index.js

'use strict';

const Module = require('module');
const dirname = require('path').dirname;
const join = require('path').join;
const resolve = require('path').resolve;
const pathsep = require('path').sep;
const getCallerFile = require('get-caller-file');
const normalize = require('normalize-path');
const originalLoader = Module._load;

let mockExports = {};
let pendingMockExports = {};

Module._load = function(request, parent) {
  if (!parent) return originalLoader.apply(this, arguments);

  const fullFilePath = getFullPathNormalized(request, parent.filename);

  if (pendingMockExports.hasOwnProperty(fullFilePath)) {
    mockExports[fullFilePath] = typeof pendingMockExports[fullFilePath] === 'string' ?
      require(pendingMockExports[fullFilePath]) :
      pendingMockExports[fullFilePath];

    delete pendingMockExports[fullFilePath];
  }

  return mockExports.hasOwnProperty(fullFilePath)
    ? mockExports[fullFilePath]
    : originalLoader.apply(this, arguments);
};

function startMocking(path, mockExport) {
  const calledFrom = getCallerFile();

  if (typeof mockExport === 'string') {
    mockExport = getFullPathNormalized(mockExport, calledFrom);
  }

  pendingMockExports[getFullPathNormalized(path, calledFrom)] = mockExport;
}

function stopMocking(path) {
  const calledFrom = getCallerFile();
  const fullPath = getFullPathNormalized(path, calledFrom);
  delete pendingMockExports[fullPath];
  delete mockExports[fullPath];
}

function stopMockingAll() {
  mockExports = {};
  pendingMockExports = {};
}

function reRequire(path) {
  const module = getFullPathNormalized(path, getCallerFile());
  delete require.cache[require.resolve(module)];
  return require(module);
}

function isInNodePath(resolvedPath) {
  if (!resolvedPath) return false;

  return Module.globalPaths
    .map((nodePath) => {
      return resolve(process.cwd(), nodePath) + pathsep;
    })
    .some((fullNodePath) => {
      return resolvedPath.indexOf(fullNodePath) === 0;
    });
}

function getFullPath(path, calledFrom) {
  let resolvedPath;
  try {
    resolvedPath = require.resolve(path);
  } catch (e) {
    // do nothing
  }

  const isLocalModule = /^\.{1,2}[/\\]?/.test(path);
  const isInPath = isInNodePath(resolvedPath);
  const isExternal = !isLocalModule && /[/\\]node_modules[/\\]/.test(resolvedPath);
  const isSystemModule = resolvedPath === path;

  if (isExternal || isSystemModule || isInPath) {
    return resolvedPath;
  }

  if (!isLocalModule) {
    return path;
  }

  const localModuleName = join(dirname(calledFrom), path);
  try {
    return Module._resolveFilename(localModuleName);
  } catch (e) {
    if (isModuleNotFoundError(e)) { return localModuleName; } else { throw e; }
  }
}

function getFullPathNormalized(path, calledFrom) {
  return normalize(getFullPath(path, calledFrom));
}

function isModuleNotFoundError(e) {
  return e.code && e.code === 'MODULE_NOT_FOUND';
}

module.exports = startMocking;
module.exports.stop = stopMocking;
module.exports.stopAll = stopMockingAll;
module.exports.reRequire = reRequire;