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Comparison

Best tools to migrate JavaScript to TypeScript

There is no single best tool, because migrations differ in size, in how much you trust automation and in how much data you can share with an AI model. Here are the real options, what each does well and where each stops. We build CodeMorph, which is on the list, so weigh our view accordingly.

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How to migrate step by step

Windows 10/11 · Free during beta · Bring your own Claude key or use Ollama

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At a glance

The options compared

ToolWhat it isCostBest forWatch out for
The TypeScript compiler (tsc)The compiler, used by handFreeSmall projects and learning the codebaseSlow at scale, and nothing sets the order of work
Editor quick fixesTypeScript language service suggestions in your editorFreeTyping one function or file at a timeLocal suggestions, not a migration plan
ts-migrateCommand-line codemod toolkit from AirbnbFree, open sourceGetting a large codebase compiling fastLeaves any and // @ts-expect-error to fix later
TypeStatCommand-line tool that adds and improves typesFree, open sourceInferring types from how code is usedCheck its repository for current activity
AI coding agentsChat and agent tools such as Claude Code, Cursor and GitHub CopilotTheir own pricingSingle files and modules, and hard casesContext per file, and checking is up to you
CodeMorphDesktop editor with a dependency-ordered, type-checked AI migrationFree during the betaReal types with every batch checked and reversibleBeta, Windows and Mac only, needs an AI key or Ollama

The order is not a ranking.

The details

Each option in more detail

1. The TypeScript compiler, by hand

Add a tsconfig.json with allowJs, rename files one at a time and let tsc tell you what is wrong. It is the baseline every other tool builds on.

Strengths
Full control, nothing to install beyond TypeScript, and you learn how the code fits together.
Limits
Slow on a large codebase, and migrations often stall halfway. You have to decide the order of work yourself.
Best for
Small projects, and a few modules you want to get exactly right.

TypeScript: Migrating from JavaScript

2. Editor quick fixes

TypeScript's language service, available in editors such as VS Code, offers quick fixes like inferring a parameter's type from how it is used.

Strengths
Free, instant, and built into the editor you already use.
Limits
They work on one spot at a time. They suggest types but do not plan a migration or check the whole project.
Best for
Typing a function or a file you are already editing.

3. ts-migrate

An open-source command-line tool from Airbnb. Its README says it takes a JavaScript, or partial TypeScript, project in and gives a compiling TypeScript project out.

Strengths
Free, offline, scriptable, and fast on a big codebase. No AI model needed. It commits after each major step in the full migration.
Limits
By its README, there will be lots of // @ts-expect-error and any to fix over time. The result builds, but a follow-up is required to improve type safety.
Best for
A very large codebase where compiling soon matters more than first-pass type quality.

ts-migrate on GitHubCodeMorph vs ts-migrate

4. TypeStat

An open-source command-line tool that describes itself as converting JavaScript to TypeScript and TypeScript to better TypeScript. It infers types to fix noImplicitAny problems and adds missing null and undefined types for strict null checks.

Strengths
Free, and its built-in fixers only add or remove types, so they do not change runtime behavior. It is a good second pass after a first conversion.
Limits
You set it up with a configuration file. Check its repository for current activity before you rely on it.
Best for
Tightening types after files are already TypeScript, or inferring types from usage.

TypeStat on GitHub

5. AI coding agents and assistants

Tools such as Claude Code, Cursor and GitHub Copilot can convert a file or a module when you point them at it, and an agent can run tsc and iterate on the errors.

Strengths
They understand intent, so they can write real types and sensible names, and they handle awkward cases that a codemod cannot.
Limits
They work with the context you give them, so a model can invent the shape of an import it cannot see. Order, whole-project verification and a way back are up to you.
Best for
Single files and modules, tricky code, and small projects.

6. CodeMorph

A desktop code editor built for this one job. It maps your dependency graph, plans leaf-first batches, converts each batch against imports that are already typed and type-checks the whole project after every batch.

Strengths
Ordered, checked and reversible: every file is a reviewable diff, and Undo all restores the project byte for byte from a checkpoint saved first. Free during the beta.
Limits
It is a beta for Windows and Mac, needs an AI provider key or local models with Ollama, and is an editor rather than a CI tool. Running your tests and build from CodeMorph is coming soon.
Best for
A real codebase where you want an AI migration with real types and every step checked.

How CodeMorph migratesCodeMorph vs ts-migrate

Choosing

How to choose

Size of the codebase

Under a few dozen files, do it by hand or with an AI assistant. Into the hundreds or thousands, you want a tool that plans the order of work and checks the whole project.

Quality you need on day one

If compiling soon is enough, ts-migrate is built for that. If you want real types from the start, look at an AI migration that type-checks every batch.

Where your code can go

ts-migrate and TypeStat run entirely on your machine. AI tools send code to a model provider, unless you run a local model, for example with Ollama.

How much checking you want to do

A chat assistant leaves the checking to you. A tool that runs the compiler after each batch, and lets you review and undo, takes that work on.

Want the full walkthrough? Read the step-by-step guide.

Questions about migration tools

What is the best tool to migrate JavaScript to TypeScript?
It depends on the project. For a very large codebase that just needs to compile, ts-migrate. For real types with every batch checked and reversible, an AI migration tool such as CodeMorph. For a small project or a single module, the compiler by hand or an AI assistant.
What tool converts JS to TS automatically?
ts-migrate and TypeStat are open-source command-line tools. AI coding agents can convert files you point them at. CodeMorph plans an AI migration in dependency order and type-checks every batch.
Is there a free tool to convert JavaScript to TypeScript?
Yes. ts-migrate and TypeStat are free and open source, and the TypeScript compiler is free. CodeMorph is free during the beta, and you pay your AI provider for model usage, or use local models with Ollama.
Can AI convert JavaScript to TypeScript?
Yes. A model can convert a file well when it has the context. For a whole project, use a tool or a workflow that fixes the order of work and runs the compiler over the whole project after each batch.
Do these tools replace code review?
No. A type-check proves types are consistent, not that behavior is unchanged. Review the changes and run your tests after each batch.

Try CodeMorph on a copy of one folder.

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Download CodeMorph (Beta)

Leave your email, then pick your computer. We'll only write about the beta and new releases.

Read the docs

Windows 10/11 · Free during beta · Bring your own Claude key or use Ollama

Windows may show 'unknown publisher'. Click More info, then Run anyway.