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JavaScript Reference

A comprehensive reference covering JavaScript fundamentals, advanced features, DOM manipulation, network requests, and modern frameworks. Consolidated from MDN Web Docs and other educational sources.

What is JavaScript?
Adding JavaScript to a Page
Script Loading Strategies
Comments
Variables
Numbers and Math
Strings
Useful String Methods
Arrays
Conditionals
Loops
Functions
Building Custom Functions
Function Return Values
Events
Object Basics
DOM Scripting
Network Requests
Working with JSON
JavaScript Frameworks: Main Features
Getting Started with React
React Components

What is JavaScript?

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/What_is_JavaScript

JavaScript is a scripting or programming language that allows you to implement complex features on web pages. It enables dynamic content updates, interactive maps, animated graphics, scrolling video jukeboxes, and much more.

The Three Layers of Web Technologies

JavaScript is the third layer of standard web technologies:

HTML: Markup language for structuring and giving meaning to web content
CSS: Language of style rules for applying styling to HTML content
JavaScript: Scripting language that creates dynamically updating content, controls multimedia, and animates images

What Can JavaScript Do?

Core JavaScript features allow you to:

Store useful values in variables
Perform operations on text (strings) -- combining and manipulating text
Run code in response to events -- detect user interactions like clicks, keyboard input, etc.
Access and manipulate HTML and CSS via the DOM (Document Object Model)

Practical Example

<button>Player 1: Chris</button>

function updateName() {
  const name = prompt("Enter a new name");
  button.textContent = `Player 1: ${name}`;
}

const button = document.querySelector("button");
button.addEventListener("click", updateName);

Browser APIs

Application Programming Interfaces (APIs) provide ready-made code building blocks that enable powerful functionality:

API	Description
DOM API	Manipulate HTML and CSS dynamically; create, remove, and change HTML elements
Geolocation API	Retrieves geographical information
Canvas and WebGL APIs	Create animated 2D and 3D graphics
Audio and Video APIs	Play audio and video in web pages; capture video from web cameras

Third-Party APIs

Not built into browsers by default; requires grabbing code from the web:

Google Maps API: Embed custom maps into websites
OpenStreetMap API: Add mapping functionality
Social media APIs: Display posts on your website

How JavaScript Runs

JavaScript runs top to bottom in the order it appears
Each browser tab has its own separate execution environment
JavaScript is interpreted (though modern interpreters use just-in-time compiling for performance)
Client-side JavaScript runs on the user's computer in the browser
Server-side JavaScript runs on the server (e.g., Node.js environment)
Dynamic code updates display depending on circumstances; static code shows the same content all the time

Adding JavaScript to a Page

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/What_is_JavaScript

1. Internal JavaScript

Add JavaScript directly in your HTML file using <script> tags:

<body>
  <button>Click me!</button>

  <script>
    function createParagraph() {
      const para = document.createElement("p");
      para.textContent = "You clicked the button!";
      document.body.appendChild(para);
    }

    const buttons = document.querySelectorAll("button");
    for (const button of buttons) {
      button.addEventListener("click", createParagraph);
    }
  </script>
</body>

2. External JavaScript (Recommended)

Keep JavaScript in a separate file for better organization and reusability:

HTML file:

<script type="module" src="script.js"></script>

script.js file:

function createParagraph() {
  const para = document.createElement("p");
  para.textContent = "You clicked the button!";
  document.body.appendChild(para);
}

const buttons = document.querySelectorAll("button");
for (const button of buttons) {
  button.addEventListener("click", createParagraph);
}

3. Inline JavaScript Handlers (Not Recommended)

<button onclick="createParagraph()">Click me!</button>

Avoid inline handlers because they pollute HTML with JavaScript, are inefficient, and are harder to maintain.

Comparison

Method	Location	Best For	Pros	Cons
Internal	`<script>` in body	Small projects	Simple, self-contained	Not reusable
External	`<script src="">`	Most projects	Reusable, organized	Requires HTTP server
Inline	`onclick=""`	Not recommended	Quick testing	Hard to maintain, pollutes HTML
Module	`<script type="module">`	Modern projects	Safe timing, organized	Requires HTTP server

Script Loading Strategies

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/What_is_JavaScript

Place `<script>` at Bottom of Body

<body>
  <h1>My Page</h1>
  <p>Content here</p>
  <script src="script.js"></script>
</body>

Use `<script type="module">` in `<head>` (Recommended)

<head>
  <script type="module" src="script.js"></script>
</head>

Browser waits for all HTML to process before executing.

Use `defer` Attribute

<head>
  <script defer src="script.js"></script>
</head>

Script downloads in parallel while HTML continues parsing; executes only after HTML is fully parsed. Scripts with defer execute in order.

Use `async` Attribute (for non-dependent scripts)

<script async src="analytics.js"></script>

Script downloads in parallel and executes immediately when ready. Does not guarantee execution order. Use only for scripts that do not depend on DOM elements.

Wrap Internal Scripts in `DOMContentLoaded`

document.addEventListener('DOMContentLoaded', function() {
  const button = document.querySelector("button");
  button.addEventListener("click", updateName);
});

Comments

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/What_is_JavaScript

Single Line Comments

// This is a single line comment
const name = "Chris"; // Can also go at end of line

Multi-Line Comments

/*
  This is a multi-line comment.
  It can span multiple lines.
  Useful for longer explanations.
*/

Best Practices

Use comments to explain why code does something, not what it does
Variable names should be intuitive -- don't comment obvious operations
More comments are usually better than fewer, but avoid overdoing it
Keep comments up-to-date as code changes

Variables

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/Variables

A variable is a container for a value, like a number or string. Variables are essential because they allow your code to remember and manipulate data.

Declaring Variables

Use the let keyword to create a variable:

let myName;
let myAge;

After declaration, the variable exists but has no value (undefined).

Initializing Variables

Assign a value using the equals sign (=):

myName = "Chris";
myAge = 37;

Or declare and initialize together:

let myDog = "Rover";

Variable Types

Numbers:

let myAge = 17;           // integer
let temperature = 98.6;   // floating point number

Strings:

let dolphinGoodbye = "So long and thanks for all the fish";

Booleans:

let iAmAlive = true;
let test = 6 < 3;  // returns false

Arrays:

let myNameArray = ["Chris", "Bob", "Jim"];
let myNumberArray = [10, 15, 40];
myNameArray[0];    // "Chris" (zero-indexed)
myNumberArray[2];  // 40

Objects:

let dog = { name: "Spot", breed: "Dalmatian" };
dog.name;  // "Spot"

Variable Naming Rules

Use Latin characters (0-9, a-z, A-Z) and underscores only
Use lower camel case: myAge, initialColor, finalOutputValue
Make names intuitive and descriptive
Variables are case-sensitive: myage is not the same as myAge
Don't start with underscore or numbers
Don't use reserved keywords (var, function, let, etc.)

