---
title: "Migrating a Large Flow Monorepo to TypeScript"
url: https://daily.dev/posts/migrating-a-large-flow-monorepo-to-typescript-nax4shetx
source_url: https://engineeringblog.yelp.com/2026/08/migrating-a-large-flow-monorepo-to-typescript.html
type: article
source: "Yelp Engineering"
published: 2026-08-04T18:07:49.926Z
updated: 2026-08-04T18:42:45.588Z
tags: ["general-programming", "typescript"]
reading_time: 12
upvotes: 3
comments: 1
language: en
---

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# Migrating a Large Flow Monorepo to TypeScript

**[Yelp Engineering](https://daily.dev/sources/yelp)** · 12 min read · 3 upvotes · 1 comments

## Summary

Yelp's Webcore team spent three and a half years migrating over 1.4 million lines of code from Flow to TypeScript across a large Lerna monorepo. The migration preserved type safety throughout by converting one package at a time, using open-source tools like flowts and Stripe's flow-to-typescript-codemod, and generating Flow-compatible type headers from TypeScript declarations via flowgen to maintain interop. Extensive dogfooding, documentation, and tooling automation preceded the general rollout in May 2023. A dependency-graph scoring system helped prioritize high-impact packages when momentum slowed. The migration concluded in February 2026 with 96.44% type coverage (up from 83.15% with Flow), roughly 6,000 suppression directives, and 82% of engineers reporting productivity gains.

## Full article

daily.dev links to this article rather than hosting it. Read it at the original source: <https://engineeringblog.yelp.com/2026/08/migrating-a-large-flow-monorepo-to-typescript.html>

## Community discussion

Top comments from developers on daily.dev.

**@pdfopsdev** · 0 upvotes

> 3.5 years for 1.4M lines tracks. we've seen flow->ts migrations stall for years when teams try to keep both type checkers green at once instead of doing lossy any-casts and backfilling later. curious if they hand rewrote the trickier generics or leaned on a codemod.

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---

Tags: [#general-programming](https://daily.dev/tags/general-programming), [#typescript](https://daily.dev/tags/typescript)

[View this post on daily.dev](https://daily.dev/posts/migrating-a-large-flow-monorepo-to-typescript-nax4shetx)