Import a Function from Another file using TypeScript

avatar

Borislav Hadzhiev

Thu Mar 03 20223 min read

Import a Function from Another file using TypeScript #

To import a function from another file in TypeScript:

  1. Export the function from file A, e.g. export function sum() {}.
  2. Import the function in file B as import { sum } from './another-file'.
  3. Use the imported function in file B.

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

another-file.ts
// 👇️ named export export function sum(a: number, b: number) { return a + b; } // (arrow function) // export const sum = (a: number, b: number) => { // return a + b; // };

The syntax is the same when using arrow functions, all you have to do is use the export keyword.

Here is how we would import the sum function in a file called index.ts.

index.ts
// 👇️ named import import { sum } from './another-file'; console.log(sum(50, 10)); // 👉️ 60

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 {sum} from '../another-file'.

We wrapped the name of the function 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 a function 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 a function that was exported using a default export.

Here are the contents of another-file.ts.

another-file.ts
// 👇️ default export export default function sum(a: number, b: number) { return a + b; } // (arrow function) // const sum = (a: number, b: number) => { // return a + b; // }; // export default sum;

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

index.ts
// 👇️ default import import sum from './another-file'; console.log(sum(50, 10)); // 👉️ 60

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

We could have also used a different name when importing the function, e.g. foo.

index.ts
import foo from './another-file'; console.log(foo(50, 10)); // 👉️ 60

This works, but is confusing and should be avoided.

If you are exporting a variable (or an arrow function) as a default export, you have to declare it on 1 line and export it on the next. You can't declare and default export a variable on the same line.

index.ts
const multiply = (a: number, b: number) => { return a * b; }; export default multiply;
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 function sum(a: number, b: number) { return a + b; } // 👇️ named export export const multiply = (a: number, b: number) => { return a * b; };

And here is how you would import the two functions.

index.ts
// 👇️ default and named imports import sum, { multiply } from './another-file'; console.log(sum(50, 10)); // 👉️ 60 console.log(multiply(50, 10)); // 👉️ 500

We used a default import to import the sum function and a named import to import the multiply function.

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

Use the search field on my Home Page to filter through my more than 1,000 articles.