diff --git a/src/IOption.ts b/src/IOption.ts
new file mode 100644
index 0000000..368b116
--- /dev/null
+++ b/src/IOption.ts
@@ -0,0 +1,15 @@
+/**
+ * A command line option
+ */
+export default interface IOption {
+  /** The default value to use, if this argument is missing. If set to `undefined`, this argument is considered to be required and the process will terminate whenever it is missing. */
+  default?: string;
+  /** The short version option as an alias for the long option, must be a single character. Can be set to an empty string or `undefined` if there should be no short option. */
+  short?: string;
+  /** A human readable description of this option, for display in the usage message. Not displayed if `undefined` or an empty string. */
+  description?: string;
+  /** A function that verifies if the given value is a valid value for this option. If set to `undefined`, any value is accepted for this option. */
+  verify?: (value: string) => boolean;
+  /** A function returning a custom error message that is shown if the verify function returns false. If the function is `undefined`, or it returns `undefined` or an empty string, a default error message is shown instead. */
+  message?: (value: string) => string;
+}
diff --git a/src/index.ts b/src/index.ts
index cd9c44c..7789aee 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,20 +1,7 @@
-import failWithError from './failWithError';
-
-/**
- * A command line option
- */
-interface IOption {
-  /** The default value to use, if this argument is missing. If set to `undefined`, this argument is considered to be required and the process will terminate whenever it is missing. */
-  default?: string;
-  /** The short version option as an alias for the long option, must be a single character. Can be set to an empty string or `undefined` if there should be no short option. */
-  short?: string;
-  /** A human readable description of this option, for display in the usage message. Not displayed if `undefined` or an empty string. */
-  description?: string;
-  /** A function that verifies if the given value is a valid value for this option. If set to `undefined`, any value is accepted for this option. */
-  verify?: (value: string) => boolean;
-  /** A function returning a custom error message that is shown if the verify function returns false. If the function is `undefined`, or it returns `undefined` or an empty string, a default error message is shown instead. */
-  message?: (value: string) => string;
-}
+import initState from './initState';
+import IOption from './IOption';
+import runPreParseHook from './runPreParseHook';
+import verifyAndStoreOptionValue from './verifyAndStoreOptionValue';
 
 /**
  * Checks the command line arguments against the given specification, and returns the arguments as a JavaScript object, as well as a failure function that prints the usage instructions before terminating.
@@ -26,102 +13,15 @@ export default function parseArguments(
     spec: { [key: string]: IOption },
     preParseHook?: (args: string[], fail: (message: string) => void) => (string[] | undefined),
   ): { fail: (errorMessage: string) => void, args: { [key: string]: string } } {
-  //The parsed arguments that are returned by this function
-  const outputArgs: { [key: string]: string } = {};
-
-  //Create a lookup table to find a long option when given the short option, and to check if all required options are set
-  const shortToLongLookup: { [key: string]: string } = {};
-  const requiredOptions: { [key: string]: boolean } = {};
-  for (const longOption in spec) {
-    if (spec.hasOwnProperty(longOption)) {
-      if (longOption === '') {
-        failWithError('', `The long option may not be an empty string. This is a bug in the source code of the script that you tried to call.`);
-      }
-      if (!longOption.match(/^[a-z0-9]+(-[a-z0-9]+)*$/)) {
-        failWithError('', `The long option "${longOption}" must only contain alpha-numeric characters, optionally separated by hyphens. This is a bug in the source code of the script that you tried to call.`);
-      }
-
-      const shortOption = spec[longOption].short;
