complete documentation

lib/Sources/EnvSource.ts

@@ -11,10 +11,20 @@ import { OptionSource } from './OptionSource';
 
 export class EnvSource extends OptionSource {
   public async parse (opt: Option, val:OptionValue): Promise<void> {
-    if (
-      typeof opt.env !== 'undefined'
-        && typeof process.env[opt.env] !== 'undefined'
-    )
-      await this.assign_arg (opt, val, process.env[opt.env]);
+    if (typeof opt.env === 'undefined')
+      return;
+
+    if (typeof process.env[opt.env] === 'undefined') {
+      if (typeof this.error_callback !== 'undefined') {
+        this.error_callback (
+          opt.name,
+          null,
+          new Error ('environment variable does not exist')
+        );
+      }
+      return;
+    }
+
+    await this.assign_arg (opt, val, process.env[opt.env]);
   }
 }