Generic interface for collections 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. It allows the interface to work with any data type while keeping type safety.

beginner

How do you declare a generic interface for a collection of items in TypeScript?

You declare it using angle brackets with a type parameter, for example: interface Collection<T> { add(item: T): void; get(index: number): T; } where T is the generic type.

beginner

Why use a generic interface for collections instead of a specific type?

Using a generic interface lets you reuse the same code for different types of collections, like numbers, strings, or custom objects, without losing type safety or writing duplicate code.

beginner

Example: What does this interface mean?
interface Collection<T> { add(item: T): void; get(index: number): T; }

It means the Collection interface works with any type T. It has an add method to add an item of type T, and a get method to get an item of type T by index.

beginner

How do you create a collection of strings using the generic interface Collection<T>?

You specify the type when using the interface, like <code>let stringCollection: Collection&lt;string&gt;</code>. This means the collection will only hold strings.

What does the T represent in interface Collection<T>?

A A function name

B A specific type like string

C A placeholder for any type

D A variable name

T is a placeholder that stands for any type you want to use when you create a collection.

How do you specify the type when using a generic interface?

A By writing the type inside angle brackets after the interface name

B By writing the type before the interface name

C By using a special keyword 'type'

D You cannot specify the type

You specify the type by putting it inside <> after the interface name, like Collection<number>.

Which of these is a benefit of using generic interfaces for collections?

A Reuse code for different data types safely

B Make code run faster

C Avoid using any types

D Automatically convert types

Generic interfaces let you reuse the same code for many types while keeping type safety.

If you have let numbers: Collection<number>, what type of items can you add?

A Only strings

B Only numbers

C Any type

D Only objects

The collection is set to hold numbers only, so you can add only numbers.

What happens if you try to add a string to Collection<number>?

A The program crashes

B It works fine

C The string is converted to number automatically

D TypeScript shows an error

TypeScript will show a type error because the collection expects numbers only.

Explain what a generic interface for collections is and why it is useful.

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

Describe how you would create and use a generic collection interface to store strings.

Focus on how to tell TypeScript the collection holds strings.

Practice

1. What does the <T> represent in a generic interface in TypeScript?

easy

A. A function inside the interface

B. A fixed type like number or string

C. A placeholder for any type that will be specified later

D. A variable name for a property

Solution

1. Step 1: Understand generic syntax

   The <T> in TypeScript generics is a placeholder representing any type that will be provided when the interface is used.

2. Step 2: Differentiate from fixed types

   Unlike fixed types like number or string, T is flexible and can represent any type, making the interface reusable.

3. Final Answer:

   A placeholder for any type that will be specified later -> Option C

4. Quick Check:

   Generic <T> = placeholder type [OK]

Hint: Remember <T> means 'any type' placeholder [OK]

Common Mistakes:

• Thinking T is a fixed type
• Confusing T with a function
• Assuming T is a variable name

2. Which of the following is the correct syntax to declare a generic interface named Collection with a type parameter T?

easy

A. interface Collection<T> { items: T[]; }

B. interface Collection { items: T[]; }

C. interface Collection<T> { items: Array; }

D. interface Collection<T> { items: T; }

Solution

1. Step 1: Check generic declaration

   The correct syntax to declare a generic interface uses <T> after the interface name, like interface Collection<T>.

2. Step 2: Verify property type

   The property items should be an array of type T, so items: T[] is correct.

3. Final Answer:

   interface Collection<T> { items: T[]; } -> Option A

4. Quick Check:

   Generic interface syntax = interface Collection<T> { items: T[]; } [OK]

Hint: Generic interface needs <T> and array type T[] [OK]

Common Mistakes:

• Omitting <T> after interface name
• Using Array without type parameter
• Declaring items as single T instead of array

3. Given the interface and usage below, what is the output of console.log(numbers.items[1])?

interface Collection<T> {
  items: T[];
}
const numbers: Collection<number> = { items: [10, 20, 30] };
console.log(numbers.items[1]);

medium

A. 10

B. 20

C. 30

D. undefined

Solution

1. Step 1: Understand the generic interface usage

   The interface Collection<T> is used with number, so items is an array of numbers: [10, 20, 30].

2. Step 2: Access the second element in the array

   Arrays are zero-indexed, so items[1] is the second element, which is 20.

3. Final Answer:

   20 -> Option B

4. Quick Check:

   Array index 1 = 20 [OK]

Hint: Remember arrays start at index 0 [OK]

Common Mistakes:

• Confusing index 1 with index 0
• Assuming generic changes array order
• Expecting undefined for valid index

4. Identify the error in this code snippet using a generic interface:

interface Collection<T> {
  items: T[];
}
const strings: Collection<string> = { items: [1, 2, 3] };

medium

A. The array items contain numbers, but the interface expects strings

B. The interface is missing the generic type parameter

C. The variable strings should be declared with type number

D. The interface property items should not be an array

Solution

1. Step 1: Check the generic type used

   The variable strings is declared as Collection<string>, so items must be an array of strings.

2. Step 2: Check the assigned array type

   The assigned array is [1, 2, 3], which is an array of numbers, not strings, causing a type mismatch error.

3. Final Answer:

   The array items contain numbers, but the interface expects strings -> Option A

4. Quick Check:

   Type mismatch in generic array = The array items contain numbers, but the interface expects strings [OK]

Hint: Check if array elements match generic type [OK]

Common Mistakes:

• Ignoring type mismatch between generic and array
• Forgetting to specify generic parameter
• Assuming interface property can be non-array

5. You want to create a generic interface KeyValueCollection that stores key-value pairs where keys are strings and values can be any type T. Which interface definition correctly achieves this?

hard

A. interface KeyValueCollection<T> { [T: string]: string; }

B. interface KeyValueCollection<T> { keys: string[]; values: T[]; }

C. interface KeyValueCollection<T> { key: string; value: T; }

D. interface KeyValueCollection<T> { [key: string]: T; }

Solution

1. Step 1: Understand key-value pair requirement

   The interface should allow any string as a key and associate it with a value of generic type T.

2. Step 2: Check index signature syntax

   Using [key: string]: T; means any string key maps to a value of type T, which matches the requirement.

3. Step 3: Eliminate other options

   interface KeyValueCollection<T> { keys: string[]; values: T[]; } separates keys and values arrays, which is less direct. interface KeyValueCollection<T> { key: string; value: T; } defines a single pair, not a collection. interface KeyValueCollection<T> { [T: string]: string; } has incorrect syntax.

4. Final Answer:

   interface KeyValueCollection<T> { [key: string]: T; } -> Option D

5. Quick Check:

   Index signature with generic value = interface KeyValueCollection<T> { [key: string]: T; } [OK]

Hint: Use index signature [key: string]: T for generic key-value [OK]

Common Mistakes:

• Using arrays for keys and values separately
• Defining only one key-value pair instead of collection
• Incorrect index signature syntax