Migrating from JavaScript to TypeScript | A Practical Guide

Have you ever asked yourself, “Why are so many dev teams suddenly talking about TypeScript?” Or wondered if our last blog on TypeScript vs. JavaScript still applies today, especially now in 2026?

Here’s the honest snapshot: JavaScript still runs virtually every website in the world. But as of 2026, TypeScript has grown in influence faster than almost any language in history. 

In August 2025, TypeScript overtook both Python and JavaScript to become the most used language by contributors on GitHub, adding over 1 million new developers this year alone – a 66% year-over-year growth that signals something big is happening in the ecosystem. 

So, let’s pause and ask a few blunt questions:

• Is sticking with plain JavaScript still enough for your team?
• Does it matter that major frameworks now scaffold new apps in TypeScript by default?
• And critically, can you really transition your existing codebase without massive disruption?

These questions matter because the way teams write, test, and ship code is changing fast. TypeScript isn’t just a “nice to have” anymore. It’s becoming the baseline choice for scalable, maintainable, and safer code, especially as generative AI takes a bigger role in how we build software. 

In your previous blog, you explored how TypeScript compares to JavaScript- what it is, where it shines, and where it doesn’t. In this follow-up, we’re going one step further. This time, we’re talking about actual migration: what it looks like in the real world; how to do it incrementally; how to avoid common pitfalls; and how to balance ongoing development with the transition.

If you’re ready to migrate JavaScript to TypeScript, not just talk about it, you’re in the right place. 

Let’s dive into a practical, step-by-step plan that keeps your app stable and your team productive.

Why Teams Are Moving from JavaScript to TypeScript?

Before jumping into tools and steps, it’s worth slowing down and asking a simple question: why are so many teams actually making this move?

Most migrations don’t start because TypeScript is “trendy.” They start because JavaScript, at scale, begins to show cracks. What works fine for small scripts often becomes painful in large, long-lived applications.

Here are the real reasons teams decide to migrate.

1. Fewer runtime errors (and fewer production surprises)

JavaScript fails at runtime. That means errors often show up only after code is deployed. By then, users are already affected.

TypeScript shifts many of those failures earlier. It flags missing properties, wrong function arguments, and unsafe assumptions while you write code, not after it ships. Developers see problems instantly in their editor or during builds.

For teams maintaining large applications, this alone is a strong reason to migrate JavaScript to TypeScript. Catching issues early saves debugging time, reduces hotfixes, and lowers the risk of production outages.

2. A noticeably better developer experience

This is one of those benefits that’s hard to explain until you feel it.

With TypeScript, editors like VS Code understand your code deeply. You get:

• Accurate autocomplete
• Inline type hints
• Clear warnings before something breaks
• Safer and faster refactoring

Instead of guessing what a function expects, developers see it immediately. Instead of searching documentation, the documentation lives in the code itself.

Over time, this improves focus and confidence. Teams spend less energy figuring things out and more time building features. That productivity gain is a big reason companies choose to migrate existing JavaScript project to TypeScript.

3. Easier long-term maintenance as projects grow

JavaScript codebases tend to age badly. As more people touch the code, assumptions pile up. Function behavior becomes unclear. Side effects sneak in.

TypeScript adds structure without forcing rigid patterns. Types act like living documentation. They explain how data flows through the system and what shapes objects are expected to have.

When someone new joins the team, they don’t have to guess how things work. They can read the types and understand the intent quickly. This matters a lot for long-running products where maintainability often matters more than initial development speed.

4. Safer refactoring in active codebases

Refactoring JavaScript can feel risky. You change one file and silently break another. Sometimes you don’t find out until much later.

TypeScript changes that dynamic. When you rename a property, change a function signature, or remove unused logic, the compiler tells you exactly what breaks. You get a clear checklist of fixes instead of hidden bugs.

For growing products with frequent updates, this safety net is essential. It’s one of the strongest reasons teams decide to move forward and migrate JavaScript to TypeScript instead of staying with plain JavaScript.

Step-by-Step: How to Migrate JavaScript to TypeScript?

Migrating doesn’t mean stopping development or rewriting everything. In practice, successful teams treat migration as a controlled, low-risk process. They move in small steps, validate constantly, and keep the app working at every stage.

Here’s how to do it in a way that actually works.

Step 1: Install TypeScript Without Changing Your Code

Start by adding TypeScript to your project. This step does not affect runtime behavior or existing JavaScript files.

At this stage, nothing breaks. Your JavaScript still runs as it always has. This is intentional. You’re preparing the ground before making changes.

Think of this step as setting up safety rails before climbing higher.

Step 2: Configure TypeScript for Migration, Not Perfection

Many teams fail here by enabling strict rules too early.

When you first migrate JavaScript to TypeScript, your goal is compatibility, not correctness. You want TypeScript to accept your existing JavaScript while gently pointing out issues.

Use a permissive configuration:

What this does:

• Allows JavaScript and TypeScript files to coexist
• Prevents TypeScript from blocking builds
• Lets you migrate incrementally

This setup is what makes it realistic to migrate existing JavaScript project to TypeScript without disrupting daily work.

Step 3: Rename One File, Not the Whole Project

Now comes the first real conversion.

Pick a small, low-risk file. Rename it from .js to .ts.

That’s it.

You don’t need to add types immediately. The file may compile with warnings, and that’s fine. The key is to prove that TypeScript can live inside your project without causing chaos.

This small win builds confidence and sets the tone for the rest of the migration.

Step 4: Add Types Where They Deliver Immediate Value

Once a file is converted, start adding types selectively.

