById("pokeball");
  pokeballImg.src = "images/mystery.gif";
}

JS

Click me!

output

Putting it All Together

// listener attached before page is loaded
window.addEventListener("load", init);

function init() {
  // this code is ran after page is loaded!
  let demoButton = document.getElementById("demo-btn");
  demoButton.addEventListener("click", changeImage);
}

function changeImage() {
  let pokeballImg = document.getElementById("pokeball");
  pokeballImg.src = "images/mystery.gif";
}

button-example.js (commented example)

You can see the HTML output here. On Wednesday, we'll add one more important component to our JS files: the module pattern.

Resources and Tips

Review programming basics: using variables, arrays, loops, if-statements, and functions

Go over some JavaScript tutorials - there are many great ones!

• Mozilla's JavaScript Basics Tutorial
• Codecademy's JavaScript Baics course (free)

Practice! We now have JavaScript problems on Practice-It (80+ problems) and its sister site CodeStepByStep (200+ problems).

Check out cool examples of JavaScript on the web!

• Dennis Music Video
• Robby Leonardi Mario-style Resume
• Species in Pieces
• Rainbow Worm
• OMFGDOGS