-      if (shortOption !== undefined && shortOption !== '') {
-        if (shortOption.length > 1) {
-          failWithError('', `Short options must only be one character long but the short option "${shortOption}" for "${longOption}" was ${shortOption.length} characters long. This is a bug in the source code of the script that you tried to call.`);
-        }
-        shortToLongLookup[shortOption] = longOption;
-      }
-
-      //If a default value is given, use the default value (it can be overridden later), otherwise mark argument as being required
-      const defaultValue = spec[longOption].default;
-      if (defaultValue !== undefined) {
-        outputArgs[longOption] = defaultValue;
-      } else {
-        requiredOptions[longOption] = false;
-      }
-    }
-  }
-
-  //---------------------------------------------------------------
-
-  let argumentOption = '';
-
-  const usage = `./${process.argv[1].substr(process.argv[1].lastIndexOf('/') + 1)} [options]\nOPTIONS\n${
-    Object.entries(spec).map(([key, {default: defaultValue, short, description}]) => (
-      `  ${(defaultValue === undefined) ? '' : '['}${(short !== undefined && short !== '') ? `-${short}, ` : ''}--${key} <value>${(defaultValue === undefined) ? '' : `], defaults to "${defaultValue}"`}${(description !== undefined && description !== '') ? `\n      ${description}` : ''}`
-    )).join('\n')
-  }`;
-
-  const failFunction = failWithError.bind(null, usage);
-
-  //The original arguments from the command line
-  let args = process.argv.slice(2);
-
-  //If defined, run the pre-parse hook to modify the arguments
-  if (preParseHook !== undefined) {
-    try {
-      const modifiedArgs = preParseHook(args, failFunction);
-      if (modifiedArgs !== undefined) {
-        args = modifiedArgs;
-      }
-    } catch (error) {
-      failFunction(`Could not run pre-parse hook, encountered error: ${String(error)}.`);
-    }
-  }
-
-  function parseValue(argumentValue: string) {
-    //Verify value for correctness
-    const verifyFunc = spec[argumentOption].verify;
-    const messageFunc = spec[argumentOption].message;
-    try {
-      if (verifyFunc !== undefined && !verifyFunc(argumentValue)) {
-        if (messageFunc !== undefined) {
-          try {
-            const customMessage = messageFunc(argumentValue);
-            if (customMessage !== undefined && customMessage !== '') {
-              failFunction(customMessage);
-            }
-          } catch (error) {
-            failFunction(`Invalid value set for option "${argumentOption}": "${argumentValue}". Could not show custom error message: ${String(error)}`);
-          }
-        }
-        failFunction(`Invalid value set for option "${argumentOption}": "${argumentValue}".`);
-      }
-    } catch (error) {
-      failFunction(`Invalid value set for option "${argumentOption}", the validation of "${argumentValue}" failed: ${String(error)}`);
-    }
-
-    //Remember the value, and if it is a required argument, mark that we have encountered it
-    outputArgs[argumentOption] = argumentValue;
-    if (requiredOptions[argumentOption] === false) {
-      requiredOptions[argumentOption] = true;
-    }
-    argumentOption = '';
-  }
-
-  //---------------------------------------------------------------
+  //Initialize state
+  const { outputArgs, shortToLongLookup, requiredOptions, failFunction } = initState(spec);
+  const args = runPreParseHook({ args: process.argv.slice(2), preParseHook, fail: failFunction });
 
   //Iterate through all command line arguments. When we have read both name and value, verify the argument for correctness.
   //Show error if a name has no value afterwards, or a value has no name in front of it.
+  let option = '';
   for (const arg of args) {
-    if (argumentOption === '') {
+    if (option === '') {
       /**  We expect a name, must be one of:
        * - `--name value`
        * - `--name=value`
@@ -129,53 +29,53 @@ export default function parseArguments(
        * - `-n=value`
        */
       if (arg === '-' || arg === '--') {
-        //TODO: what about the -- option? skip parsing of all following options?
