feat(core): enhance repl 'help' command with nestjs functions

packages/core/repl/repl-native-commands.ts

@@ -0,0 +1,51 @@
+import type { REPLServer } from 'repl';
+
+/**
+ * Displays the list of available commands in the REPL alonside with their
+ * descriptions.
+ * (c) This code was inspired by the 'help' command from Node.js core:
+ * {@link https://github.com/nodejs/node/blob/58b60c1393dd65cd228a8b0084a19acd2c1d16aa/lib/repl.js#L1741-L1759}
+ */
+/* istanbul ignore next */
+function listAllCommands(replServer: REPLServer) {
+  Object.keys(replServer.commands)
+    .sort()
+    .forEach(name => {
+      const cmd = replServer.commands[name];
+      if (cmd) {
+        replServer.output.write(`${name}\t${cmd.help || ''}\n`);
+      }
+    });
+}
+
+export function defineDefaultCommandsOnRepl(replServer: REPLServer): void {
+  replServer.defineCommand('help', {
+    help: 'Show REPL options',
+    action(name?: string) {
+      this.clearBufferedCommand();
+
+      if (name) {
+        // Considering native commands before native nestjs injected functions.
+        const nativeCommandOrFunction =
+          this.commands[name] || this.context[name];
+        // NOTE: If the command was retrieve from the context, it will have a `help`
+        // getter property that outputs the helper message and returns undefined.
+        // But if the command was retrieve from the `commands` object, it will
+        // have a `help` property that returns the helper message.
+        const helpMessage = nativeCommandOrFunction?.help;
+        if (helpMessage) {
+          this.output.write(`${helpMessage}\n`);
+        }
+      } else {
+        listAllCommands(this);
+        this.output.write('\n\n');
+        this.context.help();
+        this.output.write(
+          '\nPress Ctrl+C to abort current expression, Ctrl+D to exit the REPL\n',
+        );
+      }
+
+      this.displayPrompt();
+    },
+  });
+}

packages/core/repl/repl.ts

@@ -5,6 +5,7 @@ import { assignToObject } from './assign-to-object.util';
 import { REPL_INITIALIZED_MESSAGE } from './constants';
 import { ReplContext } from './repl-context';
 import { ReplLogger } from './repl-logger';
+import { defineDefaultCommandsOnRepl } from './repl-native-commands';
 
 export async function repl(module: Type | DynamicModule) {
   const app = await NestFactory.createApplicationContext(module, {
@@ -23,5 +24,7 @@ export async function repl(module: Type | DynamicModule) {
   });
   assignToObject(replServer.context, replContext.globalScope);
 
+  defineDefaultCommandsOnRepl(replServer);
+
   return replServer;
 }