Mocking and stubbing

mocking.js

/* This is my file I'll be testing foo.js */

import fs from 'fs'
import { promisify } from 'util'

const readFileAsync = promisify(fs.readFile)

export function readJsonFile (filePath) {
 return readFileAsync(filePath).then(JSON.parse)
}

/* This is my test file foo.test.js */

import fs from 'fs'
import test from 'ava';
import { stub } from 'sinon'
import proxyquire from 'proxyquire'

test('readJsonFile with proxyquire', async function (t) {
  t.plan(2)
  
  /* fs.readFile is overwritten for this import of foo.js */
  const { readJsonFile } = proxyquire('./foo.js', {
    fs: {
      readFile(filePath, callback) {
        t.is(filePath, 'myTestFile')
        
        return callback(null, '{ success: true }')
      }
    }
  })
  
  const results = await readJsonFile('myTestFile')
  t.deepEqual(results, { success: true })
})

test('readJsonFile with sinon', async function (t) {
  t.plan(1)
  
  /* fs.readFile is overwritten everywhere */
  const fsStub = stub(fs, 'readFile')
    .withArgs('myTestFile')
    .callsArg(2, null, '{ success: true }')
  
  const results = await readJsonFile('myTestFile')
  t.deepEqual(results, { success: true })
  
  // Won't happen if test fails :(
  fsStub.restore()
})