Generic interface for collections in Typescript - Mini Project: Build & Apply

Generic interface for collections

📖 Scenario: You are building a simple program to manage collections of items. You want to create a generic interface that can work with any type of collection, like numbers or strings.

🎯 Goal: Create a generic interface called Collection with a method to add items and a method to get all items. Then use this interface with a number collection and a string collection.

📋 What You'll Learn

Create a generic interface called Collection with a type parameter T.

Add a method add(item: T): void to add an item to the collection.

Add a method getAll(): T[] to return all items in the collection.

Create a variable numberCollection of type Collection<number>.

Create a variable stringCollection of type Collection<string>.

💡 Why This Matters

🌍 Real World

Generic interfaces help you write flexible code that works with many types of data, like lists of numbers, strings, or custom objects.

💼 Career

Understanding generics is important for TypeScript developers to create reusable and type-safe components and libraries.

1

Create the generic interface

Create a generic interface called Collection with a type parameter T. Inside it, declare a method add(item: T): void and a method getAll(): T[].

Typescript

// Your code here

Hint

Use interface Collection<T> to create a generic interface with type parameter T.

2

Create number and string collections

Create a variable called numberCollection of type Collection<number> and a variable called stringCollection of type Collection<string>. For now, assign them empty objects {}.

Typescript

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

// Your code here

Hint

Use const numberCollection: Collection<number> = {} and similarly for strings.

3

Implement the collections with arrays

Implement numberCollection and stringCollection as objects with an internal array called items. Add the add(item: T): void method to push items to the array, and the getAll(): T[] method to return the array.

Typescript

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

// Your code here to implement numberCollection and stringCollection

Hint

Use an array property items inside each object to store the collection items.

4

Add items and print collections

Add the numbers 10 and 20 to numberCollection using add. Add the strings "apple" and "banana" to stringCollection using add. Then print the results of getAll() for both collections using console.log.

Typescript

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

const numberCollection: Collection = {
    items: [] as number[],
    add(item: number): void {
        this.items.push(item);
    },
    getAll(): number[] {
        return this.items;
    }
};

const stringCollection: Collection = {
    items: [] as string[],
    add(item: string): void {
        this.items.push(item);
    },
    getAll(): string[] {
        return this.items;
    }
};

// Your code here to add items and print collections

Hint

Use numberCollection.add(10) and console.log(numberCollection.getAll()) to add and print.

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