🎨 Set return type of fail function to never

src/failWithError.ts

@@ -1,7 +1,10 @@
-export default function failWithError(usage: string, msg?: string) {
+/** Prints the given error message and usage instructions, then terminates with a non-zero exit code. */
+export default function failWithError(usage: string, msg?: string): never {
   if (msg !== undefined) {
     process.stderr.write(`Error: ${String(msg).trim()}\n`);
   }
-  process.stderr.write(`Usage: ${usage} \n`);
+  if (usage !== '') {
-  process.exit(1);
+    process.stderr.write(`Usage: ${usage} \n`);
+  }
+  return process.exit(1);
 }

src/index.ts

@@ -12,7 +12,7 @@ import runPreParseHook from './runPreParseHook';
 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 } } {
+  ): { fail: (errorMessage: string) => never, args: { [key: string]: string } } {
   //Initialize state
   const { outputArgs, shortToLongLookup, requiredOptions, fail } = initState(spec);
   const args = runPreParseHook({ args: process.argv.slice(2), preParseHook, fail });

src/initState.ts

@@ -1,7 +1,7 @@
 import failWithError from './failWithError';
 import IOption from './IOption';
 
-const failFunction = failWithError.bind(null, '');
+const failFunction: (error?: string) => never = failWithError.bind(null, '');
 
 export default function initState(spec: { [key: string]: IOption }) {
   //The parsed arguments that are returned by this function
@@ -43,7 +43,7 @@ export default function initState(spec: { [key: string]: IOption }) {
       )).join('\n')
   }`;
 
-  const failWithUsage = failWithError.bind(null, usage);
+  const failWithUsage: (error?: string) => never = failWithError.bind(null, usage);
 
   return { outputArgs, shortToLongLookup, requiredOptions, fail: failWithUsage };
 }

src/parseArgument.ts

@@ -4,7 +4,7 @@ import verifyAndStoreOptionValue from './verifyAndStoreOptionValue';
 export default function parseArgument(
   { spec, fail, outputArgs, requiredOptions, shortToLongLookup }: {
     spec: { [key: string]: IOption },
-    fail: (errorMessage: string) => void,
+    fail: (errorMessage: string) => never,
     outputArgs: { [key: string]: string },
     requiredOptions: { [key: string]: boolean },
     shortToLongLookup: { [key: string]: string },

src/runPreParseHook.ts

@@ -1,7 +1,8 @@
+/** Runs the given pre-parse hook, if defined, to modify the command line arguments. Returns the modified command line arguments. */
 export default function runPreParseHook({ args, preParseHook, fail }: {
   args: string[],
   preParseHook?: (args: string[], fail: (message: string) => void) => (string[] | undefined),
-  fail: (errorMessage: string) => void,
+  fail: (errorMessage: string) => never,
 }): string[] {
   if (preParseHook === undefined) {
     return args;

src/verifyAndStoreOptionValue.ts

@@ -13,7 +13,7 @@ export default function verifyAndStoreOptionValue({
   value: string,
   verify: IOption['verify'],
   message: IOption['message'],
-  fail: (errorMessage: string) => void,
+  fail: (errorMessage: string) => never,
   outputArgs: { [key: string]: string },
   requiredOptions: { [key: string]: boolean },
 }) {