TS1448: '{0}' resolves to a type-only declaration and must be re-exported using a type-only re-export when 'isolatedModules' is enabled

Understanding TypeScript and Types

TypeScript is a strongly typed programming language that builds on JavaScript by adding static types. It is a "superset" of JavaScript, which means that any valid JavaScript code is also valid TypeScript code, with the added benefit of allowing developers to define strict rules for how data is structured and passed around in an application. This enforceable type-checking increases code reliability and reduces runtime errors by catching mistakes during development.

But what exactly are "types"? A type in TypeScript defines the shape and behavior of data in your application. For example, a type can specify whether a value is a number, a string, or a more complex object. Here’s a simple example of a type in TypeScript:

let userName: string = "Alice"; // 'userName' must always be a string.

If you’d like to deepen your knowledge of TypeScript, or if you're just starting to learn programming concepts, consider subscribing to my blog! Additionally, you can use AI-based tools like gpteach.us to guide you in learning how to code.

TS1448: '{0}' resolves to a type-only declaration and must be re-exported using a type-only re-export when 'isolatedModules' is enabled

Now, let's discuss this specific TypeScript error: TS1448: '{0}' resolves to a type-only declaration and must be re-exported using a type-only re-export when 'isolatedModules' is enabled.

This error might seem daunting at first, but once you break it down, it becomes easier to understand. The error predominantly appears in projects where the isolatedModules flag is activated in your TypeScript configuration.

What is isolatedModules?

The isolatedModules flag enforces rules making sure each file in your TypeScript project can be transpiled on its own — independently — without depending on other files. This is most commonly used for supporting tools like Babel or for ensuring your TypeScript code is transpile-safe. The constraint comes with stricter rules for managing and exporting types.

Why does this error occur?

The error occurs when you're exporting something that is "type-only" (a declaration that exists purely for type-checking purposes) without explicitly marking it as such. In general, TypeScript distinguishes between values (things like functions and variables that exist at runtime) and types (things like interfaces, type aliases, and enums that purely serve a compile-time purpose). When working with isolatedModules, type-only declarations must use a special syntax (export type {}) to avoid confusion.

Example of the Error and How to Fix It

Here’s a minimal code example that demonstrates this error:

// types.ts
export interface User {
  name: string;
  age: number;
}
// main.ts
import { User } from "./types";

export { User }; // ⚠️ Causes TS1448 error when 'isolatedModules' is enabled

When the isolatedModules flag is set to true (in your tsconfig.json), TypeScript throws TS1448: '{User}' resolves to a type-only declaration and must be re-exported using a type-only re-export when 'isolatedModules' is enabled.

The Fix

The solution is simple: when re-exporting a type-only declaration, use the export type syntax. Here’s how you fix the previous example:

// main.ts
import { User } from "./types";

export type { User }; // ✅ Correct: explicitly marks the export as type-only

By using export type, you’re telling TypeScript that User is a type-only declaration, avoiding potential confusion at runtime.

Important to Know!

  1. Values vs Types Remember that values exist at runtime, but types exist only during compile-time. For example: 
// This is a value
   const isAdmin: boolean = true;

   // This is a type
   type UserID = string | number;

If you're exporting something meant solely for compile-time use (like the User type above), always ensure you're using export type when isolatedModules is enabled.

  1. Enabling isolatedModules You can enable this flag in your TypeScript configuration (tsconfig.json) like so: 
{
     "compilerOptions": {
       "isolatedModules": true
     }
   }

FAQs

Q: Why does TypeScript treat type-only declarations differently under isolatedModules?

A: This is because tools like Babel or other transpilers might not understand or preserve TypeScript-only syntax. Distinguishing type-only exports ensures that these tools know what should and shouldn't exist at runtime.

Q: Can I avoid this error without enabling isolatedModules?

A: Yes, if you disable isolatedModules in your tsconfig.json, TypeScript won’t enforce this rule. However, that might not be ideal if you're working with certain tools or build setups.

Q: Does the export type syntax affect my JavaScript output?

A: No. Type-only exports (like export type) are removed during compilation, so they don’t appear in the final JavaScript output.

A List of Important Things to Know

To avoid or handle TS1448: '{0}' resolves to a type-only declaration and must be re-exported using a type-only re-export when 'isolatedModules' is enabled, keep the following in mind:

  1. Always use export type {} for type-only declarations if isolatedModules is enabled.
  2. Understand the difference between runtime values and compile-time types.
  3. Review your tsconfig.json to determine if isolatedModules is required for your project.

By understanding and following these rules, you’ll avoid encountering errors like TS1448: '{0}' resolves to a type-only declaration and must be re-exported using a type-only re-export when 'isolatedModules' is enabled. TypeScript’s stricter typing system is there to make your code safer and more predictable, so don’t shy away from these details!