send/app/experiments.js

import hash from 'string-hash';

const experiments = {};

//Returns a number between 0 and 1
// eslint-disable-next-line no-unused-vars
function luckyNumber(str) {
  return hash(str) / 0xffffffff;
}

function checkExperiments(state, emitter) {
  const all = Object.keys(experiments);
  const id = all.find(id => experiments[id].eligible(state));
  if (id) {
    const variant = experiments[id].variant(state);
    state.storage.enroll(id, variant);
    experiments[id].run(variant, state, emitter);
  }
}

export default function initialize(state, emitter) {
  emitter.on('DOMContentLoaded', () => {
    const xp = experiments[state.query.x];
    if (xp) {
      xp.run(+state.query.v, state, emitter);
    }
  });

  if (!state.storage.get('testpilot_ga__cid')) {
    // first ever visit. check again after cid is assigned.
    emitter.on('DOMContentLoaded', () => {
      checkExperiments(state, emitter);
    });
  } else {
    const enrolled = state.storage.enrolled;
    enrolled.forEach(([id, variant]) => {
      const xp = experiments[id];
      if (xp) {
        xp.run(variant, state, emitter);
      }
    });
    // single experiment per session for now
    if (enrolled.length === 0) {
      checkExperiments(state, emitter);
    }
  }
}