Overview

The Builder pattern decomposes the construction of a complex object into discrete steps, exposed by a builder object. The client calls the steps it cares about in any order, then asks the builder for the final result. It shines when a constructor would otherwise need a dozen parameters, half of them optional.

When to use it

• An object has many optional fields or configuration steps and constructor overloading would explode.
• You want the same construction process to produce different representations (e.g., a query that can render as SQL or as a parameterized object).
• You want construction to be fluent and self-documenting at the call site.

Example

class Pizza {
  size: string = "M";
  cheese = false;
  pepperoni = false;
  mushrooms = false;
}

class PizzaBuilder {
  private pizza = new Pizza();

  size(size: string) { this.pizza.size = size; return this; }
  addCheese() { this.pizza.cheese = true; return this; }
  addPepperoni() { this.pizza.pepperoni = true; return this; }
  addMushrooms() { this.pizza.mushrooms = true; return this; }

  build(): Pizza { return this.pizza; }
}

const pizza = new PizzaBuilder()
  .size("L")
  .addCheese()
  .addMushrooms()
  .build();