Lammert Westerhoff, Developer in Puebla, Mexico
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Lammert Westerhoff

Verified Expert  in Engineering

Full-stack Developer

Puebla, Mexico
Toptal Member Since
January 11, 2021

Lammert is a full-stack developer with an entrepreneurial mindset specializing in iOS development. Due to collaboration with clients such as the Dutch railways, ING, and Eneco, his work has reached millions of people. Additionally, he is experienced with Java, Kotlin, and React Native and has extensive knowledge of AWS. As a certified scrum master, Lammert has also led teams on several projects.


Fifty Fifty Card
Amazon Web Services (AWS), Accessibility, Agile Software Development, Android...
Evervault Limited
iOS, Kotlin, Android, Swift, Mobile App Development, Android SDK, iOS SDK...
iOS, Swift, Android, Kotlin, TypeScript, Bluetooth, iOS SDK, Bluetooth LE...




Preferred Environment

Amazon Web Services (AWS), Slack, IntelliJ IDEA, Xcode, MacOS

The most amazing...

...project I've developed was an app for Dutch railways, one of the most used apps in the Netherlands that won the award for the best transportation app.

Work Experience

CTO and Full-stack Developer

2021 - PRESENT
Fifty Fifty Card
  • Developed a native iOS and Android app with Kotlin Multiplatform Mobile (KMM) that allows users to sign up and make reservations at restaurants.
  • Created a scalable server with Kotlin and Ktor running on AWS Elastic Beanstalk and Postgres database.
  • Contributed to the design of user experience and mobile app from the ground up.
  • Created a second mobile app for restaurant managers and staff to manage reservations and restaurants.
Technologies: Amazon Web Services (AWS), Accessibility, Agile Software Development, Android, Architecture, Location Services, Core Location, iOS, Kotlin, Kotlin Multiplatform Mobile (KMM), Images, Stripe API, Stripe, Ktor, Startups, Firebase, REST APIs, SwiftUI, Mobile, Back-end, DevOps, Clean Architecture, GitHub, React, Figma, Mobile App Development, Android SDK, Encryption, JavaScript, Full-stack, SQL

iOS and Android Developer

2023 - 2023
Evervault Limited
  • Developed an iOS and Android SDK to encrypt data with the Evervault platform.
  • Created a secure Inputs library using SwiftUI for iOS and Compose for Android to process and encrypt credit card information.
  • Integrated a Rust library with the iOS and Android SDKs to create a secure connection to Evervault servers.
Technologies: iOS, Kotlin, Android, Swift, Mobile App Development, Android SDK, iOS SDK, Encryption, Data Encryption

Lead Mobile Developer

2021 - 2023
  • Developed a KMM app for iOS and Android for patient monitoring. The app connects to a wristband through Bluetooth to collect activity information, present this information in the app, and show feedback from doctors.
  • Acted as the lead developer by introducing, instructing, and guiding Android developers on KMM development.
  • Developed a KMM app for surgical planning. Surgeons can use the app to approve surgical plans by inspecting Digital Imaging and Communications in Medicine (DICOM) files and chatting with planners to approve or decline plans.
  • Created a KMM framework to manage network requests, cache, and view models.
Technologies: iOS, Swift, Android, Kotlin, TypeScript, Bluetooth, iOS SDK, Bluetooth LE, Mobile Architecture, Technical Leadership, Architecture, Accessibility, Firebase, REST APIs, Node.js, SwiftUI, Healthcare, Mobile, Back-end, DevOps, Clean Architecture, Serverless, Figma, Mobile App Development, Android SDK, JavaScript, Full-stack, SQL

Freelance iOS Developer

2019 - 2023
  • Developed a new Eneco native iOS app using KMM and Swift.
  • Launched the app eight months after the project started and attracted hundreds of thousands of users.
  • Created a tool to manage localization and strings in one place for both iOS and Android.
Technologies: iOS, Kotlin, Swift, Android, iOS SDK, Mobile Architecture, Technical Leadership, Architecture, Accessibility, Firebase, SwiftUI, Mobile, Clean Architecture, Figma, Mobile App Development

Full-stack Developer Consultant

2018 - 2019
Dutch Railways
  • Developed the NS app for iOS which was used by millions of people.
  • Developed and managed several REST API services in Kotlin and ran them on AWS.
  • Developed and launched the NS International app targeted to international travelers.
Technologies: Kotlin, Amazon Web Services (AWS), iOS, Swift, iOS SDK, GPS, Mobile Architecture, Architecture, Location Services, Accessibility, Firebase, REST APIs, Mobile, Back-end, DevOps, Serverless, GitHub, Mobile App Development, Encryption, Full-stack, SQL

Senior Developer

2016 - 2018
Nyon (now Fashion Cloud)
  • Completely migrated the Nyon OrderWriter iPad app from Objective-C to Swift.
  • Created a server-side ordering pipeline for fashion retailers to manage their orders.
  • Established the hiring process for new developers and coached junior developers.
Technologies: Amazon Web Services (AWS), Django, Python, Objective-C, Swift, iOS, iOS SDK, Mobile Architecture, Technical Leadership, Architecture, Accessibility, Firebase, REST APIs, Mobile, Back-end, DevOps, Mobile App Development, Full-stack