Focus on:

• Function parameters
• Function return values
• Shared utilities
• API response shapes

Example:

These types act like guardrails. They prevent incorrect usage and make intent clear.

You don’t need to type everything. The goal is clarity, not verbosity. This approach keeps converting js to typescript practical instead of overwhelming.

Step 5: Use any Strategically (Not Emotionally)

Many developers treat any like a failure. During migration, it’s a tool.

let config: any;

Use any when:

• You’re dealing with legacy logic
• You don’t fully understand the data yet
• You want to keep momentum

The key is intention. Leave comments. Track where any exists. Replace it gradually as understanding improves.

This flexibility is what allows teams to convert js code to typescript without stalling progress.

Step 6: Handle Third-Party Libraries Early

External libraries often cause the first real friction.

For most popular libraries, install type definitions:

npm install –save-dev @types/library-name

If types don’t exist:

• Create minimal custom types
• Use any temporarily
• Type only what you use

Ignoring this step leads to frustration later. Addressing it early keeps your migration on track.

Step 7: Gradually Increase Type Strictness

Once a significant part of the codebase is in TypeScript, it’s time to tighten rules.

Do this gradually:

• Enable noImplicitAny
• Enable strictNullChecks
• Finally, enable full strict mode

Each step increases safety without overwhelming the team.

This phase is where TypeScript’s real benefits become obvious, especially during refactoring and feature expansion.

Step 8: Convert Core Business Logic First

Not all files are equal.

Prioritize:

• Business logic
• Data models
• Shared services
• Critical workflows

UI components and edge scripts can wait. Typing the core logic first delivers faster returns and reduces risk across the system.

This strategy works especially well when migrating JavaScript applications to TypeScript in production environments.

Step 9: Keep Shipping While Migrating

This is important.

Migration should not pause feature development. Teams that succeed:

• Migrate alongside regular work
• Convert files touched by new features
• Improve types as part of normal refactoring

TypeScript becomes part of daily development, not a separate task.

Step 10: Treat Migration as a Continuous Improvement Process

Migration doesn’t end with “everything is .ts.”

Even after full conversion:

• Improve types
• Remove unnecessary any
• Simplify complex type definitions
• Align with evolving standards

This mindset keeps the codebase healthy long after you migrate JavaScript to TypeScript.

When Should You Not Migrate?

Migrating to TypeScript is a smart move for many teams, but it’s not always necessary. Every JavaScript to TypeScript migration guide should be honest about this. Migration takes time, focus, and learning effort. If those costs outweigh the benefits, staying with JavaScript may be the better decision.

Here are situations where it may not make sense to migrate JavaScript to TypeScript.

1. The project is very small and already stable

If your application is small, has limited logic, and rarely changes, migrating may offer little real value.

TypeScript shows its strengths in larger systems, especially when refactoring, scaling, or onboarding new developers. In a small codebase, the effort to convert js code to typescript may feel like extra work with minimal payoff.

In these cases, continuing with JavaScript is often more practical than trying to migrate existing JavaScript project to TypeScript.

2. No future development is planned

Migration is an investment. It pays off over time, not instantly.

If your application is in maintenance mode or close to being retired, there’s little reason to start migrating JavaScript applications to TypeScript. The team won’t benefit from improved refactoring, stronger typing, or long-term maintainability if the code isn’t evolving.

For projects with no roadmap ahead, it’s usually better to keep the code stable and avoid structural changes.

3. The team lacks TypeScript experience and timelines are tight

While TypeScript builds on JavaScript, it still introduces new concepts—types, interfaces, generics, and strictness rules.

If your team has no prior experience and is under tight deadlines, attempting to migrate JavaScript to TypeScript can slow delivery. Developers may rely too heavily on any struggle with type errors or lose momentum.

In this scenario, a better approach may be to:

• Use TypeScript for new projects
• Train the team gradually
• Plan converting js to typescript when timelines allow

Conclusion

Migrating from JavaScript to TypeScript is not about chasing trends. It’s about making your codebase safer, easier to understand, and more resilient as it grows. When done gradually, migration reduces bugs, improves developer confidence, and makes long-term maintenance far less painful.

The key takeaway is simple: you don’t need to rewrite everything. You can migrate JavaScript to TypeScript one file, one feature, and one improvement at a time. By starting small, prioritizing core logic, and increasing strictness gradually, teams can modernize their applications without disrupting ongoing development.

That said, migration is not just a technical task. It’s a planning exercise. It requires understanding your existing architecture, your team’s skill level, and your product roadmap. When those factors align, migrating JavaScript applications to TypeScript becomes a strategic upgrade rather than a risky overhaul.

If you’re planning to migrate existing JavaScript project to TypeScript and want expert guidance, EitBiz can help. Our experienced development team works with businesses to plan, execute, and optimize TypeScript migrations, without breaking production systems or slowing delivery.

Get in touch with EitBiz today to discuss your JavaScript to TypeScript migration strategy and move forward with confidence.

FAQs

1. Why should businesses migrate from JavaScript to TypeScript?

Businesses migrate to reduce bugs, improve code quality, and make large applications easier to maintain. TypeScript also helps teams onboard faster by making code easier to understand.

2. Can JavaScript be converted to TypeScript without rewriting the entire app?

Yes. TypeScript allows incremental adoption. You can convert js code to typescript file by file while keeping the rest of the app in JavaScript.

3. Does migrating from JavaScript to TypeScript impact application performance?

No. TypeScript compiles to JavaScript and does not affect runtime performance.

4. What are the biggest challenges when migrating JavaScript to TypeScript?

Common challenges include legacy code cleanup, learning TypeScript syntax, managing strictness levels, and updating build tools.