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TypeScript Express

This guide uses an example API for IMDb that introduces you to using Fern. We'll generate a TypeScript Express server and a Postman collection to implement and test our API.

👀 Sneak peak: here's what your repo will look like after completing the tutorial.

Step 0: Prerequisites

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Step 1: Set up

Create a new folder for this tutorial and cd into it. Let's create a new npm project.

npm init -y
npm install typescript express @types/express ts-node

This will set us up for a new TypeScript Express backend repo. We'll also install fern:

npm install -g fern-api

Step 2: Initialize

In the root of your backend repo, run:

fern init

When it asks you for the name of your organization, enter imdb.

This will add the following:

├─ fern.config.json # root-level configuration
└─ api/ # your API
├─ generators.yml # generators you're using
└─ definition/
├─ api.yml # configuration for your API
└─ imdb.yml # a Service, which includes some endpoints

Step 3: Add TypeScript code generator

When adding a generator, Fern defaults to the SDK (aka client). We'll see this by running:

fern add typescript

This will add:

-draft: []
+ - name: fernapi/fern-typescript
+ version: 0.0.189
+ config:
+ mode: client-v2

For this tutorial, we want to generate a TypeScript Express server, so we'll change this to:

- name: fernapi/fern-typescript
version: 0.0.189
- mode: client-v2
+ mode: server
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We're here to help! Reach out in Discord.

Step 4: Run the generator

Next we'll invoke the TypeScript server code generator by running:

fern generate

After a few seconds of processing, you'll see a message in your terminal. Here's a screenshot after the code generator ran successfully:

Successful code generation

Step 5: Install the generated package

Create a new file .npmrc at the root of our project and add the following:


This tells npm the location of your generated package. By default, packages are published to a private registry.

Next we'll install the package by running npm install @imdb-fern/api-server@0.0.3. This will add the dependency:

   "dependencies": {
"@types/express": "^4.17.14",
"express": "^4.18.1",
"ts-node": "^10.9.1",
"typescript": "^4.8.3",
+ "@imdb-fern/api-server": "@0.0.3"

Step 6: Implement the server

Create a new file server.ts at the root of our project. This will be the server which serves the API.

Copy and paste the following:

import { GetMovieErrorBody } from "@imdb-fern/api-server/model/imdb";
import { MoviesService } from "@imdb-fern/api-server/services/imdb";
import express from "express";

const app = express();

createMovie: () => {
return {
ok: true,
// We are hardcoding the movie "Iron Man 3" for this demo
// because we don't have our server wired up to a database.
body: "iron-man-3",

getMovie: () => {
return {
ok: false,
error: GetMovieErrorBody.notFoundError(),

console.log("Listening for requests...");

Step 7: Run the server

npx ts-node server.ts

In the terminal, you should see Listening for requests... Great, our server is running!

Step 8: Add the Postman generator

Open a new terminal window and run:

fern add postman
fern generate

In the api/ folder you'll see generated-postman.json was recently created.

Step 9: Hit the server from Postman

Open Postman. Click File -> Import and navigate to api/generated-postman.json. You should now see a collection called api.

Click the createMovie endpoint and then click Send. You should get iron-man-3 back from your server.


Select the getMovie endpoint and hit Send. As expected, we get a 404 response back.


Step 10: Celebrate 🎉

You've successfully implemented a IMDb TypeScript Express server using Fern. You're invited to join our Discord.