When TypeScript shows the error “Property does not exist on type…”, it usually appears in the middle of a refactor or when you are consuming an API, and can be confusing because the property clearly exists at runtime. This error is TypeScript’s way of telling you that the type you are working with does not … Read more >>
When you “check the type” of a variable in TypeScript, you’re really checking the runtime value and letting the compiler narrow the static type for you. In this tutorial, I’ll walk through the practical ways to do that using typeof, instanceof, and type guards, with examples you can drop into real projects. How to check the type … Read more >>
If you’ve ever tried to write this in TypeScript: interface User { firstName: string; lastName: string; age: number = 30; // ❌} …you’ve seen the error and thought, “Wait, how do I give this a default?” Interfaces only describe shape; they don’t exist at runtime, so they can’t hold default values. But in real projects, … Read more >>
In TypeScript, “TypeError: is not a function” is a runtime JavaScript error that appears when you try to call something with () that is not actually a function. The value might be a string, an object, undefined, null, or some other type that cannot be called. Most of the time, this happens because the variable you are calling does … Read more >>
TypeScript interfaces can absolutely have functions, and there are several useful patterns beyond the basic “add a method and implement it in a class.” In this tutorial, I’ll walk you through those patterns in a practical, no-nonsense way. Why put functions in a TypeScript interface? When I say “functions in an interface,” I’m really talking … Read more >>
If you’ve ever copied and pasted the same function logic three times just to handle string, number, and boolean separately, TypeScript generic anonymous functions are the fix. They let you write one flexible, type-safe function expression that adapts to any data type, without losing IntelliSense or compile-time safety. In this tutorial, I’ll walk you through exactly how TypeScript generic … Read more >>
TypeScript is very good at guessing (inferring) the return type of your arrow functions, but in real projects, “good enough” inference can still hide bugs. In this tutorial, I’ll walk through how to explicitly specify the return type for arrow functions in TypeScript, and more importantly, when it actually matters. We’ll start with the basics, then move … Read more >>
When I write TypeScript functions, I often want some arguments to be optional without creating multiple overloads or duplicating logic. Optional parameters are exactly for that. They let me keep function signatures flexible, while TypeScript still checks types for me. In this tutorial, I’ll walk through how optional parameters in TypeScript work, the rules you … Read more >>
During a recent TypeScript workshop, a developer asked a question I hear all the time: “Should I use type or interface here?” If you read the docs or AI-generated summaries, you’ll often see “they are mostly the same.” That’s true for simple cases, but it’s not the full story in real-world projects. In this guide, you’ll learn the practical … Read more >>
TypeScript functions can technically return only one value, but in real-world projects, we often need to return several pieces of data at once, such as the result of an operation, an error message, and some metadata. There are multiple ways to model this in TypeScript, and each option comes with different trade-offs for readability, type … Read more >>
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