Dynamic Typing

JavaScript is dynamically typed -- you don't declare variable types. A variable's type is determined by the value assigned:

let myString = "Hello";
typeof myString;           // "string"

let myNumber = "500";
typeof myNumber;           // "string"

myNumber = 500;
typeof myNumber;           // "number"

Constants with `const`

Use const for values that should not change:

const myDog = "Rover";
myDog = "Fido";  // Error: cannot reassign

For objects, you can still modify properties even with const:

const bird = { species: "Kestrel" };
bird.species = "Striated Caracara";  // OK - modifying content

`let` vs `const` vs `var`

Feature	`let`	`const`	`var`
Can reassign	Yes	No	Yes
Must initialize	No	Yes	No
Scoping	Block	Block	Function
Hoisting issues	No	No	Yes

Best Practice: Use const when possible, use let when you need to reassign. Avoid var in modern JavaScript.

Numbers and Math

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/Math

Types of Numbers

Integers: Numbers without a fractional part (e.g., 10, 400, -5)
Floating Point Numbers (Floats): Have decimal points (e.g., 12.5, 56.7786543)
JavaScript has only one data type for numbers: Number (plus BigInt for very large integers)

Arithmetic Operators

Operator	Name	Example	Result
`+`	Addition	`6 + 9`	`15`
`-`	Subtraction	`20 - 15`	`5`
`*`	Multiplication	`3 * 7`	`21`
`/`	Division	`10 / 5`	`2`
`%`	Remainder (Modulo)	`8 % 3`	`2`
`**`	Exponent	`5 ** 2`	`25`

const num1 = 10;
const num2 = 50;
9 * num1;      // 90
num1 ** 3;     // 1000
num2 / num1;   // 5

Operator Precedence

Multiply and Divide are done first (left to right)
Add and Subtract are done after (left to right)

num2 + num1 / 8 + 2;        // = 53.25  (50 + 1.25 + 2)
(num2 + num1) / (8 + 2);    // = 6      (60 / 10)

Increment and Decrement Operators

let num1 = 4;
num1++;     // Returns 4, then increments to 5
++num1;     // Increments first, then returns 6

let num2 = 6;
num2--;     // Returns 6, then decrements to 5
--num2;     // Decrements first, then returns 4

Assignment Operators

Operator	Example	Equivalent
`+=`	`x += 4;`	`x = x + 4;`
`-=`	`x -= 3;`	`x = x - 3;`
`*=`	`x *= 3;`	`x = x * 3;`
`/=`	`x /= 5;`	`x = x / 5;`

Comparison Operators

Operator	Name	Example	Result
`===`	Strict equality	`5 === 2 + 3`	`true`
`!==`	Strict non-equality	`5 !== 2 + 3`	`false`
`<`	Less than	`10 < 6`	`false`
`>`	Greater than	`10 > 20`	`false`
`<=`	Less than or equal	`3 <= 2`	`false`
`>=`	Greater than or equal	`5 >= 4`	`true`

Always use === and !== (strict versions) instead of == and !=.

Useful Number Methods

// Round to decimal places
const lotsOfDecimal = 1.7665849587;
lotsOfDecimal.toFixed(2);  // "1.77"

// Convert string to number
let myNumber = "74";
myNumber = Number(myNumber) + 3;  // 77

// Check data type
typeof 5;      // "number"
typeof 6.667;  // "number"

// Math object methods
Math.random();           // Random number between 0 and 1
Math.floor(2.9);         // 2 (rounds down)
Math.ceil(2.1);          // 3 (rounds up)
Math.pow(5, 2);          // 25 (5 to the power of 2)

Strings

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/Strings

Creating Strings

Strings must be surrounded by quotes:

const single = 'Single quotes';
const double = "Double quotes";
const backtick = `Backtick`;

The same character must be used for the start and end of a string.

Template Literals

Template literals (backtick-wrapped strings) have two special features:

1. Embedding JavaScript Expressions:

const name = "Chris";
const greeting = `Hello, ${name}`;
console.log(greeting); // "Hello, Chris"

const song = "Fight the Youth";
const score = 9;
const highestScore = 10;
const output = `I like the song ${song}. I gave it a score of ${
  (score / highestScore) * 100
}%.`;
// "I like the song Fight the Youth. I gave it a score of 90%."

2. Multiline Strings:

const newline = `One day you finally knew
what you had to do, and began,`;

With regular strings, use \n for line breaks:

const newline2 = "One day you finally knew\nwhat you had to do, and began,";

String Concatenation

// Using + operator
const greeting = "Hello" + ", " + "Bob";  // "Hello, Bob"

// Using template literals (recommended)
const name = "Ramesh";
console.log(`Howdy, ${name}`);  // "Howdy, Ramesh"

Escaping Characters

const bigmouth = 'I\'ve got no right to take my place...';

Useful String Methods

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/Useful_string_methods

Finding String Length

const browserType = "mozilla";
browserType.length;  // 7

Retrieving Characters

browserType[0];                          // "m" (first character)
browserType[browserType.length - 1];     // "a" (last character)

Testing for Substrings

const browserType = "mozilla";

browserType.includes("zilla");     // true
browserType.startsWith("zilla");   // false
browserType.endsWith("zilla");     // true

Finding Position of a Substring

const tagline = "MDN - Resources for developers, by developers";
tagline.indexOf("developers");     // 20
tagline.indexOf("x");             // -1 (not found)

// Finding subsequent occurrences
const first = tagline.indexOf("developers");           // 20
const second = tagline.indexOf("developers", first + 1); // 35

Extracting Substrings

const browserType = "mozilla";
browserType.slice(1, 4);  // "ozi"
browserType.slice(2);     // "zilla" (from index 2 to end)

Changing Case

const radData = "My NaMe Is MuD";
radData.toLowerCase();  // "my name is mud"
radData.toUpperCase();  // "MY NAME IS MUD"

Replacing Parts of a String

// Replace first occurrence
const browserType = "mozilla";
const updated = browserType.replace("moz", "van");  // "vanilla"

// Replace all occurrences
let quote = "To be or not to be";
quote = quote.replaceAll("be", "code");  // "To code or not to code"

Important: String methods return new strings; they don't modify the original unless you reassign.

