diff --git a/src/functions.ts b/src/functions.ts
new file mode 100644
index 00000000..2494f019
--- /dev/null
+++ b/src/functions.ts
@@ -0,0 +1,98 @@
+// Copyright 2019 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import * as express from 'express';
+
+export interface HttpFunction {
+  // tslint:disable-next-line:no-any express interface.
+  (req: express.Request, res: express.Response): any;
+}
+export interface EventFunction {
+  // tslint:disable-next-line:no-any
+  (data: {}, context: Context): any;
+}
+export interface EventFunctionWithCallback {
+  // tslint:disable-next-line:no-any
+  (data: {}, context: Context, callback: Function): any;
+}
+export type HandlerFunction =
+  | HttpFunction
+  | EventFunction
+  | EventFunctionWithCallback;
+
+/**
+ * The Cloud Functions context object for the event.
+ *
+ * @link https://cloud.google.com/functions/docs/writing/background#function_parameters
+ */
+export interface CloudFunctionsContext {
+  /**
+   * A unique ID for the event. For example: "70172329041928".
+   */
+  eventId?: string;
+  /**
+   * The date/time this event was created. For example: "2018-04-09T07:56:12.975Z"
+   * This will be formatted as ISO 8601.
+   */
+  timestamp?: string;
+  /**
+   * The type of the event. For example: "google.pubsub.topic.publish".
+   */
+  eventType?: string;
+  /**
+   * The resource that emitted the event.
+   */
+  resource?: string;
+}
+
+/**
+ * The CloudEvents v0.2 context object for the event.
+ *
+ * @link https://github.com/cloudevents/spec/blob/v0.2/spec.md#context-attributes
+ */
+export interface CloudEventsContext {
+  /**
+   * Type of occurrence which has happened.
+   */
+  type?: string;
+  /**
+   * The version of the CloudEvents specification which the event uses.
+   */
+  specversion?: string;
+  /**
+   * The event producer.
+   */
+  source?: string;
+  /**
+   * ID of the event.
+   */
+  id?: string;
+  /**
+   * Timestamp of when the event happened.
+   */
+  time?: string;
+  /**
+   * A link to the schema that the event data adheres to.
+   */
+  schemaurl?: string;
+  /**
+   * Content type of the event data.
+   */
+  contenttype?: string;
+
+  // tslint:disable-next-line:no-any CloudEvents extension attributes.
+  [key: string]: any;
+}
+
+export type Context = CloudFunctionsContext | CloudEventsContext;
diff --git a/src/index.ts b/src/index.ts
index ed6ef53c..9c357efd 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -30,12 +30,9 @@
 import * as minimist from 'minimist';
 import { resolve } from 'path';
 
-import {
-  ErrorHandler,
-  SignatureType,
-  getServer,
-  getUserFunction,
-} from './invoker';
+import { getUserFunction } from './loader';
+
+import { ErrorHandler, SignatureType, getServer } from './invoker';
 
 // Supported command-line flags
 const FLAG = {
diff --git a/src/invoker.ts b/src/invoker.ts
index 74f800b6..59f16dbf 100644
--- a/src/invoker.ts
+++ b/src/invoker.ts
@@ -29,89 +29,14 @@ import * as onFinished from 'on-finished';
 import { FUNCTION_STATUS_HEADER_FIELD } from './types';
 import { logAndSendError } from './logger';
 import { isBinaryCloudEvent } from './cloudevents';
-
-/**
- * The Cloud Functions context object for the event.
- *
- * @link https://cloud.google.com/functions/docs/writing/background#function_parameters
- */
-export interface CloudFunctionsContext {
-  /**
-   * A unique ID for the event. For example: "70172329041928".
-   */
-  eventId?: string;
-  /**
-   * The date/time this event was created. For example: "2018-04-09T07:56:12.975Z"
-   * This will be formatted as ISO 8601.
-   */
-  timestamp?: string;
-  /**
-   * The type of the event. For example: "google.pubsub.topic.publish".
-   */
-  eventType?: string;
-  /**
-   * The resource that emitted the event.
-   */
-  resource?: string;
-}
-
-/**
- * The CloudEvents v0.2 context object for the event.
- *
- * @link https://github.com/cloudevents/spec/blob/v0.2/spec.md#context-attributes
- */
-export interface CloudEventsContext {
-  /**
-   * Type of occurrence which has happened.
-   */
-  type?: string;
-  /**
-   * The version of the CloudEvents specification which the event uses.
-   */
-  specversion?: string;
-  /**
-   * The event producer.
-   */
-  source?: string;
-  /**
-   * ID of the event.
-   */
-  id?: string;
-  /**
-   * Timestamp of when the event happened.
-   */
-  time?: string;
-  /**
-   * A link to the schema that the event data adheres to.
-   */
-  schemaurl?: string;
-  /**
-   * Content type of the event data.
-   */
-  contenttype?: string;
-
-  // tslint:disable-next-line:no-any CloudEvents extension attributes.
-  [key: string]: any;
-}
-
-export type Context = CloudFunctionsContext | CloudEventsContext;
-
-export interface HttpFunction {
-  // tslint:disable-next-line:no-any express interface.
-  (req: express.Request, res: express.Response): any;
-}
-export interface EventFunctionWithCallback {
-  // tslint:disable-next-line:no-any
-  (data: {}, context: Context, callback: Function): any;
-}
-export interface EventFunction {
-  // tslint:disable-next-line:no-any
-  (data: {}, context: Context): any;
-}
-export type HandlerFunction =
-  | HttpFunction
-  | EventFunction
-  | EventFunctionWithCallback;
+import {
+  HttpFunction,
+  EventFunction,
+  EventFunctionWithCallback,
+  HandlerFunction,
+  CloudFunctionsContext,
+  CloudEventsContext,
+} from './functions';
 
