Understanding import type in TypeScript: The Right Way to Import Types

When writing TypeScript code, it’s common to import types from other files to structure and annotate your code more effectively. However, there's a subtle but powerful distinction between a regular import and an import type statement that many developers overlook. Mastering this feature can lead to cleaner, more efficient, and safer TypeScript codebases—especially when targeting tree-shaking and optimized bundling.

What is import type?

In TypeScript, you can import types using the import type syntax:

import type { User } from './models'

This tells the TypeScript compiler that the User being imported is used for type annotations only—not at runtime. As a result, this import is completely erased from the final JavaScript output.

Why You Should Use import type

Here are some key reasons to use import type instead of regular imports when dealing with types:

1. Smaller JavaScript Output

When compiling TypeScript to JavaScript, regular imports will remain in the final JS, even if they’re only used for type annotations. This can cause unnecessary code to be bundled, especially when the imported file contains side effects or large dependencies.

Using import type guarantees the import is removed from the output.

2. Avoid Runtime Side Effects

Some modules include side effects—code that runs as soon as the file is loaded (e.g., polyfills, DOM manipulation, modifying global variables). If you import such a file using a regular import just for its types, you risk executing unwanted code.

With import type, you’re safe: no runtime code is loaded.

3. Prevent Circular Dependencies

Large codebases often suffer from circular dependencies, which are difficult to debug and can cause runtime errors. Since import type does not include runtime code, it helps prevent circular import issues by keeping the dependency graph cleaner.

4. Improved Build Performance

Using import type can improve incremental build times because the compiler doesn't need to follow through the entire module to resolve runtime values—just type declarations.

Common Misuse: Mixing Type and Value Imports

TypeScript allows you to write:

import { MyType, myFunction } from './utils'

But this imports the entire module at runtime, even if you only use MyType for type checking. To optimize this, split your imports:

import type { MyType } from './utils'
import { myFunction } from './utils'

This ensures only what’s necessary is bundled.

Limitations and Considerations

- Not usable in dynamic imports

You cannot use import type with dynamic imports (import() syntax), as it’s purely a compile-time construct.

- No TypeScript code transformation

Tools like Babel that strip types might not preserve import type behavior unless properly configured. Always verify your build pipeline supports it.

- Ambient Modules & Declaration Files

When importing from .d.ts files or external type declarations (like @types/* packages), import type may not offer much benefit, since these imports are already stripped out during compile time.

Best Practices

• ✅ Use import type for interfaces, types, and aliases.
• ✅ Keep imports clean and separated: value imports vs. type-only imports.
• ✅ Configure your linter or formatter (like ESLint with @typescript-eslint/consistent-type-imports) to enforce consistent usage.
• ✅ Don’t mix type and runtime imports in one statement.
• ❌ Don’t use import type for values like functions, constants, or classes.

Finally

Using import type may seem like a small thing, but it can have a significant impact on the performance and maintainability of your TypeScript project. It encourages you to clearly separate your runtime and compile-time concerns, which is crucial as your codebase grows.

If you haven’t already, consider doing a type audit in your project—check where you can replace regular imports with import type. Your future self (and your bundler) will thank you.

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