Testing Express.js Applications (Jest + Supertest)

Back to Express.js
Arjit Sharma
01 Min

Testing ensures that your application behaves correctly and continues working as new features are added.

Backend testing commonly verifies:

  • API responses
  • Status codes
  • Request validation
  • Authentication flows
  • Database operations

In Express.js applications, a popular testing setup is:

  • Jest → Test runner and assertion library
  • Supertest → HTTP testing for Express APIs

Installing Testing Dependencies

npm install --save-dev jest supertest

Configuring Jest

Update package.json:

"scripts":{
  "test":"jest"
}

Now tests can be executed using:

npm test

Structuring the Express App for Testing

A common practice is separating the Express app from the server startup.

Example

// app.js
import express from "express";

const app = express();

app.use(express.json());

app.get("/users", (req, res) => {
    res.json([
        { id: 1, name: "Arjit" }
    ]);
});

export default app;

Then start the server separately:

// server.js
import app from "./app.js";

app.listen(3000, () => {
    console.log("Server running on port 3000");
});

This allows tests to import the app directly without starting a real server.

Writing Your First Test

// tests/users.test.js
import request from "supertest";

import app from "../app.js";

describe("GET /users", () => {

    it("should return list of users", async () => {

        const res = await request(app)
            .get("/users")
            .expect(200);

        expect(Array.isArray(res.body)).toBe(true);
    });

});
  • describe() - Groups related tests together.
  • it() - Defines an individual test case.
  • request(app) - Supertest sends HTTP requests directly to the Express app.
  • expect() - Jest assertions verify behavior.
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