When working with external JavaScript libraries that don't provide TypeScript definitions, maintaining type safety in your codebase becomes challenging. Here’s how you can safely use those libraries:

1. Use TypeScript’s declare Keyword
   • You can declare the library and its methods as any to suppress errors, but it's better to add as much detail as possible.
   • Example:

 declare module 'legacy-lib' {
 export function doSomething(param: string): number;
 }

1. Write Custom Type Declarations
   • Create a *.d.ts file with more precise type information based on the library’s documentation or source code.
   • Example:

 // legacy-lib.d.ts
 declare module 'legacy-lib' {
 export function initialize(config: object): void;
 export function shutdown(): void;
 }

1. Use Type Assertions When Necessary
   • If you know the type of a result, but TypeScript can't infer it, use type assertions.
   • Example:

 const lib = require('legacy-lib') as {
 doSomething: (param: string) => number
 };
 const result = lib.doSomething('input');

1. Search for Community Types

   • Sometimes, the type definitions exist under @types/<library> on npm.
   • Example:
     • Install types using: npm install --save-dev @types/legacy-lib

2. Use any as a Last Resort

   • Avoid using any for entire libraries unless absolutely unavoidable, as this defeats the purpose of TypeScript.

Best Practice: Always incrementally improve your custom typings as you use more of the library.

Summary of Approach

• Create custom .d.ts files when types do not exist
• Prefer explicit type definitions to any
• Regularly update types as usage evolves

By carefully declaring types and updating them, you ensure your TypeScript project remains robust even when using untyped libraries.