Arrays

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/Arrays

Creating Arrays

const shopping = ["bread", "milk", "cheese", "hummus", "noodles"];
const sequence = [1, 1, 2, 3, 5, 8, 13];
const random = ["tree", 795, [0, 1, 2]];  // Mixed types allowed

Finding Array Length

shopping.length;  // 5

Accessing and Modifying Items

shopping[0];           // "bread" (zero-indexed)
shopping[0] = "tahini"; // Modify first item

// Multidimensional arrays
const random = ["tree", 795, [0, 1, 2]];
random[2][2];  // 2

Finding Index of Items

const birds = ["Parrot", "Falcon", "Owl"];
birds.indexOf("Owl");     // 2
birds.indexOf("Rabbit");  // -1 (not found)

Adding Items

const cities = ["Manchester", "Liverpool"];

// Add to end
cities.push("Cardiff");
cities.push("Bradford", "Brighton");  // Add multiple

// Add to start
cities.unshift("Edinburgh");

Removing Items

const cities = ["Manchester", "Liverpool", "Edinburgh", "Carlisle"];

// Remove from end
cities.pop();       // Returns removed item

// Remove from start
cities.shift();     // Returns removed item

// Remove from specific index
const index = cities.indexOf("Liverpool");
if (index !== -1) {
  cities.splice(index, 1);    // Remove 1 item at index
}
cities.splice(index, 2);      // Remove 2 items starting at index

Iterating Over Arrays

for...of Loop:

const birds = ["Parrot", "Falcon", "Owl"];
for (const bird of birds) {
  console.log(bird);
}

map() -- Transform Items:

const numbers = [5, 2, 7, 6];
const doubled = numbers.map(number => number * 2);
// [10, 4, 14, 12]

filter() -- Select Matching Items:

const cities = ["London", "Liverpool", "Totnes", "Edinburgh"];
const longer = cities.filter(city => city.length > 8);
// ["Liverpool", "Edinburgh"]

Converting Between Strings and Arrays

// String to Array
const data = "Manchester,London,Liverpool";
const cities = data.split(",");
// ["Manchester", "London", "Liverpool"]

// Array to String
const commaSeparated = cities.join(",");
// "Manchester,London,Liverpool"

// Simple toString (always uses comma)
const dogNames = ["Rocket", "Flash", "Bella"];
dogNames.toString();  // "Rocket,Flash,Bella"

Conditionals

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/Conditionals

if...else Statements

if (condition) {
  /* code to run if condition is true */
} else {
  /* run some other code instead */
}

else if Statements

if (choice === "sunny") {
  para.textContent = "Wear shorts!";
} else if (choice === "rainy") {
  para.textContent = "Take a rain coat.";
} else if (choice === "snowing") {
  para.textContent = "It is freezing!";
} else {
  para.textContent = "";
}

Logical Operators

AND (&&) -- All conditions must be true:

if (choice === "sunny" && temperature < 86) {
  para.textContent = "Nice and sunny. Let's go to the beach!";
}

OR (||) -- At least one condition must be true:

if (iceCreamVanOutside || houseStatus === "on fire") {
  console.log("You should leave the house quickly.");
}

NOT (!) -- Negates an expression:

if (!(iceCreamVanOutside || houseStatus === "on fire")) {
  console.log("Probably should just stay in then.");
}

Common Mistake:

// WRONG - will always evaluate to true
if (x === 5 || 7 || 10 || 20) { }

// CORRECT
if (x === 5 || x === 7 || x === 10 || x === 20) { }

Switch Statements

switch (choice) {
  case "sunny":
    para.textContent = "Wear shorts!";
    break;
  case "rainy":
    para.textContent = "Take a rain coat.";
    break;
  case "snowing":
    para.textContent = "It is freezing!";
    break;
  default:
    para.textContent = "";
}

Ternary Operator

const greeting = isBirthday
  ? "Happy birthday Mrs. Smith!"
  : "Good morning Mrs. Smith.";

Loops

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/Loops

for...of Loop

Used to iterate through collections:

const cats = ["Leopard", "Serval", "Jaguar", "Tiger"];
for (const cat of cats) {
  console.log(cat);
}

Standard for Loop

for (let i = 1; i < 10; i++) {
  console.log(`${i} x ${i} = ${i * i}`);
}

Components: initializer (let i = 1), condition (i < 10), final-expression (i++).

Looping Through Arrays with for

const cats = ["Leopard", "Serval", "Jaguar"];
for (let i = 0; i < cats.length; i++) {
  console.log(cats[i]);
}

while Loop

let i = 0;
while (i < cats.length) {
  console.log(cats[i]);
  i++;
}

do...while Loop

Code executes at least once, then checks condition:

let i = 0;
do {
  console.log(cats[i]);
  i++;
} while (i < cats.length);

break and continue

break -- Exit Loop Immediately:

for (const contact of contacts) {
  const splitContact = contact.split(":");
  if (splitContact[0].toLowerCase() === searchName) {
    console.log(`${splitContact[0]}'s number is ${splitContact[1]}.`);
    break;
  }
}

continue -- Skip to Next Iteration:

for (let i = 1; i <= num; i++) {
  let sqRoot = Math.sqrt(i);
  if (Math.floor(sqRoot) !== sqRoot) {
    continue;  // Skip non-perfect squares
  }
  console.log(i);
}

Which Loop Type to Use?

Loop Type	Best For
`for...of`	Iterating collections when you don't need index
`for`	General purpose loops; full control of iteration
`while`	When initializer before loop makes sense
`do...while`	When code must run at least once
`map()`	Transforming array items
`filter()`	Selecting specific array items

Warning: Always ensure loops terminate. Infinite loops crash browsers.

Functions

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/Functions

What Are Functions?

Functions are reusable blocks of code that perform a single task. They allow you to store code in a defined block and call it whenever needed.

Built-in Browser Functions

const myText = "I am a string";
const newString = myText.replace("string", "sausage");  // "I am a sausage"

const myArray = ["I", "love", "chocolate", "frogs"];
const madeAString = myArray.join(" ");  // "I love chocolate frogs"

const myNumber = Math.random();  // Random number between 0 and 1

Custom Functions

function myFunction() {
  alert("hello");
}

myFunction();  // Calls the function

Function Parameters and Default Parameters

function hello(name = "Chris") {
  console.log(`Hello ${name}!`);
}

hello("Ari");  // "Hello Ari!"
hello();       // "Hello Chris!"

