react-app-rewired-preact/scripts/utils/rewireJestConfig.js

Tweak the create-react-app webpack config(s) without using 'eject' and without creating a fork of the react-scripts

'use strict';

const path = require('path');
const paths = require('./paths');

module.exports = (config) => {
  // TODO: remove in major release
  Object.keys(config.transform).forEach((key) => {
    if (config.transform[key].endsWith('babelTransform.js')) {
      config.transform[key] = path.resolve(__dirname + '/babelTransform.js');
    }
  });
  const overrides = Object.assign({}, require(paths.appPackageJson).jest);
  
  // Jest configuration in package.json will be added to the the default config
  Object.keys(overrides)
    .forEach(key => {
      //We don't overwrite the default config, but add to each property if not a string
      if(config[key]) {
        if(typeof overrides[key] === 'string') {
          config[key] = overrides[key];
        } else if(Array.isArray(overrides[key])) {
          config[key] = overrides[key].concat(config[key]);
        }
        else if(typeof overrides[key] === 'object') {
          config[key] = Object.assign({}, overrides[key], config[key]);
        }
      } else {
        config[key] = overrides[key];
      }
    });
  return config;
};