On Sunday March 6, we migrated Stripe’s largest JavaScript codebase (powering the Stripe Dashboard) from Flow to TypeScript. In a single pull reques

Migrating millions of lines of code to TypeScript

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2022-05-20 21:00:05

On Sunday March 6, we migrated Stripe’s largest JavaScript codebase (powering the Stripe Dashboard) from Flow to TypeScript. In a single pull request, we converted more than 3.7 million lines of code. The next day, hundreds of engineers came in to start writing TypeScript for their projects.

Seriously unreal. I remember a short time ago laughing at the idea of typescript ever landing at Stripe, and then I woke up Christmas Monday morning and it was here.

TypeScript is the de facto standard for JavaScript type checking, and our engineers have been overjoyed by this migration. We’re sharing our TypeScript conversion tool on GitHub to help others perform similar migrations.

Stripe has built large-scale frontend applications since 2012, including stripe.com, Stripe JS, and the Stripe Dashboard. As our company grew, we increased the quality and reliability of our products by type checking our JS code. In 2016, we were an early adopter of Flow, an optional type system for JavaScript developed at Meta (then Facebook). Since then, Flow has provided type safety for the majority of our frontend applications.

However, engineers had trouble working with Flow. The type checker’s memory usage would lock up laptops, and the in-editor integration was frequently slow and unreliable. Meanwhile TypeScript, an alternative type system developed at Microsoft, exploded in popularity thanks to its tooling and robust community. TypeScript availability became a top request among engineers at Stripe.