Anonymous Functions

Functions without names, often passed as parameters:

textBox.addEventListener("keydown", function (event) {
  console.log(`You pressed "${event.key}".`);
});

Arrow Functions

Modern syntax using =>:

// Full syntax
textBox.addEventListener("keydown", (event) => {
  console.log(`You pressed "${event.key}".`);
});

// Single parameter - parentheses optional
textBox.addEventListener("keydown", event => {
  console.log(`You pressed "${event.key}".`);
});

// Single return statement - implicit return
const originals = [1, 2, 3];
const doubled = originals.map(item => item * 2);  // [2, 4, 6]

Function Scope

Variables inside functions are locked to function scope and unreachable from outside:

const x = 1;        // global scope - accessible everywhere

function myFunc() {
  const y = 2;      // function scope - only inside myFunc
  console.log(x);   // Can access global x
}

console.log(x);     // Can access global x
console.log(y);     // ReferenceError: y is not defined

Block Scope (let/const)

if (x === 1) {
  const c = 4;      // block scope
}
console.log(c);     // ReferenceError: c is not defined

Building Custom Functions

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/Build_your_own_function

Function Structure

function displayMessage() {
  // Function body code goes here
}

Key components:

function keyword declares a function definition
Function name follows variable naming conventions
Parentheses () hold parameters
Curly braces {} contain the code that runs when called

Complete Practical Example

function displayMessage(msgText, msgType) {
  const body = document.body;

  const panel = document.createElement("div");
  panel.setAttribute("class", "msgBox");
  body.appendChild(panel);

  const msg = document.createElement("p");
  msg.textContent = msgText;
  panel.appendChild(msg);

  const closeBtn = document.createElement("button");
  closeBtn.textContent = "x";
  panel.appendChild(closeBtn);

  closeBtn.addEventListener("click", () => body.removeChild(panel));

  if (msgType === "warning") {
    msg.style.backgroundImage = 'url("icons/warning.png")';
    panel.style.backgroundColor = "red";
  } else if (msgType === "chat") {
    msg.style.backgroundImage = 'url("icons/chat.png")';
    panel.style.backgroundColor = "aqua";
  } else {
    msg.style.paddingLeft = "20px";
  }
}

Calling Functions

// Direct invocation
displayMessage("Your inbox is almost full", "warning");

// As event handler (no parentheses)
btn.addEventListener("click", displayMessage);

// With parameters via anonymous function
btn.addEventListener("click", () =>
  displayMessage("Woo, this is a different message!"),
);

Important: Don't include parentheses when passing a function as a callback:

btn.addEventListener("click", displayMessage);    // Correct
btn.addEventListener("click", displayMessage());  // Wrong - calls immediately

Parameters vs Arguments

Parameters are the named variables in the function definition
Arguments are the actual values passed when calling the function

Function Return Values

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/Return_values

What Are Return Values?

Return values are the values that a function returns when it completes execution.

const myText = "The weather is cold";
const newString = myText.replace("cold", "warm");  // "The weather is warm"

Using the return Keyword

function random(number) {
  return Math.floor(Math.random() * number);
}

When a function is called, the return value substitutes for the function call:

ctx.arc(random(WIDTH), random(HEIGHT), random(50), 0, 2 * Math.PI);
// If random() calls return 500, 200, 35:
ctx.arc(500, 200, 35, 0, 2 * Math.PI);

Creating Functions with Return Values

function squared(num) {
  return num * num;
}

function cubed(num) {
  return num * num * num;
}

function factorial(num) {
  if (num < 0) return undefined;
  if (num === 0) return 1;
  let x = num - 1;
  while (x > 1) {
    num *= x;
    x--;
  }
  return num;
}

Using Return Values

input.addEventListener("change", () => {
  const num = parseFloat(input.value);
  if (isNaN(num)) {
    para.textContent = "You need to enter a number!";
  } else {
    para.textContent = `${num} squared is ${squared(num)}. `;
    para.textContent += `${num} cubed is ${cubed(num)}. `;
    para.textContent += `${num} factorial is ${factorial(num)}. `;
  }
});

Concept	Description
return keyword	Returns a value and exits the function immediately
No return value	Functions without explicit return return `undefined`
Variable storage	Return values can be saved to variables for later use

Events

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/Events

What Are Events?

Events are signals fired by the browser when something significant happens. They allow your code to react to user interactions and system activities.

Using addEventListener() (Recommended)

const btn = document.querySelector("button");

function random(number) {
  return Math.floor(Math.random() * (number + 1));
}

btn.addEventListener("click", () => {
  const rndCol = `rgb(${random(255)} ${random(255)} ${random(255)})`;
  document.body.style.backgroundColor = rndCol;
});

Using Named Functions

function changeBackground() {
  const rndCol = `rgb(${random(255)} ${random(255)} ${random(255)})`;
  document.body.style.backgroundColor = rndCol;
}

btn.addEventListener("click", changeBackground);

Removing Event Listeners

btn.removeEventListener("click", changeBackground);

Adding Multiple Listeners

myElement.addEventListener("click", functionA);
myElement.addEventListener("click", functionB);
// Both functions run when element is clicked

Common Event Types

Event	Description
`click`	User clicks an element
`dblclick`	User double-clicks an element
`focus`	Element receives focus
`blur`	Element loses focus
`mouseover`	Mouse pointer hovers over element
`mouseout`	Mouse pointer leaves element
`keydown`	User presses a key
`submit`	Form is submitted
`play`	Media begins playing
`pause`	Media is paused

Event Objects

Event handler functions receive an event object automatically:

function bgChange(e) {
  const rndCol = `rgb(${random(255)} ${random(255)} ${random(255)})`;
  e.target.style.backgroundColor = rndCol;
}

btn.addEventListener("click", bgChange);

Keyboard Events:

const textBox = document.querySelector("#textBox");
const output = document.querySelector("#output");

textBox.addEventListener("keydown", (event) => {
  output.textContent = `You pressed "${event.key}".`;
});

Preventing Default Behavior

const form = document.querySelector("form");
const fname = document.getElementById("fname");
const lname = document.getElementById("lname");

form.addEventListener("submit", (e) => {
  if (fname.value === "" || lname.value === "") {
    e.preventDefault();
    para.textContent = "You need to fill in both names!";
  }
});

Event Handler Properties (Not Recommended for Multiple Handlers)

btn.onclick = () => {
  document.body.style.backgroundColor = rndCol;
};

Cannot add multiple listeners -- subsequent assignments overwrite previous ones.

Object Basics

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/Object_basics

What Are Objects?

