Top mistakes of beginner TypeScript developers: from A to Z

Top mistakes of beginner TypeScript developers: from A to Z

TypeScript is a powerful tool that helps you write more reliable and predictable code. However, at the start, many developers (especially those coming from JavaScript) make typical mistakes. In this article, we will analyze the 10 most common problems that beginners face and show how to avoid them. Let's dive into the world of TypeScript from A to Z.

1. Overusing any

This is perhaps the most popular mistake. When a developer doesn't know what data type to expect, they write any. This kills the whole point of TypeScript — static typing. You lose editor hints and protection from errors during compilation.

Bad example:

function processData(data: any) {\n    return data.name.toLowerCase(); // Error will only appear at runtime\n}\n

How to fix: Use more specific types, unknown, or generics.

function processData(data: { name: string }) {\n    return data.name.toLowerCase(); // TypeScript will check this at compile time\n}\n

If the type is truly unknown, use unknown — it will force you to check the data before using it.

2. Ignoring strict mode

Many beginners disable strict: true in tsconfig.json to "avoid unnecessary errors." This is a big mistake. Without strict mode, you lose checks for null and undefined, which leads to Cannot read property of undefined in production.

// Without strict: true — no error\nconst name: string = null; // Works, but will crash later\n\n// With strict: true — compilation error\nconst name: string = null; // ❌ Type 'null' is not assignable to type 'string'\n

Tip: Always enable strict: true. This will save you from hundreds of bugs.

3. Incorrect use of types and interfaces

Beginners often confuse type and interface, and also use them where it's not needed. For example, they create an interface for a single function, when they could simply describe the type directly in the parameter.

Bad example:

interface UserData {\n    name: string;\n    age: number;\n}\nfunction greet(user: UserData) {\n    console.log(`Hello, ${user.name}`);\n}\n

This is redundant if the interface is only used once.

Better approach:

function greet(user: { name: string; age: number }) {\n    console.log(`Hello, ${user.name}`);\n}\n

Use interface for objects that are extended or implemented by classes, and type for unions and intersections.

4. Forgetting about Union Types

One of the most powerful features of TypeScript is union types. Beginners often write redundant checks or use any when they could simply specify multiple valid types.

// Bad\nfunction getLength(value: any) {\n    return value.length;\n}\n\n// Good\nfunction getLength(value: string | any[]) {\n    return value.length;\n}\n

TypeScript itself will suggest which methods are available for each type in the union.

5. Mistakes with null and undefined

TypeScript is strict about null and undefined, but beginners often forget to check for them. This leads to runtime errors.

function printName(name: string | null) {\n    console.log(name.toUpperCase()); // ❌ Object is possibly 'null'\n}\n

How to fix: Use type guards:

function printName(name: stri