Function Declaration

[00:00] Okay. Let's make this function declaration. So we're gonna make function called get message and return our value. And yep, AI knows exactly what to do here. So what is this?

[00:12] What are we looking at? This is called a function declaration. Starts with the keyword function. You give it a space to separate function from the name of the function. And then you give these parenthesis.

[00:22] We'll talk about what those parenthesis can be used for later, what can go in between them. And then we have this return value that we're specifying. So it's a colon and then the the type of value that we are returning. And then, we have our block. This is what we actually call a closure in JavaScript.

[00:39] So variables defined in here, will exist as long as, this function exists. And it's a pretty interesting, thing to consider with garbage collection, all this stuff that we're gonna get into later. But inside of this closure, we can do whatever we want to. But at the very end, we're gonna have a return statement, that here we're saying hello functions and that has to match our return value. So if I save this, we're gonna get our log because of this log and then we'll see our export, Just a function called get message.

[01:09] And, if we were to try to return number here, we're gonna get a type error because that doesn't match our return value. If I change this to number, then now we're gonna get that return value, in return. And, yeah. That is, the very basics of our function. We've we've got lots more to talk about with functions.

[01:30] They're really powerful, primitive of, JavaScript or or really any language being able to reuse this stuff and and everything. So, this is very important part of learning how to develop software. And this is that that piece. The cool thing about this too is we can, do this as many times as we want to and now we get hello functions. And then we can say, you know what?

[01:53] I wanna be really excited about this. And so we just change it there once and we get it all, applied everywhere else that we're calling the function. That might look a little bit like a loop and, yeah, you could definitely use this in a loop. But the the point is that you don't have to use it in a loop. You can, call it the function from wherever, that function is accessible.

[02:14] And I think that's pretty cool. So there you go. That is your first introduction to functions. Hello, functions.

[00:00] Great. We're gonna get into functions now. This is where you can take a so like so far we've checked out programs and we can skip lines and stuff. Now you can take a piece of your program and put it somewhere and then have other parts of your program go over there and just like, go do that, go do that, go that, give me that, you know, here's do this, whatever. So that's the idea of functions.

[00:21] We're gonna start with a really basic one where you're just getting a value, and then we'll log that to the console. And we'll also export that function so that the test can use it. So I want you to jump into this one.

Learning to program can feel overwhelming. There are countless tutorials, boot camps, and YouTube videos all pulling you in different directions, and most of them assume you already know something.

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## Expressions & Output

Every program needs a way to tell you what it's doing. Without output, you're flying blind-- writing code with no way to verify it works. This is the very first skill you need as a developer.

In this section, you'll get your programs talking back to you while learning the building blocks of every piece of code you'll ever write:

- Outputting values with `console.log`
- String expressions and concatenation
- Number expressions and arithmetic operators
- Escaping special characters in strings
- Template literals with interpolation and multiline support

## Variables

Hardcoded values can only get you so far. Real programs need to store, reference, and update data as they run. Without a solid understanding of how variables work, you'll constantly be confused by unexpected behavior.

Variables are foundational to every program you'll ever write. Here you'll learn how to declare them, when to use each type, and a critical nuance that trips up even experienced developers:

- Declaring variables with `let` and `const`
- Naming conventions and casing (camelCase, SCREAMING_SNAKE_CASE)
- Reassigning values with `let` and compound operators like `+=`
- The important distinction between reassignment and mutation
- Why `const` doesn't always mean the value can't change

## Primitive Types

JavaScript has types whether you think about them or not. Every value in every variable carries a type, and ignoring that fact leads to bugs that are hard to track down. TypeScript makes these types explicit, catching entire categories of errors before your code ever runs.

This section introduces you to all seven primitive types and how TypeScript helps you work with them safely:

- Explicit type annotations with `number`, `string`, and `boolean`
- The difference between `null` and `undefined`
- Special types: `bigint` for arbitrarily large numbers and `symbol` for unique identifiers
- Type coercion and why it can surprise you
- Floating-point precision and practical considerations for working with numbers

## Comparisons & Boolean Logic

Making decisions is at the heart of every program. Should this button be enabled? Does this user have access? Is the cart empty? All of these questions boil down to comparisons and boolean logic.

Getting comparisons wrong leads to subtle bugs that are painful to debug. This section gives you the tools to write conditions that behave exactly as you'd expect:

- Loose equality (`==`) vs. strict equality (`===`) and why you should almost always use strict
- Not-equal, greater-than, and less-than-or-equal-to operators
- Combining conditions with logical AND (`&&`), OR (`||`), and NOT (`!`)
- Order of operations with logical operators
- Using parentheses to make complex conditions explicit

## Truthiness & Falsiness

JavaScript has a unique feature where every value has an inherent "truthiness" or "falsiness." This isn't a bug-- it's one of the most practical features of the language once you understand it. But misunderstanding it leads to some of the most confusing behavior new developers encounter.

Here you'll learn which values are truthy, which are falsy, and how to use this knowledge to write cleaner conditional logic:

- Coercing values to booleans with the `Boolean` constructor
- The double-not (`!!`) operator and when to use it
- Falsy values: `0`, `""`, `null`, `undefined`, `false`, and `NaN`
- Practical patterns like checking array length and optional values
- Avoiding common pitfalls with logical operators and coercion

## Functions

Up to this point, your programs run top to bottom and that's it. Functions change everything. They let you take a chunk of code, put it somewhere reusable, and call it from anywhere-- with different inputs each time.

Functions are the single most important building block for organizing and scaling your programs. This section takes you from writing your first function through passing functions as arguments to other functions:

- Function declarations with the `function` keyword
- Accepting parameters and specifying their types
- Return values and TypeScript's return type inference
- Arrow functions and implicit returns
- Passing functions as arguments to other functions
- Documenting your functions with JSDoc

## Type Inference & Arrow Functions

TypeScript is smart. You don't have to type everything explicitly-- TypeScript can often figure out the types for you by looking at your code. Knowing when to lean on inference and when to be explicit is a skill that makes your code cleaner and your development faster.

This section also covers arrow functions, a concise alternative syntax you'll see everywhere in modern JavaScript and TypeScript:

- How TypeScript infers return types from your function body
- When explicit return types help catch bugs earlier
- Converting function declarations to arrow functions
- Implicit returns for single-expression functions
- The hoisting difference between declarations and expressions
- Function expressions as first-class values

## Void & Never

Not every function returns something useful, and some code paths should never be reached. TypeScript has dedicated types for both of these situations, and understanding them helps you write more precise, self-documenting code.

This section introduces two TypeScript-specific concepts that complete your understanding of the type system's fundamentals:

- The `void` type for functions that don't return a value
- How `void` relates to `undefined` in practice
- The `never` type for functions that throw errors or never return
- Using `never` to enforce exhaustive checks
- Throwing errors to halt execution and signal failures
- Practical patterns for input validation with error throwing

Programming Foundations

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Function Declaration

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