feat: allow to access output of default completion in custom completion

lib/completion.ts

@@ -193,7 +193,8 @@ export class Completion implements CompletionInstance {
       return (this.customCompletionFunction as FallbackCompletionFunction)(
         current,
         argv,
-        () => this.defaultCompletion(args, argv, current, done),
+        (onCompleted = done) =>
+          this.defaultCompletion(args, argv, current, onCompleted),
         completions => {
           done(null, completions);
         }
@@ -278,7 +279,7 @@ interface FallbackCompletionFunction {
   (
     current: string,
     argv: Arguments,
-    defaultCompletion: () => any,
+    completionFilter: (onCompleted?: CompletionCallback) => any,
     done: (completions: string[]) => any
   ): any;
 }

test/completion.cjs

@@ -341,16 +341,16 @@ describe('Completion', () => {
       r.logs.should.include('success!');
     });
 
-    it('allows the custom completion function to use the standard one', done => {
+    it('allows the custom completion function to use the default completion w/o filter', done => {
       checkUsage(
         () => {
           yargs(['./completion', '--get-yargs-completions'])
             .command('foo', 'bar')
             .command('apple', 'banana')
             .completion(
               'completion',
-              (current, argv, defaultCompletion, done) => {
-                defaultCompletion();
+              (current, argv, completionFilter, done) => {
+                completionFilter();
               }
             )
             .parse();
@@ -365,6 +365,35 @@ describe('Completion', () => {
       );
     });
 
+    it('allows custom completion to be combined with default completion, using filter', done => {
+      checkUsage(
+        () => {
+          yargs(['./completion', '--get-yargs-completions'])
+            .command('foo', 'bar')
+            .command('apple', 'banana')
+            .completion(
+              'completion',
+              (current, argv, completionFilter, done) => {
+                completionFilter((err, completions) => {
+                  const filteredCompletions = completions.filter(
+                    completion => completion === 'foo'
+                  );
+                  done(filteredCompletions);
+                });
+              }
+            )
+            .parse();
+        },
+        null,
+        (err, r) => {
+          if (err) throw err;
+          r.logs.should.include('foo');
+          r.logs.should.not.include('apple');
+          return done();
+        }
+      );
+    });
+
     it('allows calling callback instead of default completion function', done => {
       checkUsage(
         () => {
@@ -373,7 +402,7 @@ describe('Completion', () => {
             .command('apple', 'banana')
             .completion(
               'completion',
-              (current, argv, defaultCompletion, done) => {
+              (current, argv, completionFilter, done) => {
                 done(['orange']);
               }
             )