Improved Solution

promise.js

import pdefer from "https://unpkg.com/p-defer@4.0.0/index.js";

/** @returns {Promise<Record<string, boolean>} */
async function fetchAllFeatures() {
  console.log("Making network call...");
  return new Promise((res, rej) => {
    setTimeout(() => {
      res({});
    }, 500);
  });
}

let promise 

/** @param {string} flag */
async function getFeatureFlag(flag) {
  if (!promise) {
    promise = new Promise(res => {
      res(fetchAllFeatures())
    })
  }

  return promise.then((flags) => {
    return flags[flag] ?? false
  })
}