 // We optionally annotate the express Request with a rawBody field.
 // Express leaves the Express namespace open to allow merging of new fields.
@@ -141,95 +66,9 @@ function isHttpFunction(
   return functionSignatureType === SignatureType.HTTP;
 }
 
-/**
- * Returns user's function from function file.
- * Returns null if function can't be retrieved.
- * @return User's function or null.
- */
-export function getUserFunction(
-  codeLocation: string,
-  functionTarget: string
-): HandlerFunction | null {
-  try {
-    const functionModulePath = getFunctionModulePath(codeLocation);
-    if (functionModulePath === null) {
-      console.error('Provided code is not a loadable module.');
-      return null;
-    }
-
-    const functionModule = require(functionModulePath);
-    let userFunction = functionTarget
-      .split('.')
-      .reduce((code, functionTargetPart) => {
-        if (typeof code === 'undefined') {
-          return undefined;
-        } else {
-          return code[functionTargetPart];
-        }
-      }, functionModule);
-
-    // TODO: do we want 'function' fallback?
-    if (typeof userFunction === 'undefined') {
-      if (functionModule.hasOwnProperty('function')) {
-        userFunction = functionModule['function'];
-      } else {
-        console.error(
-          `Function '${functionTarget}' is not defined in the provided ` +
-            'module.\nDid you specify the correct target function to execute?'
-        );
-        return null;
-      }
-    }
-
-    if (typeof userFunction !== 'function') {
-      console.error(
-        `'${functionTarget}' needs to be of type function. Got: ` +
-          `${typeof userFunction}`
-      );
-      return null;
-    }
-
-    return userFunction as HandlerFunction;
-  } catch (ex) {
-    let additionalHint: string;
-    // TODO: this should be done based on ex.code rather than string matching.
-    if (ex.stack && ex.stack.includes('Cannot find module')) {
-      additionalHint =
-        'Did you list all required modules in the package.json ' +
-        'dependencies?\n';
-    } else {
-      additionalHint = 'Is there a syntax error in your code?\n';
-    }
-    console.error(
-      `Provided module can't be loaded.\n${additionalHint}` +
-        `Detailed stack trace: ${ex.stack}`
-    );
-    return null;
-  }
-}
-
 // Response object for the most recent request.
 let latestRes: express.Response | null = null;
 
