Never Type

[00:00] Okay. So never types, typically show up when we're throwing errors. You can also use them for some pretty advanced typescripty things. But, yeah. Pretty much when we're, throwing errors and stuff.

[00:12] So we're gonna create a throw error function, and Marty the money bag gave this to us because, it would just be a waste of time to have you type it all out. I I mean, I guess you could type it all out. It's good reps and stuff. But, yeah. Here's what a function that is typed to return never looks like.

[00:30] So it's literally just a function that throws an error and that's that's it. So the reason that it returns never is because it just never returns. There's nothing that it there's no situation where, calling throw error, would result in a value because before it gets to here, the error has been thrown. So it's gotta evaluate this side first to determine what this value variable should, be assigned to. But because we're throwing an error, once it hits this line of code, it stops execution.

[01:02] It's done. That's what throwing errors does is it's just we're done executing. We're not going to evaluate this side of the equal sign because we stop execution once we hit a throw. Now, we can do try catches and stuff we'll talk about later, but, yeah. Throw, throwing like this, just stops all execution and that's why value is never And the value of this, is that, if I tried to use it, like value, yeah, too fixed or whatever, I'm gonna get a typing error here because it's saying this could never be.

[01:33] It's impossible for you to actually have a value in the value variable. So that is, that's basically never. And here, would you look at that, TypeScript is, telling us void. So it is sometimes useful to explicitly type your, functions as having returned never. Now in our case, it's it's fine because, we're going to end up with, too fixed here anyway.

[01:58] If I try to return, hi or whatever, that is going to be all grayed out and our return value is still actually going to be a string. So you might argue that, TypeScript could be improved here because it should know, like, you're throwing an error. You're never gonna get to that return value, so that return value should be string. So this is where, like, explicitly typing it as never can be useful. But, yeah.

[02:22] I just I don't really run into that, all that often. Sometimes, you could say or string right here and then, you can return a string. You still get the grayed out thing right here in your editor. You typically wouldn't do this. You'd say, if this, then throw an error.

[02:38] Otherwise, do this thing. And in that case, it, will the types will flow a lot better. But, anyway, that is, the, never type. Let's continue from here just to get a couple more reps and and look at a couple other things. So here, in this case, we've got our parse number.

[02:58] We have our value string. We're gonna return a number, and we're going to determine if the number is not a number. So if this, value because not all strings can be turned into numbers. Like if I were to put, whatever, that's not a number. Right?

[03:14] So we're past the value into the number, constructor, here and that gives us the number primitive. And then if that if the number constructor here says that this is not actually a number, then we're gonna throw an error. Hey. The value you gave me is not a valid number. Otherwise, we'll return the result.

[03:35] And here we're typing it as a number return, so that's what we're gonna get back. So just interesting, even though you're throwing doesn't necessarily mean that your return value is going to be, never. K. Great. You and so you could actually end up in a situation where you say const, one equals parse number one.

[03:56] And then you say one to fixed and TypeScript is totally fine with that. But the reason that that's fine in this case is because, once let's, yeah, TypeScript doesn't know that the string one is not going to evaluate to a number. But it does know that if it doesn't, it's gonna be throwing this error, here. So we're not even gonna get to, continue the rest of the program if, this doesn't evaluate to be a number anyway. And so we don't need to worry about this because this does actually also return a number and in that case, one will be a number and we'll be happy.

[04:34] So this is a happy case and the rest of the program will evaluate correctly. This is the sad case, but but the rest of the program won't even evaluate, correctly anyway, so we don't need to worry about checking the rest of this. It can just be a number. Okay. Great.

[04:49] And then we have our ensure positive function. So let's, just some more reps with this. Same sort of idea there. Let's add our, test here, right there. And we're gonna get invalid numbers, so let's fix this one.

[05:05] Number must be positive. Let's make that. And there we go. Tada. Everything is working as expected.

[05:12] Okay. So hopefully, now you have a little bit of an understanding of the never type if you run into it in, the code that your AI is generating or that you need to write yourself, or that you see in open source and elsewhere. That is what never is. It's just a situation that could never possibly happen. Good job.

[00:00] Now we're gonna talk about never. We're never gonna talk about oh, I sorry. I tried to come up with a funny pun. I'm a dad, but I failed. We're gonna understand the never type.

[00:10] This is a special kind of type that says the effectively, this should never happen and then TypeScript can enforce that that thing never happens in your code. So in this example, we're gonna give you a couple of situations that should never be and TypeScript enforcing that to help you understand the never type. This isn't something that you'd use a ton. I I don't use it on a day to day basis myself, at least not explicitly. But but yeah, you will run into it and hopefully, it makes sense, when we're talking about errors and stuff that we're gonna be throwing around.

[00:43] So have a good time with this one and we'll see you when you're finished with that and we can go through it together.

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.

The result? Gaps in your understanding that haunt you for years. You memorize syntax without understanding what it means. You copy-paste code that works but can't explain why. When something breaks, you're stuck because you never built the mental model to reason about what's happening underneath.

The Programming Foundations Workshop is your ground-zero start to becoming a software developer. Built with TypeScript from the very first line, it takes you from zero prior programming experience through expressions, variables, types, functions, and beyond-- equipping you with the foundational knowledge that every concept in web development builds on.

No prior programming knowledge is required. If you can open an editor and run the workshop app, you're ready. And because this is how software is built today, you'll learn alongside AI-assisted tooling from the start-- building the skills to write, review, and understand code in a modern development workflow.

## 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

Structured Data

Type Safety

Object Oriented TypeScript

Advanced TypeScript

Practical TypeScript: Foundations to Fluency

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Never Type

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