TypeScript, Jest, ECMAScript Modules, and Projen

.projenrc.ts

import { typescript, javascript } from "projen";

const project = new typescript.TypeScriptProject({
  name: "template-typescript-esm",
  defaultReleaseBranch: "main",
  projenrcTs: true,

  devDeps: ["@jest/globals"],

  minNodeVersion: "18.0.0",
  workflowNodeVersion: "18.18.2",
  tsconfig: {
    compilerOptions: {
      moduleResolution: javascript.TypeScriptModuleResolution.NODE16,
      module: "node16",
      lib: ["DOM", "ES2022"],
      target: "es2022",
    },
  },

  jestOptions: {
    jestConfig: {
      extensionsToTreatAsEsm: [".ts"],
      moduleNameMapper: {
        "^(\\.{1,2}/.*)\\.js$": "$1",
      },
    },
  },
  tsJestOptions: {
    transformPattern: "^.+\\.ts$",
    transformOptions: {
      useESM: true,
    },
  },
});

project.package.file.addOverride("type", "module");
project.testTask.env("NODE_OPTIONS", "--experimental-vm-modules");
const [{ exec: projenrcCommand = "" }] = project.defaultTask!.steps;
project.defaultTask!.reset(projenrcCommand.replace("ts-node", "ts-node-esm"));

project.synth();

README.projen-ts-esm.md

Projen + ECMAScript Modules (ESM) + TypeScript + Jest

How to configure Projen for an ESM library project on Node.js 18.

Many thanks to @danpetitt for the Gist he put together.

See the .projenrc.ts file below.

hello.test.ts

// This file exists at test/hello.test.ts

import { Hello } from "../src/index.js";
import { HELLO_WORLD } from "../src/text.js";

test("hello", () => {
  expect(new Hello().sayHello()).toBe(HELLO_WORLD);
});

index.ts

// This file exists at src/index.ts

import { HELLO_WORLD } from "./text.js";

export class Hello {
  public sayHello(): string {
    return HELLO_WORLD;
  }
}

text.ts

// This file exists at src/text.ts

export const HELLO_WORLD: string = "hello, world!";