Sometimes when writing a unit test, you know that the module you're testing imports a module that you would like to observe, or at the very least mock to prevent side effects like network activity or file system operations.

For JavaScript unit tests that run in Node, we can hijack the built-in require

This means we could replace functions with no-ops to prevent side effects, or replace them with 

In this video, we'll demonstrate the basics of using Proxyquire to prevent and observe a file system operation.

index.js:

const fs = require('fs');

exports.writeFile = (filename, contents) => {
  fs.writeFileSync(filename, contents.trim(), 
 
'utf8');
}

For unit testing, we don't want to call 'fs.writeFileSync', instead we want to using mock function.

const assert = require('assert');
const proxyquire = require('proxyquire');
const sinon = require('sinon');

const fsMock = {};
// In index.js file, we want to mock 'fs' module with our fsMock object
const main = proxyquire('./index.js', {'fs', fsMock});
describe('main.writeFile', () => {
  it('should write a trimmed string to the specififed file', () => {
    fsMock.writeFileSync = sinon.spy();
    main.writeFile('file.txt', 'string');
    assert.deepEqual(fsMock.writeFileSync.getCall(0).args, ['file.txt', 'string', 'utf8'])
  })
})