A self-taught developer's journal on modern JavaScript fundamentals

Prerequestes

Create an account on freecodecamp.org - https://www.freecodecamp.org/jefferyjjensen

BYU-Idaho Academic Catalog Dept of Computer Science and Engineering Degrees and Certificates - Web and Computer Programming (Certificate #1) and Full Stack Web Development (Certificate #2). Then, you can earn an Associate's in Software Development from BYU Pathway Worldwide.

YouTuber Catherine Li - How to Learn JavaScript FAST in 2026

install vs code

To start a JavaScript (Node.js) command-line interpreter (REPL) in VS Code, you just use VS Code's integrated terminal and run Node directly. VS Code itself doesn't add a special Node console - the REPL comes from Node.js

PS C:\bb\Barefoot Betters\BB - Documents\barefootbetters.com\blog> node -v
v24.13.1
PS C:\bb\Barefoot Betters\BB - Documents\barefootbetters.com\blog> node
Welcome to Node.js v24.13.1.
Type ".help" for more information.
> let developer = 'Jeff';
undefined
> console.log(developer);
Jeff
undefined
> developer = 'Tom';
'Tom'
> console.log(developer);
Tom
undefined
>
(To exit, press Ctrl+C again or Ctrl+D or type .exit)
>
PS C:\bb\Barefoot Betters\BB - Documents\barefootbetters.com\blog> 

Variables

Old JavaScript uses var and JavaScript ES6 (2015) uses let and const. So by default use const and use let when reassignment is needed.

// example ways to declare variables
const smsSendingLimit = 1234;     // number, const is short for constant, value never changes
const MAXAGE = 100;               // custom to declare constants all uppercase
let hasPermission = true;         // boolean
var username = 'BarefootBetters'; // use let instead of var JavaScript ES6 (2015) 
let nothing;                      // undefined - absence of a value
const nothingExplicit = null;     // object instead of null (quirk)

// output variables
console.log("smsSendingLimit is a " + typeof smsSendingLimit);
console.log("hasPermission is a " + typeof hasPermission);
console.log("username is a " + typeof username);
console.log("nothing is " + typeof nothing); // undefined
console.log(`nothingExplicit is a ${typeof nothingExplicit}`); // "object" (quirk)

Also, let is block-scoped, whereas var is global or function scope.

Primitive data types are numbers, strings, booleans, null, and undefined.

Think of strings as values, not containers. Variables can point to different strings. The string never changes (immutable) but the variable does. "Changing" a string always means creating a new one, that is the variable will reference a new string. Once a string is created, you cannot change its characters directly.

String Concatenation

String concatenation is the joining of pieces of text together using the + operator, += operator, or the String object's concat() method. Note, concat() is not a method of the variable itself. Note, concat() always returns a new string and does not modify the original. So, when you write:

let a = 'Hello';
let b = a.concat(' World'); // b = 'Hello World'
a.concat(' World'); // a = 'Hello' nothing changed because you must assign the result

What’s happening is:

1. b receives the new string
2. 'Hello' is a string primitive
3. JavaScript temporarily wraps it in a String object
4. The method String.prototype.concat() is invoked
5. A new string is returned (strings are immutable)

Template Literals (backticks)

Backticks are used for String interpolation and multiline strings

const name = 'Jeff';
const greeting = `Hello, ${name}`;

Function

A function is a reusable block of code that performs a specific task and can be called with various inputs.

Method

A method is a type of function that is associated with an object, meaning it operates on the data contained within that object. Think of the concat() method.

let str1 = 'Hello';
let str2 = 'World';

let result = str1.concat(' ', str2);
console.log(result); // Hello World