Spies and mocking with Node.js test runner (node:test)

Node.js version 20 brings a stable test runner so you can run tests inside *.test.js files with node --test command. This post covers the primary usage of it regarding spies and mocking for the unit tests.

Spies are functions that let you spy on the behavior of functions called indirectly by some other code while mocking injects test values into the code during the tests.

mock.method can create spies and mock async, rejected async, sync, chained methods, and external and built-in modules.

• Async function

import assert from 'node:assert/strict';
import { describe, it, mock } from 'node:test';

const calculationService = {
  calculate: () => // implementation
};

describe('mocking resolved value', () => {
  it('should resolve mocked value', async () => {
    const value = 2;
    mock.method(calculationService, 'calculate', async () => value);

    const result = await calculationService.calculate();

    assert.equal(result, value);
  });
});

• Rejected async function

const error = new Error('some error message');
mock.method(calculationService, 'calculate', async () => Promise.reject(error));

await assert.rejects(async () => calculateSomething(calculationService), error);

• Sync function

mock.method(calculationService, 'calculate', () => value);

• Chained methods

mock.method(calculationService, 'get', () => calculationService);
mock.method(calculationService, 'calculate', async () => value);

const result = await calculationService.get().calculate();

• External modules

import axios from 'axios';

mock.method(axios, 'get', async () => ({ data: value }));

• Built-in modules

import fs from 'fs/promises';

mock.method(fs, 'readFile', async () => fileContent);

• Async and sync functions called multiple times can be mocked with different values using context.mock.fn and mockedFunction.mock.mockImplementationOnce.

describe('mocking same method multiple times with different values', () => {
  it('should resolve mocked values', async (context) => {
    const firstValue = 2;
    const secondValue = 3;
    const calculateMock = context.mock.fn(calculationService.calculate);
    calculateMock.mock.mockImplementationOnce(async () => firstValue, 0);
    calculateMock.mock.mockImplementationOnce(async () => secondValue, 1);

    const firstResult = await calculateMock();
    const secondResult = await calculateMock();

    assert.equal(firstResult, firstValue);
    assert.equal(secondResult, secondValue);
  });
});

• To assert called arguments for a spy, use mockedFunction.mock.calls[0] value.

mock.method(calculationService, 'calculate');

await calculateSomething(calculationService, firstValue, secondValue);

const call = calculationService.calculate.mock.calls[0];
assert.deepEqual(call.arguments, [firstValue, secondValue]);

• To assert skipped call for a spy, use mockedFunction.mock.calls.length value.

mock.method(calculationService, 'calculate');

assert.equal(calculationService.calculate.mock.calls.length, 0);

• To assert how many times mocked function is called, use mockedFunction.mock.calls.length value.

mock.method(calculationService, 'calculate');

calculationService.calculate(3);
calculationService.calculate(2);

assert.equal(calculationService.calculate.mock.calls.length, 2);

• To assert called arguments for the exact call when a mocked function is called multiple times, an assertion can be done using mockedFunction.mock.calls[index] and call.arguments values.

const calculateMock = context.mock.fn(calculationService.calculate);
calculateMock.mock.mockImplementationOnce((a) => a + 2, 0);
calculateMock.mock.mockImplementationOnce((a) => a + 3, 1);

calculateMock(firstValue);
calculateMock(secondValue);

[firstValue, secondValue].forEach((argument, index) => {
  const call = calculateMock.mock.calls[index];

  assert.deepEqual(call.arguments, [argument]);
});

Running TypeScript tests

Add a new test script, --experimental-transform-types flag requires Node version >= 22.10.0

{
  "type": "module",
  "scripts": {
    "test": "node --test",
    "test:ts": "NODE_OPTIONS='--experimental-transform-types --disable-warning=ExperimentalWarning' node --test ./src/**/*.{spec,test}.ts"
  }
}

Demo

The demo with the mentioned examples is available here.