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Justworks Software Engineer Interview Guide

Justworks Software Engineer interview guide: reported rounds, questions and how candidates describe the process.

The 60-second brief

A Software Engineer at Justworks plays a critical role in building and scaling a platform that simplifies entrepreneurship by managing payroll, benefits, compliance, and HR. Because Justworks operates as a Professional Employer Organization (PEO), the software you build directly handles sensitive financial transactions, legal compliance, and critical employee data. The engineering team is tasked with translating highly complex government regulations and insurance workflows into simple, beautiful, and intuitive user experiences. The impact of this position is immense. Small and medium-sized businesses rely on Justworks to run their daily operations smoothly and ensure their employees are paid accurately and on time. As a Software Engineer, you will work on core product features, complex calculation engines, third-party integrations with financial institutions, and high-throughput APIs.

This guide is scoped to a Software Engineer candidate at Justworks.

Focus areas

  • Algorithms & Problem Solving
  • System Design
  • Python

The Justworks interview process

Justworks candidates report 3 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Recruiter Call

A conversational call with a recruiter to discuss your background and the role.

What to demonstrate

  • A conversational call with a recruiter to discuss your background and the role
  • Depth in Algorithms & Problem Solving

How to prepare

  • Be able to walk your CV end to end in two minutes, and say why this company specifically.
  • Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.

Technical Screening

Choose between a timed online assessment or a live technical phone screen.

What to demonstrate

  • Choose between a timed online assessment or a live technical phone screen
  • Depth in Algorithms & Problem Solving

How to prepare

  • Answer aloud and timed: Write a function to perform operations on a binary search tree (BST), such as finding the lowest common ancestor or validating the tree structure.
  • Answer aloud and timed: Implement an algorithm to manipulate arrays or strings efficiently within a strict 40-minute timeframe.

Onsite Interview

Participate in four to five rounds covering coding, debugging, system design, and behavioral discussions.

What to demonstrate

  • Participate in four to five rounds covering coding, debugging, system design, and behavioral discussions
  • Depth in Algorithms & Problem Solving

How to prepare

  • Answer aloud and timed: Design the database schema and API endpoints for a collaborative board like Trello or Jira. Focus on data modeling, relationships, and state transitions.
  • Answer aloud and timed: Model the database schema for an employee management platform, explaining how you would handle complex organizational hierarchies.

Where candidates lose points

Over-communicate during coding rounds

Always talk through your approach before typing any code. Explain your variable names, the data structures you are choosing, and the trade-offs of your solution.

Focus on data modeling over infrastructure

In system design interviews, prioritize building a solid, normalized database schema and clear API endpoints. Do not waste time discussing complex cloud infrastructure unless explicitly asked.

Prepare for DEI questions

Justworks takes diversity, equity, and inclusion seriously. Be prepared to share concrete, personal examples of how you contribute to an inclusive team culture.

Clarify requirements early

In both the coding and system design rounds, ask clarifying questions before writing code. This demonstrates a methodical, product-minded engineering approach.

Questions & practice

Implement a function to merge overlapping intervals from a given list of interval pairs (e.g., merge `[[1,3],[

Medium · Coding and Algorithm Challenges

Implement a function to merge overlapping intervals from a given list of interval pairs (e.g., merge [[1,3],[2,6],[8,10]] into [[1,6],[8,10]]).

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Write a function to perform operations on a binary search tree (BST), such as finding the lowest common ancest

Medium · Coding and Algorithm Challenges

Write a function to perform operations on a binary search tree (BST), such as finding the lowest common ancestor or validating the tree structure.

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Implement an algorithm to manipulate arrays or strings efficiently within a strict 40-minute timeframe.

Medium · Coding and Algorithm Challenges

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Model the database schema for an employee management platform, explaining how you would handle complex organiz

Medium · System and API Design

Model the database schema for an employee management platform, explaining how you would handle complex organizational hierarchies.