         failFunction(`Empty option "-" or "--" is not supported.`);
       } else if (arg.startsWith('--')) { //long argument
-        argumentOption = arg.substr(2);
-        let argumentValue;
+        option = arg.substr(2);
+        let value;
         //If there's a =, immediately read the value
-        if (argumentOption.indexOf('=') !== -1) {
-          [argumentOption, argumentValue] = argumentOption.split('=', 2);
+        if (option.indexOf('=') !== -1) {
+          [option, value] = option.split('=', 2);
         }
         //Check that argument name is valid
-        if (spec[argumentOption] === undefined) {
-          failFunction(`Unknown option "--${argumentOption}".`);
+        if (spec[option] === undefined) {
+          failFunction(`Unknown option "--${option}".`);
         }
         //If value was provided, check that value is correct and remove name for next loop iteration
-        if (argumentValue !== undefined) {
-          parseValue(argumentValue);
-          argumentOption = '';
+        if (value !== undefined) {
+          verifyAndStoreOptionValue({option, value, verify: spec[option].verify, message: spec[option].message, fail: failFunction, outputArgs, requiredOptions});
+          option = '';
         }
       } else if (arg.startsWith('-')) { //short argument
-        argumentOption = arg.substr(1);
-        let argumentValue;
+        option = arg.substr(1);
+        let value;
         //If there's a =, immediately read the value
-        if (argumentOption.indexOf('=') !== -1) {
-          [argumentOption, argumentValue] = argumentOption.split('=', 2);
+        if (option.indexOf('=') !== -1) {
+          [option, value] = option.split('=', 2);
         }
         //Check that argument name is valid
-        if (shortToLongLookup[argumentOption] === undefined) {
-          failFunction(`Unknown short option "-${argumentOption}".`);
+        if (shortToLongLookup[option] === undefined) {
+          failFunction(`Unknown short option "-${option}".`);
         }
-        argumentOption = shortToLongLookup[argumentOption];
+        option = shortToLongLookup[option];
         //If value was provided, check that value is correct and remove name for next loop iteration
-        if (argumentValue !== undefined) {
-          parseValue(argumentValue);
-          argumentOption = '';
+        if (value !== undefined) {
+          verifyAndStoreOptionValue({option, value, verify: spec[option].verify, message: spec[option].message, fail: failFunction, outputArgs, requiredOptions});
+          option = '';
         }
       } else {
         failFunction(`Arguments must be preceded by an option but there was no option in front of "${arg}".`);
       }
     } else { //We expect a value, can be anything
-      parseValue(arg);
+      verifyAndStoreOptionValue({option, value: arg, verify: spec[option].verify, message: spec[option].message, fail: failFunction, outputArgs, requiredOptions});
+      option = '';
     }
 
   }
 
   // argumentName must be cleared to '' after the value is read, so if it is not an empty string, the value was missing
-  if (argumentOption !== '') {
-    failFunction(`Option "${argumentOption}" was not followed by a value.`);
+  if (option !== '') {
+    failFunction(`Option "${option}" was not followed by a value.`);
   }
 
   //check if any entry in requiredArguments was not set
diff --git a/src/initState.ts b/src/initState.ts
new file mode 100644
index 0000000..fd9356f
--- /dev/null
+++ b/src/initState.ts
@@ -0,0 +1,49 @@
+import failWithError from './failWithError';
+import IOption from './IOption';
+
+const fail = failWithError.bind(null, '');
+
+export default function initState(spec: { [key: string]: IOption }) {
+  //The parsed arguments that are returned by this function
+  const outputArgs: { [key: string]: string } = {};
+
+  //Create a lookup table to find a long option when given the short option, and to check if all required options are set
+  const shortToLongLookup: { [key: string]: string } = {};
+  const requiredOptions: { [key: string]: boolean } = {};
+  for (const longOption in spec) {
+    if (spec.hasOwnProperty(longOption)) {
+      if (longOption === '') {
+        fail(`The long option may not be an empty string. This is a bug in the source code of the script that you tried to call.`);
+      }
+      if (!longOption.match(/^[a-z0-9]+(-[a-z0-9]+)*$/)) {
