Mocking a function within a closure

index.js

// import `thirdPartyFunction` as a member of the imported module
import * as myDependency from 'my-dependency'

export default myTestableFunction(arg1) {
  // and call `thirdPartyFunction` as a member of the imported module
  myDependency.thirdPartyFunction(arg1)
}

index.test.js

import { assert } from 'chai'
import sinon from 'sinon'

// import your testable module normally
import myTestableFunction from './index'

// import `thirdPartyFunction` as a member of the imported module
import * as myDependency from 'my-dependency'

describe('myTestableFunction', () => {
  let thirdPartySpy
  
  before(() => {
     // initialize the spy once
     thirdPartySpy = sinon.spy(myDependency, 'thirdPartyFunction')
  })
  
  afterEach(() => {
    // reset it after each test
    thirdPartySpy.resetHistory()
  })
  
  it('passes an argument to `thirdPartyFunction`', () => {
    myTestableFunction('foo')
    assert.isTrue(thirdPartySpy.calledWith('foo'))
  })
})