First, configure tsconfig.json that supports progressive migration, and enable key options such as allowJs, checkJs and strict; 2. Adopt file-by-file migration strategy, prioritize conversion of tool files or combine with JSDoc to obtain type checking benefits in advance; 3. Respond to common problems such as implicit any, third-party library missing types, dynamic attribute access and circular dependencies; 4. Integrate type checking into lint, editor and CI processes to ensure quality; 5. Track progress through statistics of .ts file proportions and errors, maintain team motivation, and ultimately achieve a safer and maintainable code base.

Migrating a large JavaScript codebase to TypeScript isn't about rewriting everything overnight—it's a strategic, incremental process that balances progress with productivity. The goal is to gain TypeScript's benefits (better tooling, fewer runtime errors, improved maintenance) without stalling development. Here's how to do it effectively.

1. Start with a Solid TypeScript Configuration

Before touching any files, set up a tsconfig.json that reflects your project's reality—not an ideal world. Use settings that allow graduate adoption:

 {
  "compilerOptions": {
    "target": "ES2020",
    "module": "commonjs",
    "allowJs": true,
    "checkJs": true,
    "outDir": "./dist",
    "rootDir": "./src",
    "strict": true,
    "noEmitOnError": false,
    "skipLibCheck": true,
    "esModuleInterop": true,
    "allowSyntheticDefaultImports": true,
    "forceConsistentCasingInFileNames": true
  },
  "include": ["src"],
  "exclude": ["node_modules", "dist"]
}

Key points:

• allowJs: true lets you mix .js and .ts files.
• checkJs: true enables type checking in .js files (with JSDoc support).
• strict: true is worth enabling from day one—it prevents weak typing later.
• Don't enable noEmitOnError early; it can block builds during migration.

You can start with checkJs: false if the codebase is too noisy, but turn it on once you've cleaned up the worst issues.

2. Adopt a File-by-File Migration Strategy

Trying to convert the whole codebase at once is a recipe for burnout. Instead, use one of these practical approaches:

? Incremental Conversion (Recommended)

• Rename one file at a time from .js to .ts (or .tsx for React).
• Fix the type errors TypeScript flags.
• Commit each converted file separately so issues are easier to track.

Pro tip: Start with utility files (pure functions, helpers)—they're usually self-contained and easier to type.

? Team-Based Ownership

• Assign ownership of modules or directories to teams.
• Encourage converting files as they touch them during feature work or bug fixes.
• Add a rule: “If you modify a file, consider converting it to TypeScript.”

? Use JSDoc for Early Gains

Even before renaming files, add JSDoc annotations to JavaScript files:

 /**
 * @param {string} name
 * @param {number} age
 * @returns {boolean}
 */
function canVote(name, age) {
  return age >= 18;
}

With checkJs: true , TypeScript will validate these types. This gives you 70% of the safety with minimal effort.

3. Handle Common Migration Challenges

Large codebases have patterns that trip up TypeScript. Be ready for these:

? Implicit any Everywhere

TypeScript will complain about untyped variables. You have options:

• Add explicit types (best long-term).
• Use @ts-ignore or @ts-expect-error sparingly (avoid debt).
• Temporarily allow noImplicitAny: false —but set a goal to remove it.

? Third-Party Libraries

Not all npm packages have good type definitions:

• Use @types/* when available.
• Write your own .d.ts files for internal or poorly-typed libraries.
• Declare globals as needed:

 declare module 'my-legacy-lib';

? Dynamic Object Access

JavaScript often uses dynamic keys: obj[someKey] . TypeScript may not know the shape.

• Use index signatures: { [key: string]: string }
• Or assert types carefully: (obj as Record<string, string>)[key]

? Complex or Circular Dependencies

Refactor as you go. TypeScript makes circular dependencies more obvious.

• Break large files into smaller, focused modules.
• Use barrel files ( index.ts ) to manage exports cleanly.

4. Integrate with Your Tooling

Make TypeScript part of your daily workflow:

• Add tsc --noEmit to your lint/pre-commit step to catch errors early.
• Use editors like VS Code—they highlight type errors in real time.
• Pair with ESLint ( @typescript-eslint ) for consistent style and advanced rules.

Example script:

 "scripts": {
  "type-check": "tsc --noEmit",
  "lint": "eslint src --ext .ts,.tsx"
}

Run type-check in CI to prevent regressions.

5. Measure Progress and Stay Motivated

Migration can feel endless. Track progress to stay on course:

• Count how many .ts / .tsx files you have vs. .js .
• Monitor reduction in type errors over time.
• Celebrate milestones: “50% converted!” or “No new @ts-ignore s this week!”

Avoid perfectionism. The goal is better , not perfect .

Bottom line: Migrating a large JavaScript codebase to TypeScript is a marathon, not a sprint. Start small, enforce consistency, and let the benefits compound over time. With the right setup and mindset, you'll end up with a codebase that's easier to navigate, safer to change, and more welcome to new developers.

Basically, just keep moving forward—one file at a time.

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