Small example how to stub pure functions

module1.js

"use strict";

import {module2_pureFunction} from './module2';

export function module1_pureFunction(){
  return module2_pureFunction();
}

module2.js

"use strict";

export function module2_pureFunction(){
  return "I'm return value from module 2 pure function";
}

test.js

let sinon = require("sinon");
let chai = require("chai");
let assert = chai.assert;

import {module1_pureFunction} from '../lib/module1';
import * as module2 from '../lib/module2';

"use strict";

describe("Test pure function importing", function () {

  let sinonSandbox;
  beforeEach(function () {
    sinonSandbox = sinon.sandbox.create();
  });

  afterEach(function () {
    sinonSandbox.restore();
  });

  it("should use stubs", function(){
    let result = module1_pureFunction();
    assert.equal(result, "I'm return value from module 2 pure function");

    const stubText = "I'm return value from stubbed function";
    let stub = sinonSandbox.stub(module2, 'module2_pureFunction');
    stub.returns(stubText);
    result = module1_pureFunction();
    assert.equal(result, stubText);
  });

  it("should be original value", function(){
    let result = module1_pureFunction();
    assert.equal(result, "I'm return value from module 2 pure function");
  })

});

Seems to work at least with node 4.2.1 and babel 6.1.4 (babel-plugin-transform-es2015-modules-commonjs plugin 6.1.4) and mocha 2.2.5