Mock Interview: Creational

Introduction

Mock interview: creational simulates a 45-minute senior loop on object-creation patterns. Each question includes junior, mid, and senior answers so you see the upgrade path explicitly.

The story

Candidate asked "implement Singleton" — wrote double-checked locking from memory. Follow-up: "How test code depending on it?" Froze. Singleton answer correct; test answer revealed gap. Mock practice fixes this.

The business problem

Creational questions trap candidates who memorize implementation but miss judgment:

• Singleton testability: can't mock global instance.
• Factory overuse: Abstract Factory for one product.
• Builder vs constructor: when telescoping actually matters.
• DI vs Singleton: modern alternative not mentioned.

The problem teams faced

Creational mock covers:

• Singleton — when, testability, DI alternative.
• Factory Method vs Abstract Factory — force distinction.
• Builder — complex object with optional fields.
• Prototype — clone vs reconstruct; when copy expensive.

Understanding the topic

Intent: Practice creational Q&A at three seniority levels.

• Junior — definition + naive implementation.
• Mid — force + basic trade-off.
• Senior — alternatives, testing, production story, when-not.

Internal architecture

Mock Q1: Singleton — three answers:

JUNIOR: "One instance." + writes double-checked locking.

MID: "Global state; hurts testing. Use DI scoped singleton."

SENIOR: "Creational — one instance per process. Force: expensive shared
resource. Trade-off: hidden dependency, test difficulty. Alternative: DI
container singleton scope. Story: metrics registry — legacy boundary required
hand-roll. Test: inject MetricsCollector interface; prod uses singleton impl,
tests use fake. Never for request-scoped data."

Visual explanation

Three diagrams cover answer levels, Factory vs Abstract Factory, Singleton test seam:

Informative example

Mock Q2: Builder — senior answer sketch:

// Force: Order with 12 optional fields — constructor telescoping
// Senior: "Builder when >4 optional params or validation between steps.
//  Alternative: factory function with defaults object in TS.
//  Story: checkout OrderBuilder — validate address before payment step.
//  Not Builder for 3-field DTO — YAGNI."

class OrderBuilder {
  private order: Partial<Order> = {}
  withItems(items: LineItem[]) { this.order.items = items; return this }
  withShipping(addr: Address) { /* validate */ this.order.shipTo = addr; return this }
  build(): Order { /* final validation */ return Order.create(this.order) }
}

Execution workflow

Real-world use

Senior loops at Google/Meta often start creational then pivot to system design; Stripe interviews Factory/Adapter for payment provider integration.

Production case study

Singleton freeze on test question:

• Implementation: correct double-checked locking.
• Follow-up: "How test dependents?" — no answer.
• Lesson: always pair creational with test seam.

Trade-offs

• Pro: explicit junior/mid/senior ladder accelerates learning.
• Con: mock ≠ real interviewer unpredictability.

Decision framework

• Every creational answer: force, test, when-not.
• Factory Method: one product type varies. Abstract Factory: product families.
• Builder: many optional fields + step validation.

Best practices

• Never write Singleton code without mentioning test alternative.
• Prototype: mention structuredClone vs custom clone when fields are complex.
• Object Pool: only when allocation measurably expensive.

Anti-patterns to avoid

• Abstract Factory with one family — deletion candidate.
• Singleton for database connection in serverless — wrong scope.

Common mistakes

• Confusing Factory Method (method) with Simple Factory (function).
• Builder for 2-field object — overengineering.

Debugging tips

• If stuck on Factory vs Abstract Factory — ask "one product or family?"

Optimization strategies

• Prepare 4 creational stories: Singleton removal, Factory for payments, Builder for orders, Prototype for templates.

Common misconceptions

• Simple Factory is not GoF Factory Method — know the distinction.
• DI replaces most Singleton hand-rolls — say it in interview.

Advanced interview questions

Summary

You can upgrade creational answers from junior to senior and survive the Singleton test follow-up. Complete this mock aloud — you own Creational Interview prep.