An object is a collection of related data and/or functionality, consisting of:

Properties: variables inside objects (data)
Methods: functions inside objects (functionality)

Object Literal Syntax

const person = {
  name: ["Bob", "Smith"],
  age: 32,
  bio() {
    console.log(`${this.name[0]} ${this.name[1]} is ${this.age} years old.`);
  },
  introduceSelf() {
    console.log(`Hi! I'm ${this.name[0]}.`);
  },
};

Dot Notation

person.age;           // 32
person.bio();         // Calls the method
person.name.first;    // Access nested properties

Bracket Notation

person["age"];              // 32
person["name"]["first"];    // Nested access

// When property names are stored in variables
function logProperty(propertyName) {
  console.log(person[propertyName]);
}
logProperty("name");  // ["Bob", "Smith"]

Setting Object Members

// Update existing properties
person.age = 45;

// Create new properties
person["eyes"] = "hazel";
person.farewell = function () {
  console.log("Bye everybody!");
};

// Dynamic property creation (only bracket notation)
const myDataName = "height";
const myDataValue = "1.75m";
person[myDataName] = myDataValue;

What is "this"?

The this keyword refers to the current object the code is being executed in:

const person1 = {
  name: "Chris",
  introduceSelf() {
    console.log(`Hi! I'm ${this.name}.`);
  },
};

const person2 = {
  name: "Deepti",
  introduceSelf() {
    console.log(`Hi! I'm ${this.name}.`);
  },
};

person1.introduceSelf();  // "Hi! I'm Chris."
person2.introduceSelf();  // "Hi! I'm Deepti."

Constructors

Functions called with the new keyword that create new objects:

function Person(name) {
  this.name = name;
  this.introduceSelf = function () {
    console.log(`Hi! I'm ${this.name}.`);
  };
}

const salva = new Person("Salva");
salva.introduceSelf();  // "Hi! I'm Salva."

const frankie = new Person("Frankie");
frankie.introduceSelf();  // "Hi! I'm Frankie."

DOM Scripting

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/DOM_scripting

What is the DOM?

The Document Object Model (DOM) is a tree structure representation created by the browser that enables HTML to be accessed by programming languages. Each entry in the tree is called a node.

DOM Tree Relationships

Root node: The top node (the HTML element)
Parent node: A node that has other nodes inside it
Child node: A node directly inside another node
Sibling nodes: Nodes on the same level under the same parent
Descendant node: A node anywhere inside another node

Selecting Elements

// querySelector - select first match (Recommended)
const link = document.querySelector("a");
const element = document.querySelector("#myId");
const element = document.querySelector(".myClass");

// querySelectorAll - select all matches (returns NodeList)
const paragraphs = document.querySelectorAll("p");

// Legacy methods
const elementRef = document.getElementById('myId');
const elementRefArray = document.getElementsByTagName('p');

Creating and Inserting Elements

const para = document.createElement("p");
para.textContent = "We hope you enjoyed the ride.";

const sect = document.querySelector("section");
sect.appendChild(para);

// Create a text node
const text = document.createTextNode(" -- the premier source.");
const linkPara = document.querySelector("p");
linkPara.appendChild(text);

Moving and Removing Elements

// Moving (appendChild on existing element moves it)
sect.appendChild(linkPara);

// Cloning
const clone = linkPara.cloneNode();       // Shallow clone
const deepClone = linkPara.cloneNode(true); // Deep clone

// Removing
sect.removeChild(linkPara);               // Using parent
linkPara.remove();                        // Modern method
linkPara.parentNode.removeChild(linkPara); // Older browsers

Manipulating Content

// textContent - plain text only (safer)
link.textContent = "Mozilla Developer Network";

// innerHTML - parses HTML (use with caution)
element.innerHTML = "<span>New content</span>";

Manipulating Attributes

link.href = "https://developer.mozilla.org";
element.getAttribute("class");
element.setAttribute("class", "newClass");
element.removeAttribute("id");

Manipulating Styles

Method 1: Inline Styles:

para.style.color = "white";
para.style.backgroundColor = "black";
para.style.padding = "10px";
para.style.width = "250px";
para.style.textAlign = "center";

Note: CSS hyphenated properties become camelCase in JavaScript (background-color becomes backgroundColor).

Method 2: CSS Classes (Recommended):

para.classList.add("highlight");
para.classList.remove("highlight");
para.classList.toggle("highlight");

Complete Practical Example: Dynamic Shopping List

<h1>My shopping list</h1>
<form>
  <label for="item">Enter a new item:</label>
  <input type="text" name="item" id="item" />
  <button>Add item</button>
</form>
<ul></ul>

const list = document.querySelector("ul");
const input = document.querySelector("input");
const button = document.querySelector("button");

button.addEventListener("click", (event) => {
  event.preventDefault();

  const myItem = input.value;
  input.value = "";

  const listItem = document.createElement("li");
  const listText = document.createElement("span");
  const listBtn = document.createElement("button");

  listItem.appendChild(listText);
  listText.textContent = myItem;
  listItem.appendChild(listBtn);
  listBtn.textContent = "Delete";
  list.appendChild(listItem);

  listBtn.addEventListener("click", () => {
    list.removeChild(listItem);
  });

  input.focus();
});

Key Browser Objects

Object	Description
`window`	Represents the browser tab
`document`	The page loaded in the window
`navigator`	Browser state and identity

Network Requests

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/Network_requests

The Fetch API

The main modern JavaScript API for making HTTP requests to retrieve resources from the server without full page reloads.

Basic Syntax

fetch(url)
  .then((response) => {
    if (!response.ok) {
      throw new Error(`HTTP error: ${response.status}`);
    }
    return response.text();
  })
  .then((data) => {
    // Use the data
  })
  .catch((error) => {
    console.error(`Fetch problem: ${error.message}`);
  });

Fetching Text Content

function updateDisplay(verse) {
  verse = verse.replace(" ", "").toLowerCase();
  const url = `${verse}.txt`;

  fetch(url)
    .then((response) => {
      if (!response.ok) {
        throw new Error(`HTTP error: ${response.status}`);
      }
      return response.text();
    })
    .then((text) => {
      poemDisplay.textContent = text;
    })
    .catch((error) => {
      poemDisplay.textContent = `Could not fetch verse: ${error}`;
    });
}

Fetching JSON Data

fetch("products.json")
  .then((response) => {
    if (!response.ok) {
      throw new Error(`HTTP error: ${response.status}`);
    }
    return response.json();
  })
  .then((json) => initialize(json))
  .catch((err) => console.error(`Fetch problem: ${err.message}`));

Fetching Binary Data (Blob)

fetch(url)
  .then((response) => {
    if (!response.ok) {
      throw new Error(`HTTP error: ${response.status}`);
    }
    return response.blob();
  })
  .then((blob) => showProduct(blob, product))
  .catch((err) => console.error(`Fetch problem: ${err.message}`));

Core Response Methods

Method	Use Case
`response.text()`	Plain text files
`response.json()`	JSON objects/arrays
`response.blob()`	Binary data (images, videos)

Error Handling

.then((response) => {
  if (!response.ok) {
    throw new Error(`HTTP error: ${response.status}`);
  }
  return response.json();
})
.catch((error) => {
  console.error(`Fetch problem: ${error.message}`);
});

Important: fetch() only rejects on network failures, not HTTP error statuses (404, 500). Always check response.ok or response.status.

