Generic interface declaration in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a generic interface in TypeScript?

A generic interface is a way to define an interface with a placeholder type that can be specified later, allowing the interface to work with different data types while keeping type safety.

beginner

How do you declare a generic interface with one type parameter named <T>?

You write: <br>interface MyInterface<T> {<br> value: T;<br>}<br>This means the interface has a property 'value' of type T, which will be specified when used.

beginner

Why use generic interfaces instead of fixed types?

Generic interfaces let you write flexible and reusable code that works with many types without losing the benefits of type checking.

beginner

Example: What does this interface mean?<br>interface Box<T> { content: T; }

It means 'Box' can hold any type of content. When you use it, you specify the type, like Box<string> for a box holding strings or Box<number> for numbers.

intermediate

Can a generic interface have multiple type parameters? How?

Yes. You separate them with commas inside the angle brackets. For example:<br>interface Pair<T, U> { first: T; second: U; }

How do you declare a generic interface named 'Container' with one type parameter 'T'?

A interface Container { item: T; }

B interface Container<T> { item: any; }

C interface Container<T> { item: T; }

D interface Container<T> { item: string; }

Option C correctly declares a generic interface with a type parameter T used for the item property.

What is the benefit of using a generic interface?

A It allows the interface to work with any type while keeping type safety.

B It makes the interface only work with strings.

C It disables type checking.

D It makes the interface slower.

Generic interfaces provide flexibility and type safety by allowing any type to be specified.

Which syntax correctly declares a generic interface with two type parameters?

A interface Pair<T; U> { first: T; second: U; }

B interface Pair<T, U> { first: T; second: U; }

C interface Pair<T U> { first: T; second: U; }

D interface Pair { first: T; second: U; }

Option B correctly uses commas to separate multiple type parameters inside angle brackets.

If you have interface Box<T> { content: T; }, what is the type of 'content' in let box: Box<number>;?

A number

B string

C any

D T

When Box is used with number, T becomes number, so content is of type number.

Can you use a generic interface without specifying the type parameter?

A Yes, it defaults to 'any'.

B No, generic interfaces cannot be used.

C Yes, it defaults to 'string'.

D No, you must specify the type parameter.

Option D is correct. Generic type 'Box' requires the type parameter to be specified; omitting it results in a TypeScript error.

Explain what a generic interface is and why it is useful in TypeScript.

Think about how you can write one interface that works with many types.

Write a simple generic interface named 'Result' that holds a value of any type and a boolean indicating success.

Use angle brackets <> to declare the generic type.