-/**
- * Returns resolved path to the module containing the user function.
- * Returns null if the module can not be identified.
- * @param codeLocation Directory with user's code.
- * @return Resolved path or null.
- */
-function getFunctionModulePath(codeLocation: string): string | null {
-  let path: string | null = null;
-  try {
-    path = require.resolve(codeLocation);
-  } catch (ex) {
-    try {
-      // TODO: Decide if we want to keep this fallback.
-      path = require.resolve(codeLocation + '/function.js');
-    } catch (ex) {}
-  }
-  return path;
-}
-
 /**
  * Sends back a response to the incoming request.
  * @param result Output from function execution.
diff --git a/src/loader.ts b/src/loader.ts
new file mode 100644
index 00000000..cda65891
--- /dev/null
+++ b/src/loader.ts
@@ -0,0 +1,110 @@
+// Copyright 2019 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// loader.ts
+/**
+ * This package contains the logic to load user's function.
+ * @packageDocumentation
+ */
+
+/**
+ * Import function signature type's definition.
+ */
+import { HandlerFunction } from './functions';
+
+/**
+ * Returns user's function from function file.
+ * Returns null if function can't be retrieved.
+ * @return User's function or null.
+ */
+export function getUserFunction(
+  codeLocation: string,
+  functionTarget: string
+): HandlerFunction | null {
+  try {
+    const functionModulePath = getFunctionModulePath(codeLocation);
+    if (functionModulePath === null) {
+      console.error('Provided code is not a loadable module.');
+      return null;
+    }
+
+    const functionModule = require(functionModulePath);
+    let userFunction = functionTarget
+      .split('.')
+      .reduce((code, functionTargetPart) => {
+        if (typeof code === 'undefined') {
+          return undefined;
+        } else {
+          return code[functionTargetPart];
+        }
+      }, functionModule);
+
+    // TODO: do we want 'function' fallback?
+    if (typeof userFunction === 'undefined') {
+      if (functionModule.hasOwnProperty('function')) {
+        userFunction = functionModule['function'];
+      } else {
+        console.error(
+          `Function '${functionTarget}' is not defined in the provided ` +
+            'module.\nDid you specify the correct target function to execute?'
+        );
+        return null;
+      }
+    }
+
+    if (typeof userFunction !== 'function') {
+      console.error(
+        `'${functionTarget}' needs to be of type function. Got: ` +
+          `${typeof userFunction}`
+      );
+      return null;
+    }
+
+    return userFunction as HandlerFunction;
+  } catch (ex) {
+    let additionalHint: string;
+    // TODO: this should be done based on ex.code rather than string matching.
+    if (ex.stack && ex.stack.includes('Cannot find module')) {
+      additionalHint =
+        'Did you list all required modules in the package.json ' +
+        'dependencies?\n';
+    } else {
+      additionalHint = 'Is there a syntax error in your code?\n';
+    }
+    console.error(
+      `Provided module can't be loaded.\n${additionalHint}` +
+        `Detailed stack trace: ${ex.stack}`
+    );
+    return null;
+  }
+}
+
+/**
+ * Returns resolved path to the module containing the user function.
+ * Returns null if the module can not be identified.
+ * @param codeLocation Directory with user's code.
+ * @return Resolved path or null.
+ */
+function getFunctionModulePath(codeLocation: string): string | null {
+  let path: string | null = null;
+  try {
+    path = require.resolve(codeLocation);
+  } catch (ex) {
+    try {
+      // TODO: Decide if we want to keep this fallback.
+      path = require.resolve(codeLocation + '/function.js');
+    } catch (ex) {}
+  }
+  return path;
+}
diff --git a/test/data/with_main/foo.js b/test/data/with_main/foo.js
new file mode 100644
index 00000000..e3699a83
--- /dev/null
+++ b/test/data/with_main/foo.js
@@ -0,0 +1,24 @@
+/**
+ * Test HTTP function to test function loading.
+ *
+ * @param {!Object} req request context.
+ * @param {!Object} res response context.
+ */
+function testHttpFunction (res, req) {
+  return 'PASS'
+};
+
+/**
+ * Test event function to test function loading.
+ *
+ * @param {!Object} data event payload.
+ * @param {!Object} context event metadata.
+ */
+function testEventFunction (data, context) {
+  return 'PASS';
+};
+
+module.exports = {
+  testHttpFunction,
+  testEventFunction,
+}
diff --git a/test/data/with_main/package.json b/test/data/with_main/package.json
new file mode 100644
index 00000000..438495d6
--- /dev/null
+++ b/test/data/with_main/package.json
@@ -0,0 +1,3 @@
+{
+    "main": "foo.js"
+}
\ No newline at end of file
diff --git a/test/data/without_main/function.js b/test/data/without_main/function.js
new file mode 100644
index 00000000..e3699a83
--- /dev/null
+++ b/test/data/without_main/function.js
@@ -0,0 +1,24 @@
+/**
+ * Test HTTP function to test function loading.
+ *
+ * @param {!Object} req request context.
+ * @param {!Object} res response context.
+ */
+function testHttpFunction (res, req) {
+  return 'PASS'
+};
+
+/**
+ * Test event function to test function loading.
+ *
+ * @param {!Object} data event payload.
+ * @param {!Object} context event metadata.
+ */
+function testEventFunction (data, context) {
+  return 'PASS';
+};
+
+module.exports = {
+  testHttpFunction,
+  testEventFunction,
+}
diff --git a/test/function.js b/test/function.js
deleted file mode 100644
index e1c222d0..00000000
--- a/test/function.js
+++ /dev/null
@@ -1,18 +0,0 @@
-// Copyright 2019 Google LLC
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-exports.foo = (data, context) => {
-  // Message used for function load verification in test.
-  return 'Hello from foo';
-};
diff --git a/test/invoker.ts b/test/invoker.ts
index e8659b56..e9546e80 100644
--- a/test/invoker.ts
+++ b/test/invoker.ts
@@ -14,20 +14,10 @@
 
