Generic interface declaration in Typescript - Mini Project: Build & Apply

Generic Interface Declaration

📖 Scenario: You are building a simple system to store information about different types of items. Each item has a name and a value. The value can be of any type, like a number, string, or boolean.

🎯 Goal: Create a generic interface called Item that can hold a name as a string and a value of any type. Then create two objects using this interface: one with a number value and one with a string value.

📋 What You'll Learn

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

The interface must have a name property of type string.

The interface must have a value property of type T.

Create a variable called numberItem of type Item<number> with name as "Age" and value as 30.

Create a variable called stringItem of type Item<string> with name as "Color" and value as "Blue".

Print both variables to the console.

💡 Why This Matters

🌍 Real World

Generic interfaces help you write flexible and reusable code that works with many data types, like storing different kinds of items in a shopping cart or settings.

💼 Career

Understanding generics is important for TypeScript developers to build scalable and type-safe applications, which is a valuable skill in many software development jobs.

1

Create the generic interface

Create a generic interface called Item with a type parameter T. It should have a name property of type string and a value property of type T.

Typescript

// Your code here

Hint

Use interface Item<T> to declare a generic interface with a type parameter T.

2

Create a number item

Create a variable called numberItem of type Item<number> with name set to "Age" and value set to 30.

Typescript

interface Item {
    name: string;
    value: T;
}

// Your code here

Hint

Use const numberItem: Item<number> = { name: "Age", value: 30 }; to create the variable.

3

Create a string item

Create a variable called stringItem of type Item<string> with name set to "Color" and value set to "Blue".

Typescript

interface Item {
    name: string;
    value: T;
}

const numberItem: Item = {
    name: "Age",
    value: 30
};

// Your code here

Hint

Use const stringItem: Item<string> = { name: "Color", value: "Blue" }; to create the variable.

4

Print the items

Print numberItem and stringItem to the console using two separate console.log statements.

Typescript

interface Item {
    name: string;
    value: T;
}

const numberItem: Item = {
    name: "Age",
    value: 30
};

const stringItem: Item = {
    name: "Color",
    value: "Blue"
};

// Your code here

Hint

Use console.log(numberItem); and console.log(stringItem); to print the objects.

Practice

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

easy

A. A special type that only accepts strings

B. A variable name for a function

C. A keyword to declare a constant

D. A placeholder for a type that will be specified later

Solution

1. Step 1: Understand generic syntax

   The <T> syntax in TypeScript is used to declare a generic type parameter, which acts as a placeholder for a type.

2. Step 2: Identify the role of T

   This placeholder allows the interface to be flexible and reusable with different types specified later.

3. Final Answer:

   A placeholder for a type that will be specified later -> Option D

4. Quick Check:

   Generic <T> = Type placeholder [OK]

Hint: Remember <T> means 'type to be given later' [OK]

Common Mistakes:

• Thinking T is a variable or constant
• Confusing <T> with a function parameter
• Assuming T is always a string type

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

easy

A. interface Box<T> = { content: T; }

B. interface Box<T> { content: T; }

C. interface Box<T> content: T;

D. interface Box { content: T; }

Solution

1. Step 1: Check interface declaration syntax

   In TypeScript, interfaces are declared with the keyword interface followed by the name and optional generic parameters in angle brackets.

2. Step 2: Verify correct use of braces and type parameter

   The correct syntax includes curly braces to define the interface body and uses <T> to declare the generic type parameter.

3. Final Answer:

   interface Box<T> { content: T; } -> Option B

4. Quick Check:

   Generic interface syntax = interface Box<T> { content: T; } [OK]

Hint: Generic interfaces need <T> and braces {} [OK]

Common Mistakes:

• Omitting the generic parameter <T>
• Missing curly braces around interface body
• Using equals sign instead of braces

3. Given the interface declaration:

interface Container<T> { value: T; }

What is the output of this code?

const numberContainer: Container<number> = { value: 42 };
console.log(numberContainer.value);

medium

A. Error: Type mismatch

B. undefined

C. 42

D. null

Solution

1. Step 1: Understand the generic interface usage

   The interface Container<T> expects a type parameter. Here, number is specified, so value must be a number.

2. Step 2: Analyze the assigned object and console output

   The object { value: 42 } matches the interface with value as a number. Logging numberContainer.value prints 42.

3. Final Answer:

   42 -> Option C

4. Quick Check:

   Generic with number type outputs 42 [OK]

Hint: Generic type sets property type; output matches assigned value [OK]

Common Mistakes:

• Expecting a runtime error due to generics
• Confusing generic type with runtime type enforcement
• Assuming output is undefined or null

4. Identify the error in this generic interface usage:

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

const pair: Pair = { first: 'hello', second: 'world' };

medium

A. Missing generic type argument when using Pair

B. Properties first and second should be numbers

C. Interface Pair cannot have two properties of the same type

D. Object literal syntax is incorrect

Solution

1. Step 1: Check interface usage with generics

   The interface Pair<T> requires a type argument when used. Here, Pair is used without specifying <T>.

2. Step 2: Identify the missing generic argument error

   TypeScript will report an error because the generic type parameter is missing when declaring pair.

3. Final Answer:

   Missing generic type argument when using Pair -> Option A

4. Quick Check:

   Generic interface usage needs type argument [OK]

Hint: Always specify type argument when using generic interfaces [OK]

Common Mistakes:

• Forgetting to add <Type> when using generic interfaces
• Assuming default type if omitted
• Confusing property types with generic parameter

5. You want to create a generic interface Result<T> that holds a data property of type T and a success boolean. Which of the following implementations correctly uses this interface to type a variable holding a string result?

hard

A. const res: Result<string> = { data: 'Done', success: true };

B. const res: Result<string> = { data: 123, success: true };

C. const res: Result = { data: 'Done', success: true };

D. const res: Result<number> = { data: 'Done', success: true };

Solution

1. Step 1: Understand the generic interface requirements

   The interface Result<T> expects a data property of type T and a success boolean.

2. Step 2: Check each option for correct type usage

   const res: Result<string> = { data: 123, success: true }; assigns a number to data but expects a string type, so it's incorrect.
   const res: Result<string> = { data: 'Done', success: true }; correctly assigns a string to data and includes success.
   const res: Result = { data: 'Done', success: true }; omits the generic type argument, which is required.
   const res: Result<number> = { data: 'Done', success: true }; uses number type but assigns a string, causing a type mismatch.

3. Final Answer:

   const res: Result<string> = { data: 'Done', success: true }; -> Option A

4. Quick Check:

   Generic type matches property type = const res: Result<string> = { data: 'Done', success: true }; [OK]

Hint: Match generic type with property value type exactly [OK]

Common Mistakes:

• Assigning wrong type to generic property
• Omitting generic type argument
• Mixing up generic type parameters