Import an Interface from Another file using TypeScript

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Borislav Hadzhiev

Thu Mar 03 20223 min read

Import an Interface from Another file using TypeScript #

To import an interface from another file in TypeScript:

  1. Export the interface from file A, e.g. export interface Employee {}.
  2. Import the interface in file B as import { Employee } from './another-file'.
  3. Use the interface in file B.

Here is an example of exporting an interface from a file called another-file.ts.

another-file.ts
// 👇️ named export export interface Employee { id: number; name: string; salary: number; }

Here is how we would import the Employee interface in a file called index.ts.

index.ts
// 👇️ named import import { Employee } from './another-file'; const emp: Employee = { id: 1, name: 'James', salary: 100, }; console.log(emp);

Make sure to correct the path that points to the another-file module if you have to. The example above assumes that another-file.ts and index.ts are located in the same directory.

For example, if another-file.ts was located one directory up, you'd have to import as import {Employee} from '../another-file'.

We wrapped the name of the interface in curly braces when importing it - this is called a named import.

TypeScript uses the concept of modules, in the same way that JavaScript does.

In order to be able to import an interface from a different file, it has to be exported using a named or default export.

The example above uses a named export and a named import.

The main difference between named and default exports and imports is - you can have multiple named exports per file, but you can only have a single default export.

Let's look at an example of how we would import an interface that was exported using a default export.

Here are the contents of another-file.ts.

another-file.ts
// 👇️ default export export default interface Employee { id: number; name: string; salary: number; }

And here is how we would import the interface using a default import.

index.ts
// 👇️ default import import Employee from './another-file'; const emp: Employee = { id: 1, name: 'James', salary: 100, }; console.log(emp);

Notice that we didn't wrap the import in curly braces.

We could have also used a different name when importing the interface, e.g. Foo.

index.ts
// 👇️ default import import Foo from './another-file'; const emp: Foo = { id: 1, name: 'James', salary: 100, }; console.log(emp);

This works, but is confusing and should be avoided.

In my experience, most real world codebases exclusively use named exports and imports, because they make it easier to leverage your IDE for autocompletion and auto-imports.
You also don't have to think about which members are exported with a default or named export.

You can also mix and match. Here is an example of a file that uses both a default and a named export.

another-file.ts
// 👇️ default export export default interface Employee { id: number; name: string; salary: number; } // 👇️ named export export interface Person { name: string; }

And here is how you would import the two interfaces.

index.ts
// 👇️ default and named imports import Employee, { Person } from './another-file'; const emp: Employee = { id: 1, name: 'James', salary: 100, }; console.log(emp); const person: Person = { name: 'Alice', }; console.log(person);

We used a default import to import the Employee interface and a named import to import the Person interface.

Note that you can only have a single default export per file, but you can have as many named exports as necessary.

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