An example on how to stub git-js methods

git_stubbed.js

const sinon = require('sinon');

const Git = require('simple-git/src/git');
const git = require('simple-git/promise');

sinon.stub(Git.prototype, '_run').callsFake(function (command, cb) {
  console.log('called command', command)

  // to indicate success
  cb.call(this, null, 'any message');

  return this;
});

git().pull('origin', 'master')
  .then(() => {
    console.log('all done, success');
  })
  .catch((err) => {
    console.log('error:', err);
  });