JavaScript

Joel Ross
Autumn 2024

View of the Day

  • JavaScript Intro

  • My First Script

  • Variables & Types

  • Data Structures (Arrays and Objects)

  • Control Structures (quickly)

 

Today is a syntax day!

(it will also mirror the videos)

JavaScript

(more in the course text)

!=

Hello World

/* My first program (in JavaScript) */
console.log("Hello world!"); //prints

script.js

Including JavaScript

Where?

<!DOCTYPE html>
<html>
<head>
  <!-- include here to load before page -->
  <script src="js/my-script.js"></script>
</head>
<body>
   ... content ...


   <!-- include here to load "after" html -->
   <!-- put it here for this course -->
   <script src="js/my-script.js"></script>
</body>
<html>

Include an external script file

<script src="path/to/my-script.js"></script>

JavaScript Console

Open
"terminal"

See also https://developer.chrome.com/devtools/docs/console

Structuring JavaScript

Similar to R and Python, JavaScript has no main() method. Instead, each line of code in the script file is executed one at a time, top to bottom.

It's like the entire file is the body of main()!

/* index.js */
/* This is the ENTIRE contents of the file! */
console.log("Hello world!");  //executed first
console.log("I'm doing JavaScript!");  //executed second

Practice

Use two (2) console.log() statements to say hello to your neighbor. Show them your message in your browser!

JS Syntax: C-based

foo.bar();

Key Differences from Java

  1. Dynamic Typing

  2. Objects

  3. Higher Order Functions

Next time!

JS Syntax: C-based

foo.bar();

Key Differences from Python

  1. Blocks and control structure (e.g,. loop) syntax

  2. Higher Order Functions are in both, but we'll use them more in JavaScript

Next time!

JavaScript Variables

Variables are dynamically typed, so they take on the type (Number, String, Array, etc) of their current value.

let x = 'hello'; //value is a String

x = 42; //value is now a Number

Any unassigned variable has a value of undefined 

//create a variable (not assigned)
let hoursSlept; 
console.log(hoursSlept); //=> undefined

camelCase variables

declare as variable

no let (already declared)

let and const 

const x = 4;
x = 5; // TypeError: Assignment to constant variable.

let y = 1.5;
y = 2; // ok!

Variables declared with const are constant and cannot be reassigned later (though properties can be changed).

Variables declared with let can be reassigned.

Best practice is to use const whenever you can.

Do not use var.

Use const wherever you can!

Basic Variable Types

//Numbers (no difference between int and float)
const x = 4;   //'number'
const y = 1.5; //'number'

//Strings (single or double quotes, just be consistent)
const message = "Hello world";

//Booleans (lowercase)
const likesCode = true;

//you can use `typeof` to inspect type (for debugging ONLY)
console.log(typeof "a string"); //'string'

JavaScript uses the same basic variable types as other languages.

Type Coercion

JavaScript will "coerce" operators to try and match types.

console.log('40' + 2)
console.log('40' - 4)


const num = 10
const str = '10'

// What are the values of each expression?
// (they will all be booleans, true/false)
const bool1 = (num == str)
const bool2 = (num === str)
const bool3 = ('' == 0) //empty String compare to 0

Arrays

//declare arrays using literal syntax
const lettersArray = ['a', 'b', 'c']; //literal syntax
const thingsArray = ['dogs', 2.5, true, [3,4,0]]; //can mix types
const emptyArray = []; //an array with no values in it

//access array elements using bracket notation
console.log(lettersArray[0]); //'a'
console.log(lettersArray[4]); //undefined
lettersArray[5] = 'f'; //assigning out of bounds grows array
console.log(lettersArray); //['a', 'b', 'c', , , 'f']

//Use the `.length` property to get the length of the array
//(based on the last index; undefined is considered a value!)
console.log(lettersArray.length); //6

//arrays have methods as well!
lettersArray.push('z'); //add 'z' to the end

JavaScript arrays are sequences of value, similar to arrays in Java or lists in Python.

