Type Assertions vs Type Declarations

Different ways to assign types.

What You'll Learn

Type annotation (declaration)
Type assertion
Key differences

Type Annotation

Declares what type a variable should be.

code.tsTypeScript

// TypeScript validates the value
const user: { name: string } = {
  name: "John"
};

// Error: missing property
const user2: { name: string; age: number } = {
  name: "John"
};

Type Assertion

Tells TypeScript to trust you about the type.

code.tsTypeScript

// You're telling TS "trust me"
const user = {} as { name: string };
// No error, but user.name is undefined!

// Common with DOM
const input = document.getElementById("email") as HTMLInputElement;

Key Differences

Declaration	Assertion
TypeScript validates	You assert
Compile-time check	Trust developer
Catches errors	May miss errors

Best Practice

code.tsTypeScript

// Prefer declarations
const user: User = { name: "John", age: 30 };

// Use assertions sparingly
const data = parseJSON(str) as Config;

// Type guard is often better
if (isConfig(data)) {
  // data is Config
}

What is the difference between Namespace and Module?

What are the important tsconfig.json options?