 import * as assert from 'assert';
 import * as express from 'express';
+import * as functions from '../src/functions';
 import * as invoker from '../src/invoker';
 import * as supertest from 'supertest';
 
-describe('loading function', () => {
-  it('should load the function', () => {
-    const loadedFunction = invoker.getUserFunction(
-      process.cwd() + '/test/function',
-      'foo'
-    ) as invoker.EventFunction;
-    const returned = loadedFunction({}, {});
-    assert.strictEqual(returned, 'Hello from foo');
-  });
-});
-
 describe('request to HTTP function', () => {
   interface TestData {
     name: string;
@@ -137,11 +127,14 @@ describe('GCF event request to event function', () => {
   testData.forEach(test => {
     it(`should receive data and context from ${test.name}`, async () => {
       let receivedData: {} | null = null;
-      let receivedContext: invoker.CloudFunctionsContext | null = null;
-      const server = invoker.getServer((data: {}, context: invoker.Context) => {
-        receivedData = data;
-        receivedContext = context as invoker.CloudFunctionsContext;
-      }, invoker.SignatureType.EVENT);
+      let receivedContext: functions.CloudFunctionsContext | null = null;
+      const server = invoker.getServer(
+        (data: {}, context: functions.Context) => {
+          receivedData = data;
+          receivedContext = context as functions.CloudFunctionsContext;
+        },
+        invoker.SignatureType.EVENT
+      );
       await supertest(server)
         .post('/')
         .send(test.body)
@@ -200,11 +193,14 @@ describe('CloudEvents request to event function', () => {
   testData.forEach(test => {
     it(`should receive data and context from ${test.name}`, async () => {
       let receivedData: {} | null = null;
-      let receivedContext: invoker.CloudEventsContext | null = null;
-      const server = invoker.getServer((data: {}, context: invoker.Context) => {
-        receivedData = data;
-        receivedContext = context as invoker.CloudEventsContext;
-      }, invoker.SignatureType.EVENT);
+      let receivedContext: functions.CloudEventsContext | null = null;
+      const server = invoker.getServer(
+        (data: {}, context: functions.Context) => {
+          receivedData = data;
+          receivedContext = context as functions.CloudEventsContext;
+        },
+        invoker.SignatureType.EVENT
+      );
       await supertest(server)
         .post('/')
         .set(test.headers)
diff --git a/test/loader.ts b/test/loader.ts
new file mode 100644
index 00000000..38b611b1
--- /dev/null
+++ b/test/loader.ts
@@ -0,0 +1,50 @@
+// Copyright 2019 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import * as assert from 'assert';
+import * as express from 'express';
+import * as functions from '../src/functions';
+import * as loader from '../src/loader';
+
+describe('loading function', () => {
+  interface TestData {
+    name: string;
+    codeLocation: string;
+    target: string;
+  }
+
+  const testData: TestData[] = [
+    {
+      name: 'function without "main" in package.json',
+      codeLocation: '/test/data/without_main',
+      target: 'testEventFunction',
+    },
+    {
+      name: 'function with "main" in package.json',
+      codeLocation: '/test/data/with_main',
+      target: 'testEventFunction',
+    },
+  ];
+
+  for (const test of testData) {
+    it(`should load ${test.name}`, () => {
+      const loadedFunction = loader.getUserFunction(
+        process.cwd() + test.codeLocation,
+        test.target
+      ) as functions.HttpFunction;
+      const returned = loadedFunction(express.request, express.response);
+      assert.strictEqual(returned, 'PASS');
+    });
+  }
+});