custom-test-environment-jsdom.js

// Add memoization to two string helpers in jsdom that are called
// tens of thousands of times on the same strings and slow tests
// down by up to 3x.
//
// See https://github.com/jsdom/jsdom/commit/63d24a06d04a60279599782dc97899cde59d901d#diff-a29b6d0a417180220d299dde53ac3e1f820c3ade3daec396118f27c4270e1457R137
const {createRequire} = require('module');

const envRequire = createRequire(require.resolve('jest-environment-jsdom'));

const stringHelpers = envRequire('jsdom/lib/jsdom/living/helpers/strings.js');

const memoize = (fn) => {
  const map = new Map();
  return (string) => {
    if (map.has(string)) {
      return map.get(string);
    }
    const value = fn(string);
    map.set(string, value);
    return value;
  };
};

stringHelpers.asciiUppercase = memoize(stringHelpers.asciiUppercase);
stringHelpers.asciiLowercase = memoize(stringHelpers.asciiLowercase);

module.exports = require('jest-environment-jsdom');

const {createRequire} = require('module');

const envRequire = createRequire(require.resolve('jest-environment-jsdom'));

const stringHelpers = envRequire('jsdom/lib/jsdom/living/helpers/strings.js')`;

is probably safer so you won't have to rely on on FS layout of node_modules

https://nodejs.org/api/module.html#module_module_createrequire_filename

Ah, good point – I'm operating in a place where there are multiple jsdoms.

There's an extra backtick on line 10

Thanks, copy paste mistake, fixed!