readline: allow completer to rewrite existing input

Sometimes, it makes sense for a completer to change the existing
input, e.g. by adjusting the casing (imagine a completer that
corrects `Number.isNan` to `Number.IsNaN`, for example).

This commit allows that in the readline implemention.

PR-URL: nodejs#39178
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

lib/readline.js

@@ -705,9 +705,21 @@ Interface.prototype._tabComplete = function(lastKeypressWasTab) {
     // If there is a common prefix to all matches, then apply that portion.
     const prefix = commonPrefix(ArrayPrototypeFilter(completions,
                                                      (e) => e !== ''));
-    if (prefix.length > completeOn.length) {
+    if (StringPrototypeStartsWith(prefix, completeOn) &&
+        prefix.length > completeOn.length) {
       this._insertString(StringPrototypeSlice(prefix, completeOn.length));
       return;
+    } else if (!StringPrototypeStartsWith(completeOn, prefix)) {
+      this.line = StringPrototypeSlice(this.line,
+                                       0,
+                                       this.cursor - completeOn.length) +
+                  prefix +
+                  StringPrototypeSlice(this.line,
+                                       this.cursor,
+                                       this.line.length);
+      this.cursor = this.cursor - completeOn.length + prefix.length;
+      this._refreshLine();
+      return;
     }
 
     if (!lastKeypressWasTab) {

test/parallel/test-readline-tab-complete.js

@@ -100,3 +100,39 @@ common.skipIfDumbTerminal();
   });
   rli.close();
 }
+
+{
+  let output = '';
+  class FakeInput extends EventEmitter {
+    columns = 80
+
+    write = common.mustCall((data) => {
+      output += data;
+    }, 9)
+
+    resume() {}
+    pause() {}
+    end() {}
+  }
+
+  const fi = new FakeInput();
+  const rli = new readline.Interface({
+    input: fi,
+    output: fi,
+    terminal: true,
+    completer: common.mustCall((input, cb) => {
+      cb(null, [[input[0].toUpperCase() + input.slice(1)], input]);
+    }),
+  });
+
+  rli.on('line', common.mustNotCall());
+  fi.emit('data', 'input');
+  queueMicrotask(() => {
+    fi.emit('data', '\t');
+    queueMicrotask(() => {
+      assert.match(output, /> Input/);
+      output = '';
+      rli.close();
+    });
+  });
+}