XMLHttpRequest (Legacy Alternative)

const request = new XMLHttpRequest();

try {
  request.open("GET", "products.json");
  request.responseType = "json";

  request.addEventListener("load", () => {
    initialize(request.response);
  });
  request.addEventListener("error", () => {
    console.error("XHR error");
  });

  request.send();
} catch (error) {
  console.error(`XHR error ${request.status}`);
}

Fetch is simpler and recommended over XMLHttpRequest.

Working with JSON

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/JSON

What is JSON?

JSON (JavaScript Object Notation) is a standard text-based format for representing structured data based on JavaScript object syntax, commonly used for transmitting data in web applications.

Converting a string to a native object is called deserialization
Converting a native object to a string is called serialization
JSON files use the .json extension and application/json MIME type

JSON Structure

{
  "squadName": "Super hero squad",
  "homeTown": "Metro City",
  "formed": 2016,
  "secretBase": "Super tower",
  "active": true,
  "members": [
    {
      "name": "Molecule Man",
      "age": 29,
      "secretIdentity": "Dan Jukes",
      "powers": ["Radiation resistance", "Turning tiny", "Radiation blast"]
    }
  ]
}

Valid JSON Data Types

String literals, number literals, true, false, null
Objects and arrays containing valid JSON types

Not valid in JSON:

undefined, NaN, Infinity
Functions or object types like Date, Set, Map
Single quotes (must use double quotes)
Trailing commas
Comments

Accessing JSON Data

superHeroes.homeTown;                    // "Metro City"
superHeroes.members[1].powers[2];        // Third power of second hero
superHeroes[0].powers[0];               // For top-level arrays

JSON.parse() -- String to Object

const jsonString = '{"name":"John","age":30}';
const obj = JSON.parse(jsonString);
console.log(obj.name);  // "John"

JSON.stringify() -- Object to String

let myObj = { name: "Chris", age: 38 };
let myString = JSON.stringify(myObj);
console.log(myString);  // '{"name":"Chris","age":38}'

Complete Example: Fetching and Displaying JSON

async function populate() {
  const requestURL =
    "https://mdn.github.io/learning-area/javascript/oojs/json/superheroes.json";
  const request = new Request(requestURL);

  const response = await fetch(request);
  const superHeroes = await response.json();

  populateHeader(superHeroes);
  populateHeroes(superHeroes);
}

function populateHeader(obj) {
  const header = document.querySelector("header");
  const myH1 = document.createElement("h1");
  myH1.textContent = obj.squadName;
  header.appendChild(myH1);

  const myPara = document.createElement("p");
  myPara.textContent = `Hometown: ${obj.homeTown} // Formed: ${obj.formed}`;
  header.appendChild(myPara);
}

function populateHeroes(obj) {
  const section = document.querySelector("section");
  const heroes = obj.members;

  for (const hero of heroes) {
    const myArticle = document.createElement("article");
    const myH2 = document.createElement("h2");
    myH2.textContent = hero.name;
    myArticle.appendChild(myH2);

    const myPara = document.createElement("p");
    myPara.textContent = `Secret identity: ${hero.secretIdentity}`;
    myArticle.appendChild(myPara);

    section.appendChild(myArticle);
  }
}

populate();

JavaScript Frameworks: Main Features

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Frameworks_libraries/Main_features

Domain-Specific Languages (DSLs)

JSX (JavaScript and XML):

const subject = "World";
const header = (
  <header>
    <h1>Hello, {subject}!</h1>
  </header>
);

Compiles to:

const header = React.createElement(
  "header",
  null,
  React.createElement("h1", null, "Hello, ", subject, "!"),
);

Handlebars (Ember):

<header>
  <h1>Hello, {{subject}}!</h1>
</header>

TypeScript (Angular):

function add(a: number, b: number) {
  return a + b;
}

Component Props and State

Props (External Data):

function AuthorCredit(props) {
  return (
    <figure>
      <img src={props.src} alt={props.alt} />
      <figcaption>{props.byline}</figcaption>
    </figure>
  );
}

<AuthorCredit
  src="./assets/zelda.png"
  alt="Portrait of Zelda Schiff"
  byline="Zelda Schiff is editor-in-chief of the Library Times."
/>

State (Internal Data):

function CounterButton() {
  const [count, setCount] = useState(0);
  return (
    <button onClick={() => setCount(count + 1)}>
      Clicked {count} times
    </button>
  );
}

Rendering Approaches

Approach	Used By	Description
Virtual DOM	React, Vue	Stores DOM info in JS memory, diffs with real DOM, applies changes
Incremental DOM	Angular	Doesn't create complete copy, ignores unchanged parts
Glimmer VM	Ember	Transpiles templates into bytecode

Dependency Injection

Solutions to "prop drilling" (passing data through many nesting levels):

Angular: Dependency injection system
Vue: provide() and inject() methods
React: Context API
Ember: Services

Testing Example (React Testing Library)

import { fireEvent, render, screen } from "@testing-library/react";
import CounterButton from "./CounterButton";

it("Renders a semantic button with an initial state of 0", () => {
  render(<CounterButton />);
  const btn = screen.getByRole("button");
  expect(btn).toBeInTheDocument();
  expect(btn).toHaveTextContent("Clicked 0 times");
});

it("Increments the count when clicked", () => {
  render(<CounterButton />);
  const btn = screen.getByRole("button");
  fireEvent.click(btn);
  expect(btn).toHaveTextContent("Clicked 1 times");
});

Getting Started with React

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_getting_started

What is React?

React is a library for building user interfaces. It is not a framework -- it is a reusable library used with other libraries like ReactDOM for web development. Its primary goal is to minimize bugs when building UIs through the use of components.

