Add support for a validate function to pre

README.md

@@ -81,7 +81,8 @@ const args = basicArg({
     port: { type: Number, description: 'Port to listen on', default: 25565 },
     online: { type: Boolean, description: 'Whether to run in online mode' },
     path: { type: String, description: 'Path to the server directory', default: '.' }
-  }
+  },
+  // validate (args) { return true } /* optional fn to verify the args before returning them; non-true return value will print help screen */
 })
 // ...
 ```

index.d.ts

@@ -6,6 +6,7 @@ declare module "basic-cli" {
     description?: string;
     default?: any;
   }
+  type Result = Record<string, boolean | string | number>
   // The second argument is the custom arg array if any, otherwise default to `process.argv`  
   export default function(options: {
     name: string,
@@ -14,6 +15,8 @@ declare module "basic-cli" {
     throwOnError?: boolean, // Throw an error instead of calling process.exit() with help screen (default: false)
     errorOnExtra?: boolean, // Throw an error if there are extra arguments (default: false)
     helpCommand?: string, // The -- command for opening the built-in help screen (default: help)
-    options: Option[]
-  }, arguments?: string[])
+    options: Option[],
+    // Return true here if the result is ok, otherwise a string to raise to the user along with help screen
+    validate?: (args: Result) => void | string
+  }, arguments?: string[]): Result
 }

index.js

@@ -106,7 +106,14 @@ function parse (options, args) {
     return raise(`** Extraneous options: ${extra.join(', ')}`, options)
   }
 
-  return Object.fromEntries(haves)
+  const result = Object.fromEntries(haves)
+
+  if (options.validate) {
+    const ret = options.validate(result)
+    if (ret !== true) raise(ret)
+  }
+
+  return result
 }
 
 module.exports = parse