How to Use the TypeScript Compiler’s Typechecker to Get the Resolved Type When the Types are Defined in a Different File

If you’ve been working with TypeScript, you may have encountered a situation where you need to access the resolved type of a variable or expression that is defined in a different file. In this blog post, we will explore how you can use the TypeScript compiler’s typechecker to achieve this.

Option 1: Importing the Type

One way to get the resolved type when the types are defined in a different file is by importing the type using the import statement. This allows you to use the imported type in your current file.

Let’s say you have a file named types.ts that defines a type called MyType:

// types.ts
export type MyType = {
  id: number;
  name: string;
};

In your current file, you can import the MyType type and use it as follows:

// currentFile.ts
import { MyType } from './types';

const myVariable: MyType = {
  id: 1,
  name: 'TypeScript',
};

By importing the MyType type from the types.ts file, you can now use it to define the type of myVariable.

Option 2: Using Triple-Slash Directives

Another way to get the resolved type when the types are defined in a different file is by using triple-slash directives. Triple-slash directives are special comments that instruct the TypeScript compiler to include additional files.

In your current file, you can use a triple-slash directive to reference the file that defines the types:

// currentFile.ts
/// 

const myVariable: MyType = {
  id: 1,
  name: 'TypeScript',
};

By referencing the types.ts file using a triple-slash directive, you can now use the MyType type to define the type of myVariable.

Conclusion

When working with TypeScript, it is common to encounter scenarios where you need to access the resolved type when the types are defined in a different file. In this blog post, we explored two options to achieve this: importing the type using the import statement and using triple-slash directives to reference the file that defines the types.

Both options provide a way to access the resolved type and enable you to write type-safe code in TypeScript.