Migrating Your JavaScript Project to TypeScript: A Practical 2026 Guide

Let me guess — you've got a JavaScript codebase that's grown way beyond what anyone planned, and now you're staring down bugs that a type system would've caught months ago. Yeah, we've been there. At our shop, we migrated a 200K-line Node.js project to TypeScript over six months, and I'm going to walk you through exactly how we did it without losing our minds.

Why Bother Migrating in 2026?

Look, TypeScript isn't new anymore. It's been around for over a decade. But the tooling in 2026 is lightyears ahead of what it was even two years ago. The compiler's faster, editor support is incredible, and honestly — most new npm packages ship types by default now.

But here's the real reason: developer velocity. After our migration, our team's bug rate on type-related issues dropped by about 60%. Code reviews got faster because reviewers could trust the types instead of manually tracing data flows. New hires onboarded in half the time because the types are the documentation.

That said, migration isn't free. It costs real time, and if you do it wrong, you'll make things worse before they get better.

The "Big Bang" vs. Incremental Approach

There are two schools of thought here, and I've tried both.

Big Bang Migration

You rename every .js file to .ts, crank the compiler settings to strict, and fix every error. This works if your project is under 10K lines and you've got a weekend to burn. For anything larger? Don't do it. Seriously.

Incremental Migration (The One That Actually Works)

This is what we did, and what I recommend for any team with a production codebase. The idea is simple: you set up TypeScript alongside your existing JavaScript, and you migrate file by file, module by module.

Here's the step-by-step:

• Step 1: Install TypeScript and create a tsconfig.json with allowJs: true and strict: false. This lets your existing JS files coexist with new TS files.
• Step 2: Pick a leaf module — something with few dependencies — and rename it to .ts. Add basic types. Don't overthink it.
• Step 3: Create a @types folder for declaration files. Any JS module that you haven't migrated yet gets a simple .d.ts stub so TypeScript doesn't complain.
• Step 4: Gradually work inward from the edges of your dependency graph. Utilities first, then services, then route handlers or components.
• Step 5: Once you're past 70-80% coverage, flip on strict: true and tackle the remaining issues.

Setting Up Your tsconfig.json Right

Your initial config should be permissive. I know that feels wrong if you're a TypeScript purist, but trust me — the goal is momentum, not perfection. Here's what we started with:

The key settings are allowJs (so you can mix JS and TS), checkJs set to false initially (so existing JS doesn't throw errors), and strict set to false. You'll tighten these over time.

One thing that caught us off guard: make sure your build tool supports TypeScript natively. If you're using Vite, you're golden — it handles .ts files out of the box. Webpack needs ts-loader or babel-loader with the TypeScript preset. And if you're on esbuild, it's basically zero config.

The Hardest Parts (Nobody Talks About These)

Third-Party Libraries Without Types

Even in 2026, you'll hit libraries that don't ship types. Check DefinitelyTyped first (@types/package-name), but if that fails, write a quick declaration file. Something like declare module 'some-library' is enough to unblock you. Don't spend hours writing perfect types for a library you might replace later.

The any Debate

Here's my controversial take: use any during migration. I know, I know. But hear me out. The goal of migration is to get TypeScript running across your codebase. You can come back and tighten types later. We actually tracked our any usage with a custom ESLint rule and chipped away at it over time. Started at 340 instances, got down to 12 within three months after the initial migration.

Testing During Migration

Your test suite is your safety net. Run it after every file migration. If you don't have good test coverage... well, that's a separate problem, but at minimum make sure your CI pipeline catches type errors. We added tsc --noEmit as a CI step from day one.

Common Pitfalls We Hit

• Circular dependencies surfaced by TypeScript: JS happily ignored these. TS won't. Budget time to untangle them.
• Dynamic property access: If your code does obj[someVariable] a lot, you'll need index signatures or type guards. This was our biggest time sink.
• Enum vs. union types: We started with enums, regretted it, switched to string union types. Unions are simpler and don't generate extra JavaScript.
• Overcomplicating generics: Early on, one of our devs went wild with generics. The types were technically correct but completely unreadable. Keep it simple. If a type takes more than 30 seconds to understand, it's too complex.

Tools That Made Our Lives Easier

A few tools genuinely saved us time:

• ts-migrate (from Airbnb): Automatically converts JS files to TS and adds any annotations where types can't be inferred. Great for the initial bulk conversion.
• TypeStat: Infers types from your runtime behavior and suggests type annotations. Not perfect, but a solid starting point.
• VSCode's built-in refactoring: The "Add missing properties" and "Infer type from usage" quick fixes are surprisingly good in 2026.

When NOT to Migrate

Real talk: not every project needs TypeScript. If you're building a quick prototype, a small script, or something that'll be thrown away in three months, the overhead isn't worth it. TypeScript shines in codebases that multiple people work on over extended periods. For a solo hackathon project? Plain JavaScript is fine.

The Payoff

Six months after completing our migration, here's what changed:

• Production bugs related to type mismatches: down 60%
• Average PR review time: down 25% (reviewers trust the types)
• New developer onboarding time: roughly halved
• Refactoring confidence: through the roof — rename a function and the compiler tells you every call site

Was it painful? Parts of it, absolutely. The circular dependency untangling alone took two weeks. But would I do it again? In a heartbeat.

Your Migration Checklist

• Install TypeScript, set up a permissive tsconfig.json
• Add tsc --noEmit to your CI pipeline immediately
• Start with leaf modules, work inward
• Use any as a temporary crutch — track and reduce it over time
• Don't migrate and refactor simultaneously — that's two changes at once and a recipe for bugs
• Run tests after every file migration
• Flip on strict mode once you're past 70% coverage
• Celebrate when you're done — seriously, it's a big deal

If you're staring at a JavaScript codebase and wondering whether the migration is worth it, the answer for most production projects is yes. Just do it incrementally, keep your tests green, and don't let perfect be the enemy of done.

Got questions about your specific migration scenario? Reach out to us — we've helped several teams through this exact process and we're happy to chat.