Let’s say we have an express app that doesn't have any DB connections

• Initialize a simple TS project

npm init -y
npx tsc --init

• Change tsconfig.json

{
  "compilerOptions": {
    "target": "ESNext",
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "esModuleInterop": true,
    "isolatedModules": true,
    "outDir": "dist"
  }
}

• Add dependencies

npm install -D ts-jest @jest/globals @types/express supertest @types/supertest
npm install express

• Know more about Supertest → https://www.npmjs.com/package/supertest

• update package.json

{
  "name": "sample_node_app",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "node --experimental-vm-modules node_modules/jest/bin/jest.js"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "type": "module",
  "devDependencies": {
    "@jest/globals": "^30.2.0",
    "@types/express": "^5.0.5",
    "@types/supertest": "^6.0.3",
    "supertest": "^7.1.4",
    "ts-jest": "^29.4.5"
  },
  "dependencies": {
    "express": "^5.1.0"
  }
}

• Initialize jest.config.ts

npx ts-jest config:init

update jest.config.js

Why this config Works

export default {
  preset: "ts-jest/presets/default-esm",
  testEnvironment: "node",
  extensionsToTreatAsEsm: [".ts"],
  moduleNameMapper: {
    "^(\\\\.{1,2}/.*)\\\\.js$": "$1"
  },
  transform: {
    "^.+\\\\.ts$": [
      "ts-jest",
      {
        useESM: true
      }
    ]
  }
};

• Create src/index.ts

import express from "express";

export const app = express();
app.use(express.json());

app.post("/sum", (req, res) => {
    const a = req.body.a;
    const b = req.body.b;
    const answer = a + b;

    res.json({
        answer
    })
});

• Add tests/sum.test.ts