+        fail(`The long option "${longOption}" must only contain alpha-numeric characters, optionally separated by hyphens. This is a bug in the source code of the script that you tried to call.`);
+      }
+
+      const shortOption = spec[longOption].short;
+      if (shortOption !== undefined && shortOption !== '') {
+        if (shortOption.length > 1) {
+          fail(`Short options must only be one character long but the short option "${shortOption}" for "${longOption}" was ${shortOption.length} characters long. This is a bug in the source code of the script that you tried to call.`);
+        }
+        shortToLongLookup[shortOption] = longOption;
+      }
+
+      //If a default value is given, use the default value (it can be overridden later), otherwise mark argument as being required
+      const defaultValue = spec[longOption].default;
+      if (defaultValue !== undefined) {
+        outputArgs[longOption] = defaultValue;
+      } else {
+        requiredOptions[longOption] = false;
+      }
+    }
+  }
+
+  const usage = `./${process.argv[1].substr(process.argv[1].lastIndexOf('/') + 1)} [options]\nOPTIONS\n${
+    Object.entries(spec).map(([key, {default: defaultValue, short, description}]) => (
+      `  ${(defaultValue === undefined) ? '' : '['}${(short !== undefined && short !== '') ? `-${short}, ` : ''}--${key} <value>${(defaultValue === undefined) ? '' : `], defaults to "${defaultValue}"`}${(description !== undefined && description !== '') ? `\n      ${description}` : ''}`
+      )).join('\n')
+  }`;
+
+  const failFunction = failWithError.bind(null, usage);
+
+  return { outputArgs, shortToLongLookup, requiredOptions, failFunction };
+}
diff --git a/src/runPreParseHook.ts b/src/runPreParseHook.ts
new file mode 100644
index 0000000..e364034
--- /dev/null
+++ b/src/runPreParseHook.ts
@@ -0,0 +1,17 @@
+export default function runPreParseHook({ args, preParseHook, fail }: {
+  args: string[],
+  preParseHook?: (args: string[], fail: (message: string) => void) => (string[] | undefined),
+  fail: (errorMessage: string) => void,
+}): string[] {
+  if (preParseHook === undefined) {
+    return args;
+  }
+
+  try {
+    const modifiedArgs = preParseHook(args, fail);
+    return modifiedArgs !== undefined ? modifiedArgs : args;
+  } catch (error) {
+    fail(`Could not run pre-parse hook, encountered error: ${String(error)}.`);
+    return [];
+  }
+}
diff --git a/src/tslint.json b/src/tslint.json
new file mode 100644
index 0000000..5352f67
--- /dev/null
+++ b/src/tslint.json
@@ -0,0 +1,17 @@
+{
+  "defaultSeverity": "error",
+  "extends": [
+      "tslint:recommended"
+  ],
+  "jsRules": {},
+  "rules": {
+      "comment-format": false,
+      "indent": [true, "spaces", 2],
+      "max-line-length": false,
+      "no-bitwise": false,
+      "no-console": false,
+      "quotemark": [true, "single"],
+      "object-literal-sort-keys": [true, "match-declaration-order"]
+  },
+  "rulesDirectory": []
+}
diff --git a/src/verifyAndStoreOptionValue.ts b/src/verifyAndStoreOptionValue.ts
new file mode 100644
index 0000000..f6e17ff
--- /dev/null
+++ b/src/verifyAndStoreOptionValue.ts
@@ -0,0 +1,44 @@
+import IOption from './IOption';
+
+export default function verifyAndStoreOptionValue({
+  option,
+  value,
+  verify,
+  message,
+  fail,
+  outputArgs,
+  requiredOptions,
+}: {
+  option: string,
+  value: string,
+  verify: IOption['verify'],
+  message: IOption['message'],
+  fail: (errorMessage: string) => void,
+  outputArgs: { [key: string]: string },
+  requiredOptions: { [key: string]: boolean },
+}) {
+  //Verify value for correctness
+  try {
+    if (verify !== undefined && !verify(value)) {
+      if (message !== undefined) {
+        try {
+          const customMessage = message(value);
+          if (customMessage !== undefined && customMessage !== '') {
+            fail(customMessage);
+          }
+        } catch (error) {
+          fail(`Invalid value set for option "${option}": "${value}". Could not show custom error message: ${String(error)}`);
+        }
+      }
+      fail(`Invalid value set for option "${option}": "${value}".`);
+    }
+  } catch (error) {
+    fail(`Invalid value set for option "${option}", the validation of "${value}" failed: ${String(error)}`);
+  }
+
+  //Remember the value, and if it is a required argument, mark that we have encountered it
+  outputArgs[option] = value;
+  if (requiredOptions[option] === false) {
+    requiredOptions[option] = true;
+  }
+}