Practice

Define an array that includes 3 strings: your name and the names of two neighbors. Then console log that array.

Nested Arrays

Arrays can contain any data type... including other arrays!
This is called a nested array or two-dimensional array.

// an array of different dinners available at a fancy party
// this list has 4 elements, each of which is a list of 3 elements
// the indentation is just for human readability
const dinnerOptions = [
    ['chicken', 'mashed potatoes', 'mixed veggies'],
    ['steak', 'seasoned potatoes', 'asparagus'],
    ['fish', 'rice', 'green beans'],
    ['portobello steak', 'rice', 'green beans']
];

console.log(dinnerOptions.length); //4

const fishOption = dinnerOptions[2]; // ['fish', 'rice', 'green beans']

// fishOption is an array, so can reference its elements by index
console.log(fishOption[0]); //"fish"

// Access the 2th element's 0th element
console.log(dinnerOption[2][0]); //"fish"
// an array of different dinners available at a fancy party
// this list has 4 elements, each of which is a list of 3 elements
// the indentation is just for human readability
const dinnerOptions = [
    ['chicken', 'mashed potatoes', 'mixed veggies'],
    ['steak', 'seasoned potatoes', 'asparagus'],
    ['fish', 'rice', 'green beans'],
    ['portobello steak', 'rice', 'green beans']
];









console.log(dinnerOption[2][0]); //?? What is this?

Objects

Objects are an unordered set of key-value-pairs.

  • Like a dictionary: have the word (the key) and the definition (the value). Use the key to "look up" the value.

  • a.k.a a Map or a Hash (HashMap in Java, list in R, dict in Python)

  • a.k.a. an associative array

Objects are like arrays, except the "indices" are strings!

//declare objects using literal syntax
const agesObj = {'sarah':42, 'amit':35, 'zhang':13}
const typeExamples = {'anInt':12, 'aStr':'dog', 'anArr':[1,2]}
const emptyObj = {}

////can omit quotes on keys, but they are actually strings!
const englishToSpanish = {one:'uno', two:'dos'}
const numWords = {1:'one', 2:'two', 3:'three'}

//global function to get the keys of an object (as an array)
const keysArray = Object.keys(numWords) //[ '1', '2', '3' ]

Accessing Properties

Access individual values in a dictionary by using bracket notation, using the key as the "index".

const agesObj = {alice:40, bob:35, charles:13};

//access ("look up") values
console.log( agesObj['alice'] ); //=> 40
console.log( agesObj['bob'] ); //=> 35
console.log( agesObj['charles'] ); //=> 13

//keys not in the object have undefined values
console.log( agesObj['fred']); //=> undefined

//assign values
agesObj['alice'] = 41;
console.log( agesObj['alice'] ); //=> 41

agesObj['fred'] = 19; //adds the key and assigns value

can assign to any key w/o "initializing"!

Practice

  1. Define a variable dailySleep that is an object. The object should have 2 values:
    • a string listing the day of the week (e.g., "Monday") assigned to "day" key
    • a number giving how many hours you slept that night, assigned to the "hoursSlept" key.
  2. Define a variable howMuchSleep and assign the value of the dailySleep object's "hoursSlept" key to that variable. Console log the variable.
     
  3. Extra: Define an array of objects that all of this structure, representing the sleep you earned during the entire week!

