understanding-graphs/src/js/utilities/fonts.js

import Observer from 'fontfaceobserver';
import promisesPolyfill from 'es6-promise';

import config from '../config.js';

// preload fonts
export default () => {
  const fontObservers = [];

  Object.keys(config.fonts).forEach((f) => {
    const font = config.fonts[f];

    fontObservers.push(
      new Observer(
        font.family,
        {
          weight: font.weight,
          style: font.style
        }
      ).load()
    );
  });

  if (fontObservers.length >= 1) {
    promisesPolyfill.polyfill();

    Promise.all(fontObservers)
      .then(() => {
        document.documentElement.classList.add('fonts-loaded');
      });
  }
};