ES6 + Babel + Sinon

babel-rewire.js

// credits - https://github.com/sinonjs/sinon/issues/1358#issuecomment-288566305

// first install babel-plugin-rewire
// then add plugin to .babelrc as
// "plugins" : ["rewire"]

import token, {clientToken} from '../../../../app/util/token'
import * as esat from 'esat'
import * as proxyquire from 'proxyquire'

describe("token handling", () => {

    describe("generating client token", () => {        

        before(() => {
            token.__Rewire__('generate',sinon.stub().resolves({token: "", rtk: 0}))

        });


        it('should contains token key', () => {

            return clientToken({uid:"abc"})
                .should
                .be
                .fulfilled;
        });
    })
});

proxyquire.js

// credits - https://stackoverflow.com/a/46135220/4969957

import token from '../../../../app/util/token'
import * as esat from 'esat'
import * as proxyquire from 'proxyquire'

describe("token handling", () => {

    describe("generating client token", () => {
        const stub = {};

        before(() => {
            stub.generate = (opt, key) => {
                return new Promise((resolve, reject) => {
                    resolve({token: "", rtk: 0})
                });
            };

        });

        it('should contains token key', () => {           
            const service = proxyquire.load('../../../../app/util/token', {esat: stub});
            return service.clientToken({uid: "abc"})
                .should
                .eventually
                .be
                .a('string')

        });
    })
});