Accessing Properties

const person = {
  firstName: 'Alice',
  lastName: 'Wong',
  age: 40,
  favorites: {
    music: 'jazz',
    food: 'pizza',
    numbers: [12, 42]
  }
};

const name = person['firstName']; //get value of 'firstName' key
person['lastName'] = 'Jones'; //set value of 'lastName' key
console.log(person['firstName']+' '+person['lastName']); //"Alice Jones"

const chosenValue = "food" //e.g., from user input
const favFood = person['favorites'][chosenValue]; //object in the object
                //object           //value

const firstNumber = person['favorites']['numbers'][0]; //12
person['favorites']['numbers'].push(7); //push 7 onto the Array

keys can be variables

Can reference values (called properties) with bracket notation, using the "key" as the "index"

Accessing Properties

Can also reference values using dot notation (like attributes
of a class). Similar in concept to $ notation in R.

const person = {
  firstName: 'Alice',
  lastName: 'Wong',
  age: 40,
  favorites: {
    music: 'jazz',
    food: 'pizza',
    numbers: [12, 42]
  }
};

const name = person.firstName; //get value of 'firstName' key
person.lastName = 'Jones'; //set value of 'lastName' key
console.log(person.firstName+' '+person.lastName); //"Alice Jones"

const chosenValue = "food" //e.g., from user input
const favFood = person.favorites.chosenValue; //undefined!!
              //object         //value

const firstNumber = person.favorites.numbers[0]; //12
person.favorites.numbers.push(7); //push 7 onto the Array

no variables in this syntax

destroyallsoftware.com/talks/wat

"Data Tables"

Use an array of objects with shared properties to represent a data table (like from a spreadsheet)

//Arbitrary list of people's names, heights, and weights
const people = [
    {name: 'Ada', height: 64, weight: 135},
    {name: 'Bob', height: 74, weight: 156},
    {name: 'Chris', height: 69, weight: 139},
    {name: 'Diya', height: 69, weight: 144},
    {name: 'Emma', height: 71, weight: 152}
]

Conditionals

JavaScript supports if/else statements just like Java or Python or R.

if(outsideTemperature < 60) {
  console.log("Wear a jacket");
}
else if(outsideTemperature > 90) {
  console.log("Wear sunscreen");
}
else if(outsideTemperature == 72) {
  console.log("Perfect weather!");
}
else {
  console.log("Wear a t-shirt");
}

For Loops

JavaScript supports Java-Style loops. For loops look the same, except JavaScript doesn't describe variable types.

const myArray = [1, 2, 3, 4];

//for loop to iterate through indices
for(let i=0; i<myArray.length; i++){
  const theItem = myArray[i]; //access element at index 
  console.log(theItem);
}

For Loops

JavaScript also supports a "for of" syntax (similar to enhanced for loop in Java or a Python loop). Use "for of" to iterate through arrays, "for in" is only used to iterate through an Object (by keys).

const myArray = [1, 2, 3, 4];
for(const theItem of myArray) { //loop array items
  console.log(theItem)
}


const myObject = {a: 1, b: 2, c: 3};
for(const theKey in myObject) { //loop object keys
  console.log(theKey, ":", myObject[theKey])
}

//explicit key looping - Joel prefers this
const keysArray = Object.keys(myObject);
for(const theKey of keysArray) { /*...* /}

Functions

Functions in JavaScript are like static methods in Java 

//JavaScript
function greet(greeting, name){
    return greeting + ", " + name;
}


const msg = greet("Hello", "World");
//Java
public static String greet(String greeting, String name){
    return greeting  + ", " + name;
}

public static void main(String[] args){
    String msg = greet("Hello", "World");
}

No
access modifier
or return type declared

parameters have no type

parameters are
comma-separated

Call with parens and assign result, like Java

Functions

Functions in JavaScript look like functions in Python, except with C-style blocks (using {})

//JavaScript
function greet(greeting, name){
    return greeting + ", " + name;
}


const msg = greet("Hello", "World");
# Python
def greet(greeting, name):
    return greeting  + ", " + name


msg = greet("Hello", "World")

Action Items!

Action Items!

  • Complete task list for Week 4 (all items!

  • Read: through Chapter 11

  • Problem Set 04 due Friday

  • Problem Set 05 due next week

    • Get started now!

  • Project Draft 1 due next week

 

Next: Functions and Functional Programming

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