Environment Setup

Requirements:

Node.js (v18 or later; v20 LTS recommended)
npm (comes with Node.js)

Creating a React App with Vite

npm create vite@latest moz-todo-react -- --template react
cd moz-todo-react && npm install
npm run dev -- --open --port 3000

Project Structure

moz-todo-react/
  index.html              (Entry HTML file)
  package.json            (Project metadata & dependencies)
  src/
    App.jsx               (Main component)
    App.css
    main.jsx              (Entry point - imports App)
    index.css             (Global styles)
    assets/
  vite.config.js

Understanding JSX

JSX extends JavaScript to allow HTML-like code alongside JavaScript:

const heading = <h1>Mozilla Developer Network</h1>;

Attributes use className instead of class:

<button type="button" className="primary">
  Click me!
</button>

Basic Component

import "./App.css";

function App() {
  return (
    <>
      <header>
        <h1>Hello, World!</h1>
        <button type="button">Click me!</button>
      </header>
    </>
  );
}

export default App;

Fragments

Use <> (fragments) to return multiple elements without extra <div> wrappers:

return (
  <>
    <header>Content</header>
    <main>More content</main>
  </>
);

JavaScript Expressions in JSX

Use curly braces {} to embed JavaScript expressions:

const subject = "React";

function App() {
  return (
    <>
      <h1>Hello, {subject}!</h1>
      <h1>Hello, {subject.toUpperCase()}!</h1>
      <h1>Hello, {2 + 2}!</h1>
    </>
  );
}

Props (Component Properties)

Props pass data from parent to child components (unidirectional data flow):

// main.jsx
<App subject="Clarice" />

// App.jsx
function App(props) {
  return (
    <header>
      <h1>Hello, {props.subject}!</h1>
    </header>
  );
}

Rendering the App

import { StrictMode } from "react";
import { createRoot } from "react-dom/client";
import App from "./App.jsx";

createRoot(document.getElementById("root")).render(
  <StrictMode>
    <App subject="Clarice" />
  </StrictMode>
);

React Components

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_components

Creating Components

A functional component is a JavaScript function that returns JSX:

function Todo(props) {
  return (
    <li className="todo stack-small">
      <div className="c-cb">
        <input id={props.id} type="checkbox" defaultChecked={props.completed} />
        <label className="todo-label" htmlFor={props.id}>
          {props.name}
        </label>
      </div>
      <div className="btn-group">
        <button type="button" className="btn">
          Edit <span className="visually-hidden">{props.name}</span>
        </button>
        <button type="button" className="btn btn__danger">
          Delete <span className="visually-hidden">{props.name}</span>
        </button>
      </div>
    </li>
  );
}

export default Todo;

Passing Props

<ul role="list" className="todo-list">
  <Todo name="Eat" id="todo-0" completed />
  <Todo name="Sleep" id="todo-1" />
  <Todo name="Repeat" id="todo-2" />
</ul>

Rendering Lists with map()

const DATA = [
  { id: "todo-0", name: "Eat", completed: true },
  { id: "todo-1", name: "Sleep", completed: false },
  { id: "todo-2", name: "Repeat", completed: false },
];

function App(props) {
  const taskList = props.tasks?.map((task) => (
    <Todo
      id={task.id}
      name={task.name}
      completed={task.completed}
      key={task.id}
    />
  ));

  return (
    <ul
      role="list"
      className="todo-list stack-large stack-exception"
      aria-labelledby="list-heading">
      {taskList}
    </ul>
  );
}

Unique Keys

Always provide a unique key prop to items rendered with iteration:

const taskList = props.tasks?.map((task) => (
  <Todo
    id={task.id}
    name={task.name}
    completed={task.completed}
    key={task.id}
  />
));

React uses keys to track which items have changed, been added, or removed.

Component Architecture Example

// src/components/Form.jsx
function Form() {
  return (
    <form>
      <h2 className="label-wrapper">
        <label htmlFor="new-todo-input" className="label__lg">
          What needs to be done?
        </label>
      </h2>
      <input
        type="text"
        id="new-todo-input"
        className="input input__lg"
        name="text"
        autoComplete="off"
      />
      <button type="submit" className="btn btn__primary btn__lg">
        Add
      </button>
    </form>
  );
}

export default Form;

// src/App.jsx
import Form from "./components/Form";
import FilterButton from "./components/FilterButton";
import Todo from "./components/Todo";

function App(props) {
  const taskList = props.tasks?.map((task) => (
    <Todo
      id={task.id}
      name={task.name}
      completed={task.completed}
      key={task.id}
    />
  ));

  return (
    <div className="todoapp stack-large">
      <h1>TodoMatic</h1>
      <Form />
      <div className="filters btn-group stack-exception">
        <FilterButton />
        <FilterButton />
        <FilterButton />
      </div>
      <h2 id="list-heading">3 tasks remaining</h2>
      <ul
        role="list"
        className="todo-list stack-large stack-exception"
        aria-labelledby="list-heading">
        {taskList}
      </ul>
    </div>
  );
}

export default App;

A First Splash into JavaScript

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting/A_first_splash

Thinking Like a Programmer

Programming requires breaking problems into actionable tasks, applying syntax to real-world problems, and understanding how features work together.

Number Guessing Game: Complete Example

let randomNumber = Math.floor(Math.random() * 100) + 1;

const guesses = document.querySelector(".guesses");
const lastResult = document.querySelector(".lastResult");
const lowOrHi = document.querySelector(".lowOrHi");
const guessSubmit = document.querySelector(".guessSubmit");
const guessField = document.querySelector(".guessField");

let guessCount = 1;
let resetButton;

function checkGuess() {
  const userGuess = Number(guessField.value);

  if (guessCount === 1) {
    guesses.textContent = "Previous guesses:";
  }
  guesses.textContent = `${guesses.textContent} ${userGuess}`;

  if (userGuess === randomNumber) {
    lastResult.textContent = "Congratulations! You got it right!";
    lastResult.style.backgroundColor = "green";
    lowOrHi.textContent = "";
    setGameOver();
  } else if (guessCount === 10) {
    lastResult.textContent = "!!!GAME OVER!!!";
    lowOrHi.textContent = "";
    setGameOver();
  } else {
    lastResult.textContent = "Wrong!";
    lastResult.style.backgroundColor = "red";
    if (userGuess < randomNumber) {
      lowOrHi.textContent = "Last guess was too low!";
    } else if (userGuess > randomNumber) {
      lowOrHi.textContent = "Last guess was too high!";
    }
  }

