MEAN TypeScript Fundamentals for Angular

Question 1

In TypeScript, what is the difference between an `interface` and a `type` alias when defining the shape of an object?

Accepted Answer

Interfaces can be extended and merged via declaration merging; type aliases cannot be re-opened

Answer

A key difference is declaration merging: you can declare an interface with the same name multiple times and TypeScript merges them. Type aliases are closed — redeclaring the same name is an error. Both are erased at compile time and produce no runtime code.

Question 2

What does the TypeScript `readonly` modifier do when applied to a class property?

Accepted Answer

It prevents the property from being reassigned after the constructor runs

Answer

readonly means the property can only be assigned during declaration or in the constructor. Any attempt to reassign it afterwards is a TypeScript compile-time error. It does not affect visibility (that is controlled by private/public/protected).

Question 3

Which TypeScript utility type creates a new type with all properties of `T` set to optional?

Accepted Answer

Partial<T>

Answer

Partial transforms every property of type T into an optional property (adds ?). This is commonly used for update DTOs or patch payloads where only some fields may be provided. Required does the opposite, making all properties mandatory.

Question 4

In Angular, the `@Injectable({ providedIn: 'root' })` decorator uses TypeScript decorators. What does this specific metadata tell Angular?

Accepted Answer

Create a single shared instance of this service in the root injector, available application-wide

Answer

providedIn: 'root' registers the service with Angular's root injector as a singleton, making it available throughout the entire application without needing to list it in any NgModule's providers array. Angular also tree-shakes it if unused.

Question 5

What does the TypeScript `!` non-null assertion operator do?

Accepted Answer

It tells the TypeScript compiler to treat the value as non-null and non-undefined, bypassing the null check

Answer

The ! postfix operator is a compile-time hint to TypeScript that you are certain a value is not null or undefined. It suppresses the type error without adding any runtime check. Misusing it can lead to runtime errors if the value actually is null.

MEAN - Stack Development Practice Test

MEAN - Stack Development Practice Test

MEAN TypeScript Fundamentals for Angular