diff --git a/src/index.ts b/src/index.ts
index f5e8caf..64040b3 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,5 +1,5 @@
 import initState from './initState';
-import IOption from './IOption';
+import IOption from './interfaces/IOption';
 import parseArgument from './parseArgument';
 import runPreParseHook from './runPreParseHook';
 
@@ -28,11 +28,9 @@ export default function parseArguments(
     }
 
     //check if any entry in requiredArguments was not set
-    for (const optName in requiredOptions) {
-      if (spec.hasOwnProperty(optName) && requiredOptions[optName] === false) {
-        throw new Error(`Missing option "${optName}" even though it is required.`);
-      }
-    }
+    Object.entries(requiredOptions).filter(([, wasFound]) => wasFound === false).forEach(([option]) => {
+      throw new Error(`Missing option "${option}" even though it is required.`);
+    });
   } catch (error) {
     fail((error as Error).message);
   }
diff --git a/src/initState.ts b/src/initState.ts
index c77bf8a..4355bf4 100644
--- a/src/initState.ts
+++ b/src/initState.ts
@@ -1,40 +1,19 @@
 import failWithError from './failWithError';
-import IOption from './IOption';
-
-const failFunction: (error?: string) => never = failWithError.bind(null, '');
+import IOption from './interfaces/IOption';
+import IState from './interfaces/IState';
+import parseSpecEntry from './parseSpecEntry';
 
 export default function initState(spec: { [key: string]: IOption }) {
   //The parsed arguments that are returned by this function
-  const outputArgs: { [key: string]: string } = {};
+  const outputArgs: IState['outputArgs'] = {};
 
   //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 === '') {
-        return failFunction(`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]+)*$/)) {
-        return failFunction(`The long option "${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) {
-          return failFunction(`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 shortToLongLookup: IState['shortToLongLookup'] = {};
+  const requiredOptions: IState['requiredOptions'] = {};
+  try {
+    Object.entries(spec).forEach(parseSpecEntry.bind(null, { outputArgs, shortToLongLookup, requiredOptions }));
+  } catch (error) {
+    failWithError('', (error as Error).message);
   }
 
   const usage = `./${process.argv[1].substr(process.argv[1].lastIndexOf('/') + 1)} [options]\nOPTIONS\n${
diff --git a/src/IOption.ts b/src/interfaces/IOption.ts
similarity index 100%
rename from src/IOption.ts
rename to src/interfaces/IOption.ts
diff --git a/src/interfaces/IState.ts b/src/interfaces/IState.ts
new file mode 100644
index 0000000..1100357
--- /dev/null
+++ b/src/interfaces/IState.ts
@@ -0,0 +1,8 @@
+export default interface IState {
+  /** Stores the parsed arguments that are returned by the argument parser */
+  outputArgs: { [key: string]: string };
+  /** Lookup table from the short option names to their corresponding long option nmae */
+  shortToLongLookup: { [key: string]: string };
+  /** For each required option, store if the parser already encountered it. At the end of parsing, all entries must be set to true or the parser fails. */
+  requiredOptions: { [key: string]: boolean };
+}
diff --git a/src/parseArgument.ts b/src/parseArgument.ts
index 4c07c0b..8365726 100644
--- a/src/parseArgument.ts
+++ b/src/parseArgument.ts
@@ -1,12 +1,13 @@
-import IOption from './IOption';
+import IOption from './interfaces/IOption';
+import IState from './interfaces/IState';
 import verifyAndStoreOptionValue from './verifyAndStoreOptionValue';
 
 export default function parseArgument(
   { spec, outputArgs, requiredOptions, shortToLongLookup }: {
     spec: { [key: string]: IOption },
-    outputArgs: { [key: string]: string },
-    requiredOptions: { [key: string]: boolean },
-    shortToLongLookup: { [key: string]: string },
+    outputArgs: IState['outputArgs'],
+    requiredOptions: IState['requiredOptions'],
+    shortToLongLookup: IState['shortToLongLookup'],
   },
   previousOption: string,
   arg: string,
diff --git a/src/parseSpecEntry.ts b/src/parseSpecEntry.ts
new file mode 100644
index 0000000..2806925
--- /dev/null
+++ b/src/parseSpecEntry.ts
@@ -0,0 +1,25 @@
+import IOption from './interfaces/IOption';
+import IState from './interfaces/IState';
+
+export default function parseSpecEntry({ outputArgs, requiredOptions, shortToLongLookup }: IState, [longOption, { short: shortOption, default: defaultValue }]: [string, IOption]) {
+  if (longOption === '') {
+    throw new Error(`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]+)*$/)) {
+    throw new Error(`The long option "${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.`);
+  }
+
+  if (shortOption !== undefined && shortOption !== '') {
+    if (shortOption.length > 1) {
+      throw new Error(`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
+  if (defaultValue !== undefined) {
+    outputArgs[longOption] = defaultValue;
+  } else {
+    requiredOptions[longOption] = false;
+  }
+}
diff --git a/src/verifyAndStoreOptionValue.ts b/src/verifyAndStoreOptionValue.ts
index 3b64317..d009275 100644
--- a/src/verifyAndStoreOptionValue.ts
+++ b/src/verifyAndStoreOptionValue.ts
@@ -1,4 +1,5 @@
-import IOption from './IOption';
+import IOption from './interfaces/IOption';
+import IState from './interfaces/IState';
 
 export default function verifyAndStoreOptionValue({
   option,
@@ -12,8 +13,8 @@ export default function verifyAndStoreOptionValue({
   value: string,
   verify: IOption['verify'],
   message: IOption['message'],
-  outputArgs: { [key: string]: string },
-  requiredOptions: { [key: string]: boolean },
+  outputArgs: IState['outputArgs'],
+  requiredOptions: IState['requiredOptions'],
 }) {
   //Verify value for correctness
   try {