JavaScript to TypeScript Transition

TODO: collapasable, copyable, download, etc

If I have a Typescript type but most of my code is javaScritp, how can I get the typechecking benefits of turning an any into a specific type? Can I get it to throw a helpful error if the data doesn't match?Integrating TypeScript type checking into a primarily JavaScript project to enforce specific types (like turning an any type into a specific type) and to get helpful error messages when data doesn't match expected types can significantly improve code reliability and maintainability. Here are steps and tools you can use to achieve this:

1. Use JSDoc with TypeScript Type Checking

Even if most of your codebase is in JavaScript, you can leverage TypeScript for type checking by using JSDoc annotations. TypeScript can interpret JSDoc comments, allowing you to specify types in your JavaScript files.

1. Enable TypeScript Checking in JS Files: First, you need to configure TypeScript to check your JavaScript files. Create a tsconfig.json in your project root with the following content:

   {
     "compilerOptions": {
       "allowJs": true,
       "checkJs": true,
       "noEmit": true,
       "strict": true,
       "baseUrl": "./",
       "paths": {
         "*": ["types/*"]
       }
     },
     "include": [
       "src/**/*" // Adjust this path to include your JavaScript files
     ]
   }
   

2. Use JSDoc to Annotate Types: In your JavaScript files, use JSDoc comments to specify types. For example:

   /**
    * @param {string} name - The name of the person.
    * @returns {void}
    */
   function greet(name) {
     console.log(`Hello, ${name}!`);
   }
   

3. Check Types: You can then run the TypeScript compiler (tsc) to check for type errors in your JavaScript files. TypeScript will interpret the JSDoc comments and perform type checking accordingly.

2. TypeScript for Type Definitions

For more complex scenarios where you might want to define custom types or interfaces that can be reused across your JavaScript files, you can still write these definitions in TypeScript.

1. Define Types in .d.ts Files: Create .d.ts files in your project where you define your custom types or interfaces. For example, in a types directory:

   // types/person.d.ts
   interface Person {
     name: string;
     age: number;
   }
   

2. Reference Types in JSDoc: Use the defined types in your JSDoc comments to annotate JavaScript variables or functions.

   /**
    * @param {Person} person - The person object.
    * @returns {void}
    */
   function greet(person) {
     console.log(`Hello, ${person.name}!`);
   }
   

3. Runtime Validation

TypeScript's type checking is static and happens at compile time. If you need runtime validation (for example, to validate data coming from an external API), consider using a runtime validation library like io-ts or ajv that can check data structures against defined schemas and throw errors if the data doesn't match.

4. Gradually Adopt TypeScript

If feasible, consider gradually converting parts of your JavaScript project to TypeScript. Even converting critical modules to TypeScript can provide significant benefits in type safety and can coexist with your JavaScript code.

Conclusion

By using JSDoc for type annotations, enabling TypeScript to check your JavaScript files, defining complex types in .d.ts files, and employing runtime validation libraries, you can enjoy the benefits of type safety and better error handling in a primarily JavaScript codebase. This approach allows you to gradually adopt TypeScript's features without a complete rewrite of your existing code.

JavaScript to TypeScript Transition created at Tue Jan 20 1970 18:42:51 GMT+0000 (Coordinated Universal Time)