  guessCount++;
  guessField.value = "";
  guessField.focus();
}

guessSubmit.addEventListener("click", checkGuess);

function setGameOver() {
  guessField.disabled = true;
  guessSubmit.disabled = true;
  resetButton = document.createElement("button");
  resetButton.textContent = "Start new game";
  document.body.appendChild(resetButton);
  resetButton.addEventListener("click", resetGame);
}

function resetGame() {
  guessCount = 1;

  const resetParas = document.querySelectorAll(".resultParas p");
  for (const resetPara of resetParas) {
    resetPara.textContent = "";
  }

  resetButton.parentNode.removeChild(resetButton);

  guessField.disabled = false;
  guessSubmit.disabled = false;
  guessField.value = "";
  guessField.focus();

  lastResult.style.backgroundColor = "white";

  randomNumber = Math.floor(Math.random() * 100) + 1;
}

Key Techniques Demonstrated

Math.floor(Math.random() * 100) + 1 -- generate random integer
Number() constructor -- convert input to a number
document.querySelector() -- select DOM elements
.textContent -- set text in elements
.style.backgroundColor -- change element styling
.disabled -- disable/enable form elements
document.createElement() -- create new HTML elements
.appendChild() / .removeChild() -- add/remove elements from DOM
addEventListener() -- attach event listeners
.focus() -- return focus to input field

JavaScript Learning Module Overview

Source: https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Scripting

Topics Covered in the MDN Learning Path

Core Language Fundamentals:

What is JavaScript?
A first splash into JavaScript
Troubleshooting JavaScript
Variables
Numbers and operators
Strings
Useful string methods
Arrays

Control Flow and Functions: 9. Conditionals 10. Loops 11. Functions 12. Build your own function 13. Function return values

Events and DOM Manipulation: 14. Introduction to events 15. Event bubbling 16. Object basics 17. DOM scripting

APIs and Data: 18. Making network requests 19. Working with JSON

Error Handling: 20. JavaScript debugging and error handling

Practical Challenges

Silly story generator -- Variables, math, strings, arrays
Image gallery -- Loops, functions, conditionals, events
House data UI -- JSON fetching, filtering, rendering

JavaScript Built-in Objects Quick Reference

Source: https://www.w3schools.com/jsref/jsref_reference.asp (partial -- access was restricted)

Core Objects and Common Methods

Array Methods: concat(), every(), filter(), find(), findIndex(), forEach(), from(), includes(), indexOf(), isArray(), join(), keys(), lastIndexOf(), map(), of(), pop(), push(), reduce(), reduceRight(), reverse(), shift(), slice(), some(), sort(), splice(), toString(), unshift(), values()

String Methods: charAt(), charCodeAt(), concat(), endsWith(), fromCharCode(), includes(), indexOf(), lastIndexOf(), match(), matchAll(), padEnd(), padStart(), repeat(), replace(), replaceAll(), search(), slice(), split(), startsWith(), substring(), toLowerCase(), toUpperCase(), trim(), trimEnd(), trimStart()

Number Methods: isFinite(), isInteger(), isNaN(), isSafeInteger(), parseFloat(), parseInt(), toExponential(), toFixed(), toLocaleString(), toPrecision(), toString()

Math Methods: abs(), acos(), asin(), atan(), atan2(), cbrt(), ceil(), cos(), exp(), floor(), log(), max(), min(), pow(), random(), round(), sign(), sin(), sqrt(), tan(), trunc()

Date Methods: getDate(), getDay(), getFullYear(), getHours(), getMilliseconds(), getMinutes(), getMonth(), getSeconds(), getTime(), now(), parse(), setDate(), setFullYear(), setHours(), setMilliseconds(), setMinutes(), setMonth(), setSeconds(), toDateString(), toISOString(), toJSON(), toLocaleDateString(), toLocaleString(), toLocaleTimeString(), toString(), toTimeString(), toUTCString()

JSON Methods: JSON.parse(), JSON.stringify()

Global Functions: decodeURI(), decodeURIComponent(), encodeURI(), encodeURIComponent(), eval(), isFinite(), isNaN(), Number(), parseFloat(), parseInt(), String()

Promise Methods: Promise.all(), Promise.allSettled(), Promise.any(), Promise.race(), Promise.reject(), Promise.resolve(), .then(), .catch(), .finally()

This reference was compiled from the following sources:

59 KiB Raw Blame History

JavaScript Reference

Table of Contents

What is JavaScript?

The Three Layers of Web Technologies

What Can JavaScript Do?

Practical Example

Browser APIs

Third-Party APIs

How JavaScript Runs

Adding JavaScript to a Page

1. Internal JavaScript

2. External JavaScript (Recommended)

3. Inline JavaScript Handlers (Not Recommended)

Comparison

Script Loading Strategies

Place <script> at Bottom of Body

Use <script type="module"> in <head> (Recommended)

Use defer Attribute

Use async Attribute (for non-dependent scripts)

Wrap Internal Scripts in DOMContentLoaded

Comments

Single Line Comments

Multi-Line Comments

Best Practices

Variables

Declaring Variables

Initializing Variables

Variable Types

Variable Naming Rules

Dynamic Typing

Constants with const

let vs const vs var

Numbers and Math

Types of Numbers

Arithmetic Operators

Operator Precedence

Increment and Decrement Operators

Assignment Operators

Comparison Operators

Useful Number Methods

Strings

Creating Strings

Template Literals

String Concatenation

Escaping Characters

Useful String Methods

Finding String Length

Retrieving Characters

Testing for Substrings

Finding Position of a Substring

Extracting Substrings

Changing Case

Replacing Parts of a String

Arrays

Creating Arrays

Finding Array Length

Accessing and Modifying Items

Finding Index of Items

Adding Items

Removing Items

Iterating Over Arrays

Converting Between Strings and Arrays

Conditionals

if...else Statements

else if Statements

Logical Operators

Switch Statements

Ternary Operator

Loops

for...of Loop

Standard for Loop

Looping Through Arrays with for

while Loop

do...while Loop

break and continue

Which Loop Type to Use?

Functions

What Are Functions?

Built-in Browser Functions

59 KiB

Raw Blame History

Place `<script>` at Bottom of Body

Use `<script type="module">` in `<head>` (Recommended)

Use `defer` Attribute

Use `async` Attribute (for non-dependent scripts)

Wrap Internal Scripts in `DOMContentLoaded`

Constants with `const`

`let` vs `const` vs `var`