module-name-mapper-helper.js

const path = require('path')
const matchModuleImport = /^[^?]*~/
/**
 * Resolves the path to the file/module.
 *
 * @param {String} to - the name of the file to resolve to
 * @param {String} importPath - the local path
 * @param {String} fileType - extension of the file to be resolved
 * @returns {String} path - path to the file to import
 */
function resolve(to, importPath, fileType) {
  importPath =
    path.extname(importPath) === '' ? `${importPath}.${fileType}` : importPath
  if (path.isAbsolute(importPath)) {
    return importPath
  } else if (matchModuleImport.test(importPath)) {
    return require.resolve(importPath.replace(matchModuleImport, ''))
  }
  return path.join(path.dirname(to), importPath)
}

/**
 * Applies the moduleNameMapper substitution from the jest config
 *
 * @param {String} source - the original string
 * @param {String} filePath - the path of the current file (where the source originates)
 * @param {Object} jestConfig - the jestConfig holding the moduleNameMapper settings
 * @param {Object} fileType - extn of the file to be resolved
 * @returns {String} path - the final path to import (including replacements via moduleNameMapper)
 */
module.exports = function applyModuleNameMapper(
  source,
  filePath,
  jestConfig = {},
  fileType = ''
) {
  if (!jestConfig.moduleNameMapper) return source
  // Extract the moduleNameMapper settings from the jest config. TODO: In case of development via babel@7, somehow the jestConfig.moduleNameMapper might end up being an Array. After a proper upgrade to babel@7 we should probably fix this.
  const module = Array.isArray(jestConfig.moduleNameMapper)
    ? jestConfig.moduleNameMapper
    : Object.entries(jestConfig.moduleNameMapper)

  const importPath = module.reduce((acc, [regex, replacement]) => {
    const matches = acc.match(regex)

    if (matches === null) {
      return acc
    }

    return replacement.replace(
      /\$([0-9]+)/g,
      (_, index) => matches[parseInt(index, 10)]
    )
  }, source)

  return resolve(filePath, importPath, fileType)
}