Generic interface for collections in Typescript

Introduction

We use generic interfaces to create flexible collection types that can hold any kind of data. This helps us write reusable and safe code.

When you want to create a list that can store numbers, strings, or any other type without rewriting code.

When you need a collection interface that works with different data types in your app.

When you want to ensure type safety while working with arrays or lists of items.

When building reusable components or libraries that handle collections of various types.

Syntax

Typescript

interface Collection<T> {
  add(item: T): void;
  remove(item: T): void;
  getAll(): T[];
}

The <T> means this interface works with any type you choose.

Methods use T to refer to the type of items in the collection.

Examples

This is the generic interface definition for a collection of items of type T.

Typescript

interface Collection<T> {
  add(item: T): void;
  remove(item: T): void;
  getAll(): T[];
}

This class uses the generic interface for a collection of numbers.

Typescript

class NumberCollection implements Collection<number> {
  private items: number[] = [];
  add(item: number): void { this.items.push(item); }
  remove(item: number): void { this.items = this.items.filter(i => i !== item); }
  getAll(): number[] { return this.items; }
}

This class uses the generic interface for a collection of strings.

Typescript

class StringCollection implements Collection<string> {
  private items: string[] = [];
  add(item: string): void { this.items.push(item); }
  remove(item: string): void { this.items = this.items.filter(i => i !== item); }
  getAll(): string[] { return this.items; }
}

Sample Program

This program creates a collection for numbers, adds three numbers, removes one, and then prints the remaining numbers.

Typescript

interface Collection<T> {
  add(item: T): void;
  remove(item: T): void;
  getAll(): T[];
}

class NumberCollection implements Collection<number> {
  private items: number[] = [];

  add(item: number): void {
    this.items.push(item);
  }

  remove(item: number): void {
    this.items = this.items.filter(i => i !== item);
  }

  getAll(): number[] {
    return this.items;
  }
}

const numbers = new NumberCollection();
numbers.add(10);
numbers.add(20);
numbers.add(30);
numbers.remove(20);
console.log(numbers.getAll());

Important Notes

Generic interfaces help keep your code flexible and safe by working with any data type.

Always specify the type when using the generic interface to avoid errors.

Summary

Generic interfaces let you create reusable collection types for any data.

Use <T> to represent the type inside the interface.

Implement the interface with specific types like number or string to use it.

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