Top Mistakes of Beginner TypeScript Developers

Top mistakes of beginner TypeScript developers

TypeScript is a powerful tool that helps write more reliable and maintainable code. However, like any language, it hides pitfalls, especially for beginners. Many developers come from JavaScript and continue using old habits, negating the benefits of static typing. In this article, we'll break down the 4 most common mistakes of beginner TypeScript developers, show how to fix them, and give practical advice. If you're just starting your journey with TypeScript, this article will help you avoid typical pitfalls.

1. Overusing the any type

The most common mistake is using any everywhere TypeScript can't infer the type automatically. Beginners often write like this:

function processData(data: any) {    console.log(data.name.toUpperCase());}

This completely disables type checking for the data variable. You lose all the benefits of TypeScript: autocompletion, error checking, and code documentation.

What to do instead?

Always try to explicitly describe the type, even if it's complex. Use unknown if the type is truly unknown at the time of writing the code:

function processData(data: unknown) {    if (typeof data === 'object' && data !== null && 'name' in data) {        console.log((data as { name: string }).name.toUpperCase());    }}

Or, even better, describe an interface:

interface DataItem {    name: string;    age?: number;}

function processData(data: DataItem) {    console.log(data.name.toUpperCase());}

Tip: Set noImplicitAny: true in tsconfig.json. The compiler will complain about any implicit any, and you'll get used to writing types.

2. Ignoring strictNullChecks

In JavaScript, null and undefined are normal values. In TypeScript without strict checks, you might accidentally access a property of null and get a runtime error. A typical mistake:

function getLength(str: string | null) {    return str.length; // Error: Object is possibly 'null'}

If strictNullChecks is disabled, TypeScript won't warn you, and the error will only appear at runtime.

Solution: Enable strict: true in tsconfig.json. This automatically activates strictNullChecks. Now TypeScript will force you to handle null and undefined:

function getLength(str: string | null): number {    if (str === null) {        return 0;    }    return str.length;}

// Or use optional chaining and nullish coalescing:function getLengthSafe(str: string | null): number {    return str?.length ?? 0;}

Important: Never disable strictNullChecks. This is one of the main features of TypeScript that prevents millions of bugs.

3. Incorrect work with async/await and promises

TypeScript strictly types async functions, but beginners often forget about the Promise return type. Example of a mistake:

async function fetchUser(id: number) {    const response = await fetch(`/api/users/${id}`);    return response.json(); // TypeScript might not know the exact type}

Here, response.json() returns Promise<any>, and you lose typing again. A second common problem is forgetting to handle errors:

async function loadData() {    const data = await fetchUser(1); // If fetch fails, the error is unhandled    console.log(data.name);}

How to do it correctly?

Always explicitly specify