Mocking node:child_process.spawn() using Jest + TypeScript

index.spec.ts

import { Readable } from "stream";

const mockSpawn = jest.fn(() => {
  return {
    stdout: new Readable({
      read() {
        this.push("stout data"); // write mock data to stdout
        this.push(null); // end stream
      },
    }),
    stderr: new Readable({
      read() {
        this.push("error data"); // write mock data to stderr
        this.push(null); // end stream
      },
    }),
    on: jest.fn((event, callback: any) => {
      if (event === "close") {
        callback(0);
      }
    }),
  };
});

jest.mock("node:child_process", () => {
  return {
    spawn: mockSpawn,
  };
});

test("mock child_process.spawn", async () => {

  // important: load implementation using dynamic import!
  const { runExternalCommand } = await import("./index");

  runExternalCommand(
    (data: Buffer) => {
      expect(data.toString('utf8')).toBe("stout data");
      expect(mockSpawn).toHaveBeenCalled();
    },
    (error: Buffer) => {
      expect(error.toString('utf8')).toBe("error data");
      expect(mockSpawn).toHaveBeenCalled();
    }
  );
});

index.ts

import { spawn } from "node:child_process";
export function runExternalCommand(onData: any, onError: any) {
  const out = spawn("ls", ["-l"]);
  out.stdout.on("data", onData);
  out.stderr.on("data", onError);
}

jest.config.ts

/** @type {import('ts-jest').JestConfigWithTsJest} */
module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
};

package.json

{
  "type": "module",
  "scripts": {
    "test": "jest"
  },
  "devDependencies": {
    "@jest/globals": "^29.7.0",
    "@types/jest": "^29.5.12",
    "jest": "^29.7.0",
    "ts-jest": "^29.1.2",
    "ts-node": "^10.9.2"
  }
}

tsconfig.json

{
    "compilerOptions": {
        "esModuleInterop": true,
    }
}