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Filename : proxyquire.js
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'use strict';
/*jshint laxbreak:true, loopfunc:true*/

var path = require('path')
  , Module = require('module')
  , ProxyquireError = require('./proxyquire-error')
  , is = require('./is')
  , assert = require('assert')
  , fillMissingKeys = require('fill-keys')
  , moduleNotFoundError = require('module-not-found-error')
  , hasOwnProperty = Object.prototype.hasOwnProperty
  ;

function validateArguments(request, stubs) {
  var msg = (function getMessage() {
    if (!request)
      return 'Missing argument: "request". Need it to resolve desired module.';

    if (!stubs)
      return 'Missing argument: "stubs". If no stubbing is needed, use regular require instead.';

    if (!is.String(request))
      return 'Invalid argument: "request". Needs to be a requirable string that is the module to load.';

    if (!is.Object(stubs))
      return 'Invalid argument: "stubs". Needs to be an object containing overrides e.g., {"path": { extname: function () { ... } } }.';
  })();

  if (msg) throw new ProxyquireError(msg);
}

function Proxyquire(parent) {
  var self = this
    , fn = self.load.bind(self)
    , proto = Proxyquire.prototype
    ;

  this._parent = parent;
  this._preserveCache = true;

  Object.keys(proto)
    .forEach(function (key) {
      if (is.Function(proto[key])) fn[key] = proto[key].bind(self);
    });

  self.fn = fn;
  return fn;
}

/**
 * Disables call thru, which determines if keys of original modules will be used
 * when they weren't stubbed out.
 * @name noCallThru
 * @function
 * @private
 * @return {object} The proxyquire function to allow chaining
 */
Proxyquire.prototype.noCallThru = function () {
  this._noCallThru = true;
  return this.fn;
};

/**
 * Enables call thru, which determines if keys of original modules will be used
 * when they weren't stubbed out.
 * @name callThru
 * @function
 * @private
 * @return {object} The proxyquire function to allow chaining
 */
Proxyquire.prototype.callThru = function () {
  this._noCallThru = false;
  return this.fn;
};

/**
 * Will make proxyquire remove the requested modules from the `require.cache` in order to force 
 * them to be reloaded the next time they are proxyquired.
 * This behavior differs from the way nodejs `require` works, but for some tests this maybe useful.
 *
 * @name noPreserveCache
 * @function
 * @private
 * @return {object} The proxyquire function to allow chaining
 */
Proxyquire.prototype.noPreserveCache = function() {
  this._preserveCache = false;
  return this.fn;
};

/**
 * Restores proxyquire caching behavior to match the one of nodejs `require` 
 *
 * @name preserveCache
 * @function
 * @private
 * @return {object} The proxyquire function to allow chaining
 */
Proxyquire.prototype.preserveCache = function() {
  this._preserveCache = true;
  return this.fn;
};

/**
 * Loads a module using the given stubs instead of their normally resolved required modules.
 * @param request The requirable module path to load.
 * @param stubs The stubs to use. e.g., { "path": { extname: function () { ... } } }
 * @return {*} A newly resolved module with the given stubs.
 */
Proxyquire.prototype.load = function (request, stubs) {
  validateArguments(request, stubs);

  // Find out if any of the passed stubs are global overrides
  for (var key in stubs) {
    var stub = stubs[key];

    if (stub === null) continue;

    if (typeof stub === 'undefined') {
      throw new ProxyquireError('Invalid stub: "' + key + '" cannot be undefined');
    }

    if (hasOwnProperty.call(stub, '@global')) {
      this._containsGlobal = true;
    }

    if (hasOwnProperty.call(stub, '@runtimeGlobal')) {
      this._containsGlobal = true;
      this._containsRuntimeGlobal = true;
    }
  }

  // Ignore the module cache when return the requested module
  return this._withoutCache(this._parent, stubs, request, this._parent.require.bind(this._parent, request));
};

// This replaces a module's require function
Proxyquire.prototype._require = function(module, stubs, path) {
  assert(typeof path === 'string', 'path must be a string');
  assert(path, 'missing path');

  if (hasOwnProperty.call(stubs, path)) {
    var stub = stubs[path];

    if (stub === null) {
      // Mimic the module-not-found exception thrown by node.js.
      throw moduleNotFoundError(path);
    }

    if (hasOwnProperty.call(stub, '@noCallThru') ? !stub['@noCallThru'] : !this._noCallThru) {
      fillMissingKeys(stub, Module._load(path, module));
    }

    // We are top level or this stub is marked as global
    if (module.parent == this._parent || hasOwnProperty.call(stub, '@global') || hasOwnProperty.call(stub, '@runtimeGlobal')) {
      return stub;
    }
  }

  // Only ignore the cache if we have global stubs
  if (this._containsRuntimeGlobal) {
    return this._withoutCache(module, stubs, path, Module._load.bind(Module, path, module));
  } else {
    return Module._load(path, module);
  }
};

Proxyquire.prototype._withoutCache = function(module, stubs, path, func) {
  // Temporarily disable the cache - either per-module or globally if we have global stubs
  var restoreCache = this._disableCache(module, path);

  // Override all require extension handlers
  var restoreExtensionHandlers = this._overrideExtensionHandlers(module, stubs);

  try {
    // Execute the function that needs the module cache disabled
    return func();
  } finally {
    // Restore the cache if we are preserving it
    if (this._preserveCache) {
      restoreCache();
    } else {
      delete require.cache[Module._resolveFilename(path, module)];
    }

    // Finally restore the original extension handlers
    restoreExtensionHandlers();
  }
};

Proxyquire.prototype._disableCache = function(module, path) {
  if (this._containsGlobal) {
    // empty the require cache because if we are stubbing C but requiring A,
    // and if A requires B and B requires C, then B and C might be cached already
    // and we'll never get the chance to return our stub
    return this._disableGlobalCache();
  }

  // Temporarily delete the SUT from the require cache
  return this._disableModuleCache(path, module);
};

Proxyquire.prototype._disableGlobalCache = function() {
  var cache = require.cache;
  require.cache = Module._cache = {};

  // Return a function that will undo what we just did
  return function() {
    require.cache = Module._cache = cache;
  };
};

Proxyquire.prototype._disableModuleCache = function(path, module) {
  // Find the ID (location) of the SUT, relative to the parent
  var id = Module._resolveFilename(path, module);

  var cached = Module._cache[id];
  delete Module._cache[id];

  // Return a function that will undo what we just did
  return function() {
    if (cached) {
      Module._cache[id] = cached;
    }
  };
};

Proxyquire.prototype._overrideExtensionHandlers = function(module, stubs) {
  var originalExtensions = {};
  var self = this;

  Object.keys(require.extensions).forEach(function(extension) {
    // Store the original so we can restore it later
    if (!originalExtensions[extension]) {
      originalExtensions[extension] = require.extensions[extension];
    }

    // Override the default handler for the requested file extension
    require.extensions[extension] = function(module, filename) {
      // Override the require method for this module
      module.require = self._require.bind(self, module, stubs);

      return originalExtensions[extension](module, filename);
    };
  });

  // Return a function that will undo what we just did
  return function() {
    Object.keys(originalExtensions).forEach(function(extension) {
      require.extensions[extension] = originalExtensions[extension];
    });
  };
};

module.exports = Proxyquire;