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Alexander Opalic
Alexander Opalic

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The Problem with "as" in TypeScript: Why It's a Shortcut We Should Avoid

Introduction: Understanding TypeScript and Its Challenges

TypeScript is like a helper for JavaScript, a well-known programming language. JavaScript is flexible but sometimes too loose with its rules, which can lead to mistakes. TypeScript adds stricter rules to help avoid these mistakes, but it's not perfect. It's like adding new parts to an old car to make it safer, but it's still not as good as a new car designed with safety in mind, like the programming language Haskell.

One part of TypeScript, the as keyword, is especially tricky. It's supposed to help with the new rules, but sometimes it can skip them. When I see as used a lot in TypeScript code, it makes me think there might be problems. It's like a sign that the code could be better. In this article, I'll talk about why as can be a problem and what we should think about when we see it in TypeScript.

Easy Introduction to TypeScript's as Keyword

TypeScript is a special version of JavaScript. It adds rules to make coding less error-prone and clearer. But there's a part of TypeScript, called the as keyword, that's tricky. In this article, I'll talk about why as can be a problem.

What is as in TypeScript?

as in TypeScript lets you change the type of a piece of data. For example:

let unknownInput: unknown = "Hello, TypeScript!";
let asString: string = unknownInput as string;
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Here, we're saying unknownInput is a string, even though TypeScript isn't sure. This seems helpful, but it can actually cause trouble.

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The Problem with as

The best thing about TypeScript is that it finds mistakes in your code before you even run it. But when you use as, you can skip these checks. It's like telling the computer, "I'm sure this is right," even if we might be wrong.

Using as too much is risky. It can cause errors in parts of your code where TypeScript could have helped. Imagine driving with a blindfold; that's kind of what it's like.

Why Using as Can Be Bad

  • Skipping Checks: TypeScript is great because it checks your code. Using as means you skip these helpful checks.
  • Making Code Unclear: When you use as, it can make your code hard to understand. Others (or even you later) might not know why you used as.
  • Errors Might Happen: If you use as wrong, your program can crash unexpectedly.

Better Ways Than as

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  • Type Guards: TypeScript has type guards. They help you check types safely.

    if (typeof unknownInput === "string") {
        // TypeScript is sure it's a string
  • Better Type Definitions: Sometimes, as is used because types aren't defined well. Making your type definitions better can help.

  • Your Own Type Guards: For complicated types, you can make your own checks.

    function isMyType(obj: any): obj is MyType {
        return obj && typeof obj.someProperty === "string";

Cases Where Using as is Okay

While it's important to be cautious with the as keyword, there are situations where its use is appropriate and even necessary:

  1. Integrating with Non-Typed Code: When working with JavaScript libraries or external APIs that aren't typed, as can be useful to assign a type to data coming from these sources. However, I personally believe that even in these situations, it's preferable to use type guards. Type guards offer a more robust and safer approach to ensuring data types, aligning better with TypeScript's goal of enhancing code reliability and maintainability.

  2. Casting in Tests: In unit tests, especially when mocking or setting up test data, as can be a practical way to quickly shape data into the required form.

Remember, even in these situations, it's crucial to ensure that the use of as is truly necessary and not a substitute for proper type handling.

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as can be handy in TypeScript, but be careful. It's usually better to fix type problems in other ways. By doing this, we keep our code clear and reduce mistakes. Let's use all the good parts of TypeScript without taking shortcuts.

Top comments (5)

_ndeyefatoudiop profile image
Ndeye Fatou Diop

Great post ! Totally agree with it. What is also very sad with as is that some of the assumptions in the code may change and now typescript won’t flag that to you

alexanderop profile image
Alexander Opalic

Good Point


pavelee profile image
Paweł Ciosek

Great post! 👏👏

alexanderop profile image
Alexander Opalic

Ty 🫡

610470416 profile image

In fact, we should avoid using typescript eventually.

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