Approach

  1. Name the grain you start from and join outward from it.
  2. Check whether any join is one-to-many before aggregating, or the sums inflate.
  3. Say which index the query would use, and what makes it unusable.
  4. Handle the rows that do not match: that is usually the actual question.

Follow-up

  • How does the query change if that join becomes one-to-many?
  • What happens to this when the table is ten times larger?

Write the update path that detects a concurrent edit

Medium · optimistic concurrency · transactional outbox · audit history

resource carries version INT NOT NULL DEFAULT 1. resource_revision holds revision_id, resource_id, version, actor_user_id, change_kind, patch JSONB, request_id, created_at with UNIQUE (resource_id, version). outbox_event holds aggregate_type, aggregate_id, aggregate_version, event_type, payload, status. A PUT carries the version the client read. Write the exact statements for the single transaction that applies the edit, records the revision and enqueues 'resource.updated', and give the handler's branch on zero affected rows. Then say what PostgreSQL 16 does under READ COMMITTED when two of these updates hit one row at once.

Approach

  1. One transaction, three writes, no network call inside it: UPDATE resource SET title = $3, version = version + 1, updated_at = now() WHERE resource_id = $1 AND tenant_id = $4 AND version = $2; then INSERT the resource_revision row at version $2 + 1; then INSERT the outbox_event row at the same aggregate_version. The event goes to a table rather than a broker because no transaction spans both.
  2. Branch on the affected-row count before doing anything else. Zero has three causes — stale version, wrong tenant, row gone — so re-read once and map to 409 carrying the current version, or 404 for an id outside the caller's tenant, which also stops the endpoint confirming that another tenant's id exists.
  3. State the engine behaviour instead of assuming it. Under READ COMMITTED the second UPDATE blocks on the row lock, and when the first commits PostgreSQL re-evaluates the WHERE clause against the newly committed row, so the version predicate now fails and the statement reports zero rows. Under REPEATABLE READ the identical collision raises SQLSTATE 40001 instead, so the handler must fold both shapes into one conflict response.
  4. Keep UNIQUE (resource_id, version) even though the predicate already serialises writers. It is what makes a lost update unwritable if any other path ever reaches the revision table, and it converts a logic bug into 23505 rather than into a silently missing history row.

Follow-up

  • A client sends the version it read ten minutes ago and the resource has moved three versions. What is in your 409 so it can resolve the conflict without a full re-fetch?
  • Two editors, two disjoint fields, no overlap. Does your answer still refuse the second write, and should it?

Design and implement a basic banking system that supports deposits, withdrawals, and transfers, handling multi

Medium · Coding and Algorithm Challenges

Design and implement a basic banking system that supports deposits, withdrawals, and transfers, handling multi-level complexity as requirements evolve.

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

Design the database schema and API endpoints for a collaborative board like Trello or Jira. Focus on data mode

Medium · System and API Design

Design the database schema and API endpoints for a collaborative board like Trello or Jira. Focus on data modeling, relationships, and state transitions.

Approach

  1. Say who the caller is and what they do when the call fails halfway.
  2. Define the identity of a request so a retry cannot double-apply it.
  3. Separate accepted, pending, failed and confirmed; they are different facts.
  4. Design the error taxonomy before the success shape; callers branch on it.

Follow-up

  • What happens if the caller retries after a timeout?
  • How does a client discover it is on an old version of this contract?

Design a backend system for a fictional application, detailing how you would handle data consistency, caching,

Medium · System and API Design

Design a backend system for a fictional application, detailing how you would handle data consistency, caching, and API authentication.

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

Analyze a purposefully confusing script (such as an employee-manager tree printing script) to identify logical

Medium · Debugging and Code Analysis

Analyze a purposefully confusing script (such as an employee-manager tree printing script) to identify logical bugs, fix failing tests, and optimize performance.

Approach

  1. Establish what changed and when, before forming any theory.
  2. Pick a bisection that eliminates candidates whichever way it turns out.
  3. Check the instrumentation before believing the symptom.
  4. Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

  • What would you look at first, and what would it rule out?
  • How would you tell a cause from a coincidence here?

Walk through a provided code snippet, explain what the code does, identify potential edge cases, and propose r

Medium · Debugging and Code Analysis

Walk through a provided code snippet, explain what the code does, identify potential edge cases, and propose refactoring steps to improve readability.

Approach

  1. Establish what changed and when, before forming any theory.
  2. Pick a bisection that eliminates candidates whichever way it turns out.
  3. Check the instrumentation before believing the symptom.
  4. Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

  • What would you look at first, and what would it rule out?
  • How would you tell a cause from a coincidence here?

Suggested practice checklist

Built from the rounds and topics Justworks candidates report.

Day 1 — Map the Justworks loop

  • Write out the reported sequence: Recruiter Call, Technical Screening, Onsite Interview.
  • For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.

Deliverable: A one-page map of the 3 reported rounds, with the weakest marked.

Day 2 — Work Algorithms & Problem Solving

  • Spend the session on Algorithms & Problem Solving, which Justworks candidates report being tested on.
  • Write one worked example in Algorithms & Problem Solving and time yourself on it.

Deliverable: One timed worked example in Algorithms & Problem Solving.

Day 3 — Work System Design

  • Spend the session on System Design, which Justworks candidates report being tested on.
  • Write one worked example in System Design and time yourself on it.

Deliverable: One timed worked example in System Design.

Day 4 — Work Python

  • Spend the session on Python, which Justworks candidates report being tested on.
  • Write one worked example in Python and time yourself on it.

Deliverable: One timed worked example in Python.

Day 5 — Answer out loud: Coding and Algorithm Challenges

  • Answer aloud, timed: Implement a function to merge overlapping intervals from a given list of interval pairs (e.g., merge [[1,3],[2,6],[8,10]] into [[1,6],[8,10]]).
  • Answer aloud, timed: Design and implement a basic banking system that supports deposits, withdrawals, and transfers, handling multi-level complexity as requirements evolve.

Deliverable: Spoken answers to 2 reported Coding and Algorithm Challenges question(s), under time.

Day 6 — Answer out loud: System and API Design

  • Answer aloud, timed: Design the database schema and API endpoints for a collaborative board like Trello or Jira. Focus on data modeling, relationships, and state transitions.
  • Answer aloud, timed: Model the database schema for an employee management platform, explaining how you would handle complex organizational hierarchies.

Deliverable: Spoken answers to 2 reported System and API Design question(s), under time.

Day 7 — Answer out loud: Debugging and Code Analysis

  • Answer aloud, timed: Analyze a purposefully confusing script (such as an employee-manager tree printing script) to identify logical bugs, fix failing tests, and optimize performance.
  • Answer aloud, timed: Walk through a provided code snippet, explain what the code does, identify potential edge cases, and propose refactoring steps to improve readability.

Deliverable: Spoken answers to 2 reported Debugging and Code Analysis question(s), under time.

Behavioural preparation

Behavioural rounds judge the decision you made and what it cost.

How do you incorporate or facilitate Diversity, Equity, and Inclusion (DEI) in your daily engineering work and

Medium · Behavioral and Culture Fit

How do you incorporate or facilitate Diversity, Equity, and Inclusion (DEI) in your daily engineering work and team interactions?

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Describe a time when you had a conflict with a product manager or teammate. How did you resolve it, and what w

Medium · Behavioral and Culture Fit

Describe a time when you had a conflict with a product manager or teammate. How did you resolve it, and what was the outcome?

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Walk me through a complex technical project you led. What were the trade-offs you made, and what would you do

Medium · Behavioral and Culture Fit

Walk me through a complex technical project you led. What were the trade-offs you made, and what would you do differently today?

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?