Builder pattern with generics in Typescript - Time & Space Complexity

Time Complexity: Builder pattern with generics

O(n)

Understanding Time Complexity

We want to see how the time needed to build an object changes as we add more parts using the builder pattern with generics.

How does the number of steps grow when the builder handles more properties?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Builder<T> {
  private obj: Partial<T> = {};

  set<K extends keyof T>(key: K, value: T[K]): this {
    this.obj[key] = value;
    return this;
  }

  build(): T {
    return this.obj as T;
  }
}
    

This code builds an object step-by-step by setting properties one at a time, using generics to keep track of property types.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Each call to set adds or updates one property in the object.
• How many times: The set method is called once per property you want to add.

How Execution Grows With Input

Each property added requires one step, so the total steps grow directly with the number of properties.

Input Size (n)	Approx. Operations
10	10 steps to set properties
100	100 steps to set properties
1000	1000 steps to set properties

Pattern observation: The work grows evenly as you add more properties, like counting one by one.

Final Time Complexity

Time Complexity: O(n)

This means the time to build the object grows in a straight line with the number of properties you add.

Common Mistake

[X] Wrong: "The builder sets all properties at once, so time is constant no matter how many properties."

[OK] Correct: Each property requires a separate set call, so time grows with how many properties you add.

Interview Connect

Understanding how building objects step-by-step affects time helps you explain design choices clearly and shows you think about efficiency in real code.

Self-Check

What if the build method also validated all properties before returning? How would the time complexity change?

Practice

1.

What is the main advantage of using the builder pattern with generics in TypeScript?

easy

A. It automatically generates UI components from objects.

B. It makes the code run faster by skipping type checks.

C. It allows creating different object types step-by-step with type safety.

D. It replaces all functions with classes.

Solution

1. Step 1: Understand builder pattern purpose

   The builder pattern helps create complex objects step-by-step, improving clarity and flexibility.

2. Step 2: Role of generics in TypeScript

   Generics allow the builder to work with any object type while keeping type safety, ensuring keys and values match.

3. Final Answer:

   It allows creating different object types step-by-step with type safety. -> Option C

4. Quick Check:

   Builder + generics = flexible, safe object creation [OK]

Hint: Generics + builder = reusable and type-safe object creation [OK]

Common Mistakes:

• Thinking builder auto-generates UI
• Believing it skips type checks
• Confusing builder with replacing functions

2.

Which of the following is the correct syntax to declare a generic builder class in TypeScript?

class Builder<T> {
  private obj: T;
  constructor() {
    this.obj = {} as T;
  }
  set(key: keyof T, value: T[keyof T]): this {
    this.obj[key] = value;
    return this;
  }
  build(): T {
    return this.obj;
  }
}

easy

A. class Builder<T> { private obj: T; constructor() { this.obj = {} as T; } }

B. class Builder<T> { private obj: T; constructor() { this.obj = new T(); } }

C. class Builder { private obj: T; constructor() { this.obj = {} as T; } }

D. class Builder<T> { private obj: any; constructor() { this.obj = {}; } }

Solution

1. Step 1: Check generic class declaration

   class Builder<T> { private obj: T; constructor() { this.obj = {} as T; } } correctly declares a generic class with <T> and initializes obj as {} as T.

2. Step 2: Identify incorrect initializations

   class Builder<T> { private obj: T; constructor() { this.obj = new T(); } } tries new T() which is invalid in TypeScript. class Builder { private obj: T; constructor() { this.obj = {} as T; } } misses generic <T>. class Builder<T> { private obj: any; constructor() { this.obj = {}; } } uses any losing type safety.

3. Final Answer:

   class Builder<T> { private obj: T; constructor() { this.obj = {} as T; } } -> Option A

4. Quick Check:

   Generic class with {} as T init = class Builder<T> { private obj: T; constructor() { this.obj = {} as T; } } [OK]

Hint: Use {} as T to initialize generic object safely [OK]

Common Mistakes:

• Trying to instantiate generic type with new
• Forgetting to declare generic parameter <T>
• Using any instead of generic type

3.

What will be the output of this code?

class Builder<T> {
  private obj: T;
  constructor() { this.obj = {} as T; }
  set(key: keyof T, value: T[keyof T]): this {
    this.obj[key] = value;
    return this;
  }
  build(): T { return this.obj; }
}

interface Person { name: string; age: number; }
const person = new Builder<Person>()
  .set('name', 'Alice')
  .set('age', 30)
  .build();
console.log(person);

medium

A. { name: 'Alice', age: 30 }

B. { name: 'Alice' }

C. Error: Cannot assign to 'name' of type 'T'

D. {}

Solution

1. Step 1: Understand builder usage

   The builder sets 'name' to 'Alice' and 'age' to 30 on an empty object typed as Person.

2. Step 2: Check final build output

   The build() method returns the object with both properties set, so output is { name: 'Alice', age: 30 }.

3. Final Answer:

   { name: 'Alice', age: 30 } -> Option A

4. Quick Check:

   Builder sets keys correctly = { name: 'Alice', age: 30 } [OK]

Hint: Builder sets keys stepwise, final object has all properties [OK]

Common Mistakes:

• Expecting partial object missing keys
• Thinking assignment causes error
• Assuming empty object returned

4.

Identify the error in this builder class and select the fix:

class Builder<T> {
  private obj: T;
  constructor() { this.obj = {}; }
  set(key: keyof T, value: T[keyof T]): this {
    this.obj[key] = value;
    return this;
  }
  build(): T { return this.obj; }
}

medium

A. Use any type for obj instead of T.

B. Change set method to accept key: string.

C. Remove generic <T> from class declaration.

D. Initialize obj as {} as T in constructor.

Solution

1. Step 1: Identify initialization error

   Assigning {} directly to obj: T causes a type error because {} is not guaranteed to be T.

2. Step 2: Correct initialization with type assertion

   Using {} as T tells TypeScript to treat empty object as type T safely during construction.

3. Final Answer:

   Initialize obj as {} as T in constructor. -> Option D

4. Quick Check:

   Use {} as T to fix generic object init [OK]

Hint: Use type assertion {} as T to initialize generic objects [OK]

Common Mistakes:

• Assigning {} directly without assertion
• Changing key type to string loses type safety
• Removing generics breaks flexibility

5.

You want to build a generic builder that only allows setting properties defined in the object type and prevents setting undefined keys. Which TypeScript feature helps enforce this at compile time?

hard

A. Using any type for keys to allow flexibility.

B. Using keyof T as the key type in the set method.

C. Using unknown type for values to accept any value.

D. Using Partial<T> for the object storage.

Solution

1. Step 1: Understand key restriction need

   To prevent setting keys not in the object type, the key parameter must be limited to valid keys.

2. Step 2: Use keyof T for key type

   keyof T represents all valid keys of type T, so using it restricts keys to only those defined in T.

3. Final Answer:

   Using keyof T as the key type in the set method. -> Option B

4. Quick Check:

   keyof T restricts keys to valid properties [OK]

Hint: Use keyof T to limit keys to object properties [OK]

Common Mistakes:

• Using any loses key restriction
• Using unknown does not restrict keys
• Partial<T> affects values, not keys