Generic repository pattern in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of generic repository save method

What is the output of this TypeScript code using a generic repository pattern?

Typescript

interface IEntity {
  id: number;
}

class GenericRepository<T extends IEntity> {
  private items: T[] = [];

  save(item: T): string {
    this.items.push(item);
    return `Saved item with id: ${item.id}`;
  }
}

const repo = new GenericRepository<{id: number; name: string}>();
console.log(repo.save({id: 1, name: 'Test'}));

A TypeError at runtime

B "Saved item with id: undefined"

C Compilation error due to missing id property

D "Saved item with id: 1"

💡 Hint

Check how the generic type extends IEntity and the id property is accessed.

The generic type T extends IEntity, which requires an id property. The save method pushes the item and returns a string with the id. Since the object passed has id: 1, the output is 'Saved item with id: 1'.

🧠 Conceptual

intermediate

Purpose of generic repository pattern

What is the main purpose of using a generic repository pattern in TypeScript?

A To replace all interfaces with classes

B To enforce UI styling rules across components

C To create a reusable data access layer for different entity types

D To handle HTTP requests directly in components

💡 Hint

Think about code reuse and data management.

The generic repository pattern allows writing one class to handle data operations for many entity types, improving code reuse and consistency.

🔧 Debug

advanced

Identify the error in generic repository update method

What error will this TypeScript code produce when trying to update an item in the generic repository?

Typescript

interface IEntity {
  id: number;
}

class GenericRepository<T extends IEntity> {
  private items: T[] = [];

  update(item: T): boolean {
    const index = this.items.findIndex(i => i.id === item.id);
    if (index !== -1) {
      this.items[index] = item;
      return true;
    }
    return false;
  }
}

const repo = new GenericRepository<{id: number; name: string}>();
repo.update({id: 1, name: 'Updated'});

A Returns false because item with id 1 does not exist

B Throws runtime error because items array is undefined

C Compilation error due to missing constructor

D Returns true but does not update the item

💡 Hint

Check the initial state of the items array before update.

The items array is empty initially, so findIndex returns -1. The update method returns false because no item with id 1 exists to update.

📝 Syntax

advanced

Correct generic constraint syntax

Which option correctly declares a generic repository class constrained to entities with an id property?

A class GenericRepository<T where T.id: number> {}

B class GenericRepository<T extends {id: number}> {}

C class GenericRepository<T: {id: number}> {}

D class GenericRepository<T with id: number> {}

💡 Hint

Remember TypeScript syntax for generic constraints uses extends.

Option B uses the correct TypeScript syntax for generic constraints. Other options use invalid syntax.

🚀 Application

expert

Number of items after operations in generic repository

Given this TypeScript code using a generic repository, how many items are in the repository after all operations?

Typescript

interface IEntity { id: number; }

class GenericRepository<T extends IEntity> {
  private items: T[] = [];

  save(item: T): void {
    this.items.push(item);
  }

  delete(id: number): boolean {
    const index = this.items.findIndex(i => i.id === id);
    if (index !== -1) {
      this.items.splice(index, 1);
      return true;
    }
    return false;
  }

  count(): number {
    return this.items.length;
  }
}

const repo = new GenericRepository<{id: number; name: string}>();
repo.save({id: 1, name: 'A'});
repo.save({id: 2, name: 'B'});
repo.delete(1);
repo.save({id: 3, name: 'C'});
repo.delete(4);
const total = repo.count();
console.log(total);

A 2

B 3

C 1

D 0

💡 Hint

Track each save and delete operation carefully.

After saving items with id 1 and 2, deleting id 1 removes one item, then saving id 3 adds another. Deleting id 4 does nothing. So total items are 2.