Have you had experience implementing TypeScript in an existing project?

Yes, the experience of implementing TypeScript in an existing project usually involves several stages:

1. Gradual migration — initially adding types to new modules or components, while leaving the old code in JavaScript.
2. Configuration setup — creating a tsconfig.json with parameters that allow gradually increasing typing (for example, allowJs: true, checkJs: false at the initial stage).
3. Using JSDoc and annotations — to facilitate migration, types can be added via JSDoc in the existing JS code.
4. Refactoring — as progress is made, typing becomes stricter, and type errors are fixed.
5. Integration with build tools — configuring Webpack, Babel, or other tools to support TypeScript.

Example of simple file conversion:

// Before (JavaScript)
function sum(a, b) {
  return a + b;
}

// After (TypeScript)
function sum(a: number, b: number): number {
  return a + b;
}

This approach helps minimize risks and gradually improve code quality.