Senior Software Developer Consultant

2016 - 2017
ING Netherlands
  • Developer the ING Beleggen (Trading) apps for iOS and Android using React Native.
  • Introduced a model-view-viewmodel (MVVM) architecture into the app.
  • Introduced a new design review process between developers and designers.
Technologies: React Native, iOS, iOS SDK, Architecture, Accessibility, REST APIs, Mobile, Back-end, Mobile App Development, JavaScript, Full-stack

Software Developer Consultant

2013 - 2017
  • Consulted for four big clients of Xebia, namely XS4All, ING (twice), Wehkamp, and Dutch Railways.
  • Initiated and organize dthe first Dutch AngularJS Conference: NG-NL.
  • Created a conference app and CMS for Xebia's conferences.
  • Initiated and organized the first Xebia School Students Innovation Day.
  • Completed the Certified Scrum Master training by the Scrum Alliance.
Technologies: Kotlin, React Native, AngularJS, Amazon Web Services (AWS), Swift, iOS, iOS SDK, GPS, Architecture, Location Services, Accessibility, REST APIs, Node.js, Mobile, Back-end, Clean Architecture, GitHub, MERN Stack, MongoDB, Mobile App Development, JavaScript, Full-stack, SQL

Co-founder and Senior Full-stack Developer

2012 - 2013
  • Developed an app where users could share photos of a particular outfit to get fashion advice from friends, family, and expert stylists.
  • Built a full-stack small social network on AWS with a native iOS app using Java.
  • Brainstormed with other co-founders to create a business plan, pitch deck, and initial feature set for the app.
Technologies: Accessibility, Amazon Web Services (AWS), Architecture, iOS, iOS SDK, Startups, REST APIs, Mobile, Back-end, Mobile App Development, SQL

Senior Software Developer

2009 - 2012
  • Successfully led several Scrum teams as tech lead and scrum master.
  • Worked within Infor's first mobile team and launched several iOS apps to customers.
  • Created a mobile platform and gave worldwide training to other Infor employees.
Technologies: Spring, Objective-C, iOS, .NET, Java, iOS SDK, Technical Leadership, Architecture, REST APIs, Mobile, Back-end, C++, Mobile App Development, SQL, Infor

Software Engineer

2006 - 2009
Asset Control (now Alveo)
  • Created several financial data loaders of financial data vendors under which the Bloomberg data loader.
  • Developed a platform for loading financial data instruments for various vendors.
  • Completed the training for Scrum team members and helped introduce Scrum into asset control processes.
Technologies: Java, Architecture

Eneco Energy App for iOS
A KMM and Swift-based app for Eneco customers to obtain insights into their energy usage and manage their Eneco services, such as seeing their invoices and managing their monthly payment amounts. This app version was rewritten entirely from scratch, and I was one of the two iOS developers playing a major role in determining the app's architecture.

NS Travel Planner App for iOS
One of the most used apps in the Netherlands and the winner of the best Dutch transportation app. It was completely written in Swift. I worked on both iOS and several server components written in Kotlin and running on AWS.

NS International iPhone App
This app is the little brother of the NS Travel Planner app, which I also worked on. However, the NS International app targets international travelers and has a different booking system, back end, and design. At the time of the project, I was the leading iOS developer of the app.

ING Investing - Beleggen App for iOS and Android
The official ING app for trading stocks. It is written using React Native for iOS and Android. I worked on parts of the server written in Java. Also, I contributed to other major features of the app, such as chart components, login screens, buying stocks screens, and stock portfolios.

Health Tracking and Patient Monitoring App
An iOS and Android app that connects to medical devices with Bluetooth, such as a wristband, to collect health information. This information is shared with a physician on a web portal for patient monitoring purposes. The native apps are built using SwiftUI and Compose on top of a shared layer written with KMM.

Fifty Fifty Card for iOS and Android
A native iOS and Android app built using KMM. It allows users to subscribe and make reservations at restaurants with a 50% discount on food. The back end runs on AWS using Ktor and a Postgres database. I am one of the co-founders and the sole developer of this project.

Fifty Fifty Serve for iOS and Android
As one of the co-founders and a sole developer, a built a native iOS and Android app using KMM. It is the restaurant owners' app for the Fifty Fifty Card app. With this app, restaurant owners can manage reservations and edit restaurant details. The back end is running on AWS with Ktor and a Postgres database.
2012 - 2016

Bachelor's Degree in Computer Science

Hanze University Groningen - Groningen, The Netherlands


Certified Scrum Master

Scrum Alliance


REST APIs, Node.js, React, Stripe, Stripe API


Xcode, GitHub, Slack, Figma, IntelliJ IDEA, Composer


iOS SDK, SwiftUI, Android SDK, React Native, AngularJS, Spring, Django, .NET, Core Location, Ktor


Java, Swift, Kotlin, Objective-C, JavaScript, SQL, Python, TypeScript, C++


Agile Software Development, Scrum, DevOps, Clean Architecture


iOS, Firebase, Mobile, MacOS, Amazon Web Services (AWS), Android, Bluetooth LE

Industry Expertise



MongoDB, Databases, PostgreSQL


Web Services, Scrum Master, Kotlin Multiplatform Mobile (KMM), Mobile Architecture, Technical Leadership, Architecture, Location Services, Accessibility, Back-end, Mobile App Development, Full-stack, Bluetooth, GPS, Serverless, Encryption, Startups, Images, MERN Stack, Data Encryption, Infor

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