The Lies We Tell Ourselves Using TypeScript

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How safe is TypeScript's type safety? How much can you trust your statically typed code? Can you even consider TypeScript's type system "strong"? In this talk, we look at situations where TypeScript fails badly and learn why things have to be that way. We talk about trade-offs, workarounds, and ultimately solutions for all the damn, terrible lies we tell ourselves when using TypeScript.

 Stefan Baumgartner
Stefan Baumgartner
28 min
21 Sep, 2023

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Video Summary and Transcription

The Talk discusses the limitations of TypeScript and the lies we tell ourselves about its type safety. It explores examples of unsafe operations and bending the type system using keywords like type assertion and function is dice. The Talk also covers catching syntax errors, type mismatches, and the complexity of conditional types and generics. It emphasizes the importance of understanding TypeScript's limitations and making well-informed decisions as software engineers.

1. Introduction to TypeScript and Its Limitations

Short description:

Welcome to my talk, Lies We Tell Ourselves Using TypeScript. It has been a fantastic TypeScript congress so far. Today, I talk about an example that TypeScript is totally happy with. We're just fetching some data from an API and concatenating it with something else. TypeScript says it's good code, but it will most likely crash at runtime. Let's go through the code step by step.

Hi, welcome to my talk, Lies We Tell Ourselves Using TypeScript. It has been a fantastic TypeScript congress so far. You have seen lots of fantastic speakers, people that I wanted to meet for such a long time, people that I've engaged with on social media, and who are the best of the best in TypeScript.

Now, I need to tell you that I'm sorry. I'm sorry because you had fantastic talks from fantastic speakers. Now, I'm going to crush your dreams and destroy your hopes because this is not going to be a fun talk. I've done a fair share of TypeScript development. I've written two books about it, one which is TypeScript, the good parts. This is how the type system works for types for JavaScript developers. The other one TypeScript Couple where they have 100 actual problems that we are going to solve together.

And throughout my journey, I always start with this example where TypeScript is absolutely perfect for. I have this JavaScript codes. Don't worry about what it does, but it's just JavaScript. The browser executes it. It just doesn't work. There's no error that's being flown. Nothing tells you what's going wrong. It runs, it doesn't throw any errors, but it does not produce the right results. And for examples like that, TypeScript is fantastic because TypeScript, just when I activate it, finds about, I don't know, 10 errors in 15 lines of code. That's what we're here for. That's TypeScript's purpose. But this is not what I'm talking about today.

Today, I talk about the total opposite. I'm going to show you an example that TypeScript is totally happy with. We're just fetching some data from some API and concatenating it with something else. And TypeScript says, well, the types check out. This is good code. It compiles, you can ship it, and it will most likely crash at runtime. You know, at runtime, those things that TypeScript is supposed to prevent. Let's go through the code step by step.

2. Fetching Data and Handling Errors

Short description:

The first function fetches data from the Star Wars API using function overloads. We unwrap the fetched data and handle any potential syntax errors. Then we concatenate arrays of people or species using the push method, and finally, we create a new people array and append species to it. This program has about 10 lines of code and we will now address the problems step-by-step, starting with the core of our structure.

The first function fetches data. We call it list entries, and you have two ... you have two function overloads that define an API for you and your users. If you get a kind of species, then you're getting a promise of species in return. If you're getting a kind of people, then you're getting a promise of people in return. We are accessing the Star Wars API, which is great if you want to do any rest tests. And you have a very tailor made API so that you actually know what you should get back. Then you have the third function overload, which is the actual function implementation for you.

Next, we are fetching data and since it has some meta information, we are unwrapping it. We are just interested in the results. One thing that is very interesting about this line of code is that we are doing some, sorry, that we are doing some error handling. So there's one part where we say, well, that result.JSON call on line three might go wrong. So let's better catch that syntax error. What if we don't get any JSON back? Fantastic. Five lines of code for that function, three lines of type information.

Next is one line. I just formatted a little bit for this slide. Then we are concatenating a promise of an array of people or species to another list of array of people or species. So we're concatenating those two with the part push method of an array. And finally, we are calling so we are creating a new people array and we are pending species to the people array. All right. This is our program. It's about, I don't know, 10 real lines of code, a couple of type information. And I want to ask you if you look at the entire program, do you know what's going wrong or do you know what's supposed to go wrong? So, well, I think everything. And where does it go wrong? Everywhere. And when does it go wrong? Yeah, one by one, because we're single threaded. But don't worry, we are going through it step-by-step. We are now going to look at all our problems. Problem number one in chapter one, a tail of fetch. Let's look at the core of our very first structure.

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