Builder pattern with generics in Typescript - Step-by-Step Execution

Concept Flow - Builder pattern with generics

Start Builder

↓

Set Property 1

↓

Set Property 2

↓

Set Property N

↓

Call build()

↓

Return Built Object

The builder starts empty, sets properties one by one, then builds the final object.

Execution Sample

Typescript

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 creates a generic builder that sets properties and builds the final object.

Execution Table

Step	Action	Key	Value	Object State	Return
1	Create Builder	-	-	{}	Builder instance
2	Set property	name	"Alice"	{"name": "Alice"}	Builder instance
3	Set property	age	30	{"name": "Alice", "age": 30}	Builder instance
4	Build object	-	-	{"name": "Alice", "age": 30}	{"name": "Alice", "age": 30}

💡 build() returns the final object with all set properties.

Variable Tracker

Variable	Start	After 1	After 2	Final
obj	{}	{"name": "Alice"}	{"name": "Alice", "age": 30}	{"name": "Alice", "age": 30}

Key Moments - 3 Insights

Why does set() return 'this' instead of void?

Returning 'this' allows chaining multiple set() calls in one line, as shown in steps 2 and 3 of the execution_table.

What does 'Partial<T>' mean in the builder?

'Partial<T>' means the object can have zero or more properties of T set at any time, so we can build it step-by-step before finalizing.

Why do we cast 'this.obj as T' in build()?

Because 'obj' is Partial<T> (may be incomplete), but build() promises to return a full T object after all properties are set.

Visual Quiz - 3 Questions

Look at the execution_table, what is the object state after step 2?

A {}

B {"name": "Alice"}

C {"age": 30}

D {"name": "Alice", "age": 30}

💡 Hint

Check the 'Object State' column for step 2 in the execution_table.

At which step does the build() method return the final object?

A Step 4

B Step 1

C Step 3

D Step 2

💡 Hint

Look for the 'Action' column with 'Build object' in the execution_table.

If set() did not return 'this', how would chaining calls change?

A The build() method would fail.

B Chaining would still work the same.

C Chaining would not work; you couldn't call set() multiple times in one line.

D The object would be empty.

💡 Hint

Refer to the key_moments explanation about why set() returns 'this'.

Concept Snapshot

Builder pattern with generics:
- Use a generic class Builder<T> with Partial<T> to hold properties.
- set<K extends keyof T>(key, value) sets properties and returns 'this' for chaining.
- build() returns the final object as type T.
- Allows step-by-step object creation with type safety.

Full Transcript

This example shows how a generic Builder class works in TypeScript. We start with an empty object of type Partial<T>. Each set() call adds a property and returns the builder itself, so we can chain calls. Finally, build() returns the complete object as type T. The execution table traces creating the builder, setting 'name' and 'age', and building the final object. Variables track how the internal object changes. Key moments explain why set() returns 'this', what Partial<T> means, and why we cast in build(). The quiz tests understanding of object state after steps, when build() returns, and the